Electromagnetic surveying method and device

Abstract

A device for performing an electromagnetic survey of a subsurface environment. The device comprises one or more transmitter coils and one or more receiver coils, the coils collectively configured to perform electromagnetic induction (EMI) on a material of the subsurface environment in response to application of an excitation signal to the one or more transmitter coils. The device further comprises a control circuit that is configured to: (i) generate the excitation signal at a frequency that varies over time and incrementally over a frequency band; (ii) apply the excitation signal to the one or more transmitter coils; (iii) process a response signal induced within the one or more receiver coils from the EMI to generate electromagnetic data representing the material as a function of at least the frequency of the excitation signal; and (iv) process the electromagnetic data to determine one or more material specific parameters of the subsurface environment around the coils.

Description

"Electromagnetic surveying method and device" Technical Field

[0001] The present invention relates to systems, methods and devices for performing a survey of the electromagnetic properties within and / or about an environment, and more specifically to the use of electromagnetic induction techniques for characterising material parameters for a wide range of applications in mining, environmental and industry, in particular the present invention can be used for example in identifying geological parameters of formations about or within a subsurface environment (e.g., surrounding a borehole). Background

[0002] Electromagnetic (EM) surveying methods detect the electromagnetic properties of materials in an environment by inducing an EM signal and measuring the response. There are a wide range of examples of electromagnetic surveying, including activities to characterize subsurface environments by determining the properties of various formations or structures of interest.

[0003] One specific application of EM surveying is Electromagnetic Induction (EMI) borehole logging. The term “borehole” is used to collectively refer to any of the various types of holes that may be drilled into a ground surface, whether above ground or underground, for example in order to perform resource exploration or geotechnical investigation or assessment of a site, such as a mine site, to enable the collection of soil samples, water samples or rock cores, or to install monitoring wells or piezometers.

[0004] Measurement of a borehole is typically performed during a geological survey of the borehole, and the generation, storage, and / or processing of the measurement data obtained from the survey is often referred to as “logging” the borehole. This information can feed into a geological profile that provides utility for assessing the subsurface and each individual borehole within. Such models are used to increaseefficiencies in planning and operating a mine site, for example, during the blasting process. This improves the ability to source and extract mineral and resource deposits, particularly in less accessible areas.

[0005] Another important outcome of a borehole surveying process is the ability to determine a lithology, or geological characterization, of the formations and strata surrounding one or more boreholes in a mine site. Further, it is desirable for the survey to be conducted in-situ by means of deployment of a borehole survey device or logging tool into one or more boreholes to produce a log of corresponding geological parameters (e.g., electrical resistivity) of the surrounding rock formations. The ability to determine geological parameters of a borehole via a surveying process that is performed in real-time promotes resource exploration and / or geotechnical investigation with increased automation and efficiency.

[0006] EMI borehole logging provides advantages for a borehole surveying process in that geological or other material parameters can be determined without electrical contact with formations via a conductive borehole mud or fluid. Parameters of interest for a borehole survey may include electrical resistivity, magnetic permeability and electrical permittivity (i.e., the three EM medium dependent parameters), and / or various other geological, geophysical, and / or petrophysical parameters that can be interpreted from an EM measurement.

[0007] For example, an electrical conductivity distribution may provide information about the fraction percent and type of clays present in a formation. This will have consequences for many mining operations (e.g., iron ore and bauxite). Electrical resistivity is an important baseline log that should be recovered in every hole, and is mandatory to meet standards for the logging of some subsurface environments (e.g., confined aquifers) in some jurisdictions. Magnetic susceptibility is also important as it can often be traced directly to distribution of magnetite which is a key mineral for understanding geology of a borehole or other subsurface environment.

[0008] It will be appreciated that EM surveying techniques, including EMI borehole logging, are a specific application of EM sensing where electromagnetic fields are generated in the presence of a medium and an induced signal is measured to determine properties of the medium. Many systems and methods designed for EM surveying can therefore also be adapted to perform EM sensing in other application domains (i.e., where the medium may or may not be located in a subsurface environment). Summary

[0009] There is provided a device for performing an electromagnetic survey of a subsurface environment, comprising: one or more transmitter coils and one or more receiver coils, the coils collectively configured to perform electromagnetic induction (EMI) on a material of the subsurface environment in response to application of an excitation signal to the one or more transmitter coils; and a control circuit configured to: (i) generate the excitation signal at a frequency that varies over time and incrementally over a frequency band; (ii) apply the excitation signal to the one or more transmitter coils; (iii) process a response signal induced within the one or more receiver coils from the EMI to generate electromagnetic data representing the material as a function of at least the frequency of the excitation signal; and (iv) process the electromagnetic data to determine one or more material specific parameters of the subsurface environment around the coils.

[0010] In some embodiments, the control circuit generates the excitation signal over a set of discrete frequencies that substantially span the frequency band.

[0011] In some embodiments, the set of discrete frequencies represent an oversampling of the frequency band relative to a bandwidth of the frequency band, or to a selected frequency value of the frequency band.

[0012] In some embodiments, a step interval of the discrete frequencies is limited by the highest frequency component of the frequency band.

[0013] In some embodiments, the frequency band has a minimum frequency value at or above 10 Hz and a maximum frequency value at or below 1MHz.. In other embodiments, one or more frequencies of the frequency band may be greater than 1MHz.

[0014] In some embodiments, the excitation signal is a sinusoidal voltage signal.

[0015] In some embodiments, the subsurface environment comprises a formation surrounding the borehole, the formation comprising the material, and wherein the device is deployed into the borehole during the survey.

[0016] In some embodiments, the control circuit is configured to repeat at least steps (i) to (iii) iteratively as the device is deployed through the borehole during the survey.

[0017] In some embodiments, processing the electromagnetic data comprises generating a depth-frequency map from the electromagnetic data generated at one or more depths of the device within the borehole.

[0018] In some embodiments, the processing of the response signal to generate the electromagnetic data is performed based on one or more calibrations of the control circuit.

[0019] In some embodiments, the control circuit generates the electromagnetic data without using a bucking coil to measure a component of the response signal induced by the one or more transmitter coils.

[0020] In some embodiments, the electromagnetic data is comprises a set of complex relative impedance vectors, each vector having real and imaginary impedance values for a corresponding excitation signal frequency value.

[0021] In some embodiments, the control circuit comprises: an impedance converter having a frequency generator, an analog-to-digital converter (ADC), and a digital-to- analog converter (DAC); and a microcontroller in communication with the impedanceconverter and configured to control the impedance converter to: (i) generate a digital excitation signal by operating the frequency generator at a selected frequency within the frequency band; (ii) convert the digital excitation signal to an analog signal using the DAC; (iii) measure, using the ADC and in response to application of the excitation signal to the one or more transmitter coils, a corresponding digital response signal from the response signal induced in the one or more receiver coils; and (iv) process the digital response signal to determine spectral response values at the frequency, wherein steps (i) to (iv) are repeated iteratively as the selected frequency is incrementally increased within the frequency band.

[0022] In some embodiments, the microcontroller controls the impedance converter to vary the frequency of the generated digital excitation signal synchronously with the measurement and processing of the corresponding digital response signal.

[0023] In some embodiments, determining the spectral response values involves applying a discrete Fourier transform (DFT) to the digital response signal.

[0024] In some embodiments, the microcontroller is configured to: receive one or more spectral response values, as generated at one or more discrete frequencies, from the impedance converter via a data bus; and determine the set of complex relative impedance vectors by calculating a magnitude and a phase of the spectral response values.

[0025] In some embodiments, the microcontroller is configured to process the set of complex relative impedance vectors to determine the material specific parameters of the subsurface environment.

[0026] In some embodiments, the one or more material specific parameters comprise at least one of: an electrical resistivity; a magnetic susceptibility; a magnetic permeability; an electrical permittivity; an electrical resistivity; and an electrical conductivity.

[0027] There is also provided a method for performing an electromagnetic (EM) survey of a subsurface environment using an EM survey device, comprising: (i) generating, by a control circuit of the device, an excitation signal at a frequency that varies over time and incrementally over a frequency band; (ii) applying, by the control circuit, the excitation signal to one or more transmitter coils of the device to cause the one or more transmitter coils to perform electromagnetic induction (EMI) on a material of the subsurface environment; (iii) receiving, by the control circuit, a response signal induced within one or more receiver coils of the device from the EMI; (iv) processing, by the control circuit, the response signal to generate electromagnetic data representing the material as a function of at least the frequency of the excitation signal; and (v) processing, by one or more processors, the electromagnetic data to determine one or more material specific parameters of the subsurface environment around the coils.

[0028] In some embodiments, at least steps (i) to (iv) are repeated iteratively as the survey device is deployed through the borehole during the survey.

[0029] In some embodiments, the method further comprises: (i) generating a digital excitation signal by operating a frequency generator of the control circuit at a selected frequency within the frequency band; (ii) converting the digital excitation signal to an analog signal using a digital-to-analog converter (DAC) of the control circuit; (iii) measuring, using an analog-to-digital converter (ADC) of the control circuit and in response to application of the excitation signal to the one or more transmitter coils, a corresponding digital response signal from the response signal induced in the one or more receiver coils; and (iv) processing, by a microcontroller of the control circuit, the digital response signal to determine spectral response values at the frequency, wherein steps (i) to (iv) are repeated iteratively as the selected frequency is incrementally increased within the frequency band.

[0030] In some embodiments, the method further comprises: receiving, at the microcontroller, one or more spectral response values, as generated at one or more discrete frequencies, from the impedance converter via a data bus; and determining, bythe microcontroller, the set of complex impedance vectors by calculating a magnitude and a phase of the spectral response values.

[0031] In some embodiments, the method further comprises processing, by the microcontroller, the set of complex impedance vectors to determine the one or more material specific parameters.

[0032] There is also provided a method for characterizing a formation surrounding a borehole comprising the steps of: conducting an electromagnetic survey of the borehole using the any of the methods described herein; processing, by one or more processing devices, data associated with the formation of the borehole as generated by the electromagnetic survey; and determining, by the one or more processing devices, a lithology of the formation based on the processing of the data associated with the formation.

[0033] In some embodiments, the processing of the data associated with the formation comprises at least one of: generating classification scores on the values of the data associated with the formation against one or more predetermined models of a rock type; and performing a statistical analysis of the values of the data associated with the formation.

[0034] In some embodiments, the data associated with the formation includes values of at least one of: an electrical resistivity; an electrical permittivity; a magnetic susceptibility; a magnetic permeability; and an electrical conductivity, of the formation.

[0035] There is also provided a method for characterizing a formation surrounding a borehole, the method comprising: using a survey device to perform electromagnetic induction (EMI) on the formation by: generating, by a control circuit of the survey device, an excitation signal to excite at least one transmitter coil of the survey device, wherein the excitation signal is generated at a frequency that varies over time and incrementally over a frequency band; receiving, by the control circuit from at least one receiver coil of the survey device, a response signal induced in the at least one receivercoil in response to the EMI; and processing, by the control circuit, the response signal to generate electromagnetic data representing the formation as a function of at least the frequency of the excitation signal; obtaining, by one or more processing devices, the electromagnetic data representing the formation; processing, by the one or more processing devices, the electromagnetic data to determine one or more geological parameters of the formation; and determining, by the one or more processing devices, a lithology characterizing the formation based on the one or more geological parameters.

[0036] In some embodiments, the survey device is a device according to any of the devices described herein.

[0037] There is also provide a method performed by a computing system for generating a visualization of a formation surrounding a borehole, the method comprising: obtaining data associated with the formation of the borehole by conducting an electromagnetic survey of the borehole using any of the methods described herein; processing data associated with the formation of the borehole as generated by the electromagnetic survey to determine visualization data that, when rendered on an electronic display, visually indicates at least: a frequency representation of the data associated with the formation over the frequency band; and a lithology of the formation; and rendering the visualization data on the electronic display. Brief Description of Drawings

[0038] Some embodiments are described herein below with reference to the accompanying drawings, wherein:

[0039] Fig.1a is a schematic diagram of an exemplary survey device configured to perform borehole electromagnetic induction (EMI) of the prior art;

[0040] Fig.1b is a schematic diagram of electromagnetic signals and induction current produced by the survey device of Fig.1a;

[0041] Fig.2 is a schematic diagram of an exemplary electromagnetic (EM) survey device for performing EMI on a formation surrounding a borehole, according to one implementation of the present technology;

[0042] Fig.3a is a schematic diagram of an example of the EM survey device of Fig. 2;

[0043] Fig.3b is a first schematic diagram illustrating the deployment of the survey device of Fig.2 to survey a borehole;

[0044] Fig.3c is a second schematic diagram illustrating the deployment of the survey device of Fig.2 to survey a borehole;

[0045] Fig.4 is a flow diagram of a method for performing an EM survey of the borehole using the EM survey device, according to one implementation of the present

[0046] Fig.5 is a flow diagram of a sub-method for performing a frequency scan setup operation of the EM survey method of Fig.4;

[0047] Fig.6a is a flow diagram of a method for generating the excitation signal to perform the EM survey method of Fig.4, according to one implementation of the present technology;

[0048] Fig.6b is a schematic diagram of an example frequency generator of an impedance converter configured to perform the EM survey method of Fig.4, according to one implementation of the present technology;

[0049] Fig.6c is a schematic diagram of an example digital-to-analog conversion (DAC) circuit of an impedance converter configured to perform the EM survey method of Fig.4, according to one implementation of the present technology;

[0050] Fig.7a is a flow diagram of a method for generating the impedance data during the EM survey method of Fig.4, according to one implementation of the present technology;

[0051] Fig.7b is a schematic diagram of an example analog-to-digital conversion (ADC) circuit of an impedance converter configured to perform the EM survey method of Fig.4, according to one implementation of the present technology;

[0052] Fig.7c is a schematic diagram of an example spectral analysis module of an impedance converter configured to perform the EM survey method of Fig.4, according to one implementation of the present technology;

[0053] Fig.8a is a flow diagram of a method for generating geological parameter values in the frequency and time or depth domains from impedance data, according to one implementation of the present technology;

[0054] Fig.8b is an exemplary table of the complex impedance values determined from impedance vectors of an impedance map, according to one implementation of the present technology;

[0055] Fig.9a is a flow diagram of a method for characterizing a formation surrounding a borehole, according to one implementation of the present technology.

[0056] Fig.9b is a flow diagram of a method for generating a visualization of a formation surrounding a borehole, according to the proposed technology.

[0057] Fig.10a is a graph of a standard geological parameter log generated by a conventional wireline survey of a formation;

[0058] Fig.10b is a graph of a geological parameter log derived from complex impedance values generated by performing an EM survey of the same formation of Fig. 10a with an EM survey device according to one implementation of the present technology;

[0059] Fig.10c is an illustration of an example visualization derived from the same geological parameter data (i.e., complex impedance values) of Fig.10b, over a frequency range of 40kHz to 80kHz;

[0060] Fig.10d is an illustration of the visual spectra of Fig.10c in a scaled view; and

[0061] Fig.10e is an illustration of a representation of intermediate outputs generated to produce the visualizations depicted in Figs.10c and 10d according to one implementation of the present technology. Description of Embodiments

[0062] In this specification and claims, except where the context requires otherwise due to express language or necessary implication, the following definitions apply.

[0063] “Bore hole”, “hole” and “borehole” refer to a hole drilled by a drill rig in an environment, area of interest, or bench which is to be surveyed.

[0064] “Environment” refers to any region in physical space within which electromagnetic fields can be generated to produce EM induction, such as for the purpose of performing EM surveys and / or sensing.

[0065] “Surveying” refers to the process of determining measurements of one or more parameters of an environment, such as for example a subsurface environment, by a measurement device over time.

[0066] “Geological surveying” refers to the process of determining at least geological data indicating, for example, the mineralogical, structural, or physical characteristics of formations, including those penetrated by a borehole, using a geo-sensing component of the measurement device.

[0067] “Geological data” refers to any data relating to the geophysical, petro-physical, mineralogical, hole geometry, chemistry and / or compositional data of an environmentbeing surveyed including a borehole, and / or of material in and / or surrounding strata / formation of the borehole, as described herein below. Geological data may also include any other data gathered from a geological surveying activity conducted in an environment, including a subsurface environment, such as geo-technical parameters (e.g., rock strength) or hydrogeological or hydraulic parameters (e.g., hydraulic conductivity, porosity, water chemistry etc).

[0068] “Depth data” refers to data values indicating a depth of a reference device, typically a measurement device also used to generate geological data, deployed within a subsurface environment, such as the interior of a borehole where the depth is an indication of the substantially linear distance between the position of the device, and a collar position of the borehole, along the axis of the borehole.

[0069] “Measurement data” refers to data generated during a surveying process by a measurement device at one or more time instants (e.g., as the measurement device is moved through a borehole), and may comprise, in some cases, of geological data and corresponding depth data. For example, wireline logging of a borehole involves the generation of measurement data at sampling intervals (spatial intervals).

[0070] “Logging” refers generally to making a record of geological data associated with subsurface environment, including the material parameter values of one or more formations within. In some cases, logging refers to the storage of measurement data generated during a surveying process (the “logging data”), where the storage occurs either within the measurement device obtaining the measurements (“on-device”) (e.g., when a wireline is not used), or on another device (“off-device”) (e.g., when a wireline is used to transmit the measured data to another device at the surface). In other cases, logging also refers to the collection, generation, and / or processing of the measurement data. For example, logging a borehole may involve transmitting or sending measurement data generated by a measurement device deployed within the borehole to one or more external devices or systems for subsequent recording of the data.

[0071] “Borehole profile data” refers to a collection of data that describes one or more characteristics or properties of a particular borehole, which may include, but is not limited to, logging data of the borehole. For example, borehole profile data may include logging data and additional data, such as locational data (e.g., specifying a position of the collar of the borehole in a mine site), and / or model data (e.g., representing a reconstruction or simulation of the borehole). Additionally, in some examples borehole profile data may include simulated or synthetic logging data, and / or data related to a producing a synthetic wireline log, or the borehole.

[0072] “Surface” refers to the top of the ground / formation and / or area of interest including, but not limited to, whatever earth, soil, or land that lies above superincumbent upon or about the collar of the borehole. In some examples, the surface is the top level of a body of water or other fluid in which the logging takes place.

[0073] “Sub-surface” refers to the region below the surface including, but not limited to, the region below the collar of the borehole into which the borehole (cavity) extends.

[0074] “Collar” (of a borehole) - the mouth or opening of the borehole onto the surface, typically created by a drilling operation carried out by a drill rig.

[0075] EM surveying is commonly used in subsurface environments, such as, for example, to perform EMI borehole logging. Conventional approaches to borehole logging involve conducting a geological survey during which a measurement device (or survey device) is deployed into the borehole. The survey device typically includes a set of electronic measuring instruments, or tools, that are adapted to collect data by using, for example, electrical, acoustical, nuclear and / or magnetic energy signals to stimulate the formations and strata of the borehole and measure the response. The goal of a geological survey is to generate data that enables a characterization of the formation(s) and / or strata of the borehole in terms of its geological parameters such as, for example, magnetic susceptibility, magnetic permeability, electrical permittivity, electrical conductivity and / or electrical resistivity.

[0076] One conventional approach to geological characterization of a subsurface formation involves applying an electrical excitation signal to the formation via an electrode array. A resulting response signal (e.g., a voltage) in the formation is measured at a frequency of the excitation signal. The response signal may therefore be processed to determine particular geological parameters of the formation (e.g., the electrical resistivity from the phase and the amplitude of the voltage).

[0077] There are disadvantages of electrode based surveying. First, this approach requires physical contact between the survey device and the formation (e.g., via the electrode array) to apply the excitation signal and measure the resulting response signal. Current must have a path from the transmitting electrode back to the receiving electrode (or electrode array). This is problematic if the hole is cased unless the casing material is roughly matched in conductivity of the formation material.

[0078] Second, in contrast to formation profiling activities that proceed using core samples extracted from the formation, electrode based approaches are less practical in applications where the surveying device is deployed into the borehole (e.g., via a wireline) with the objective of collecting real-time data (i.e., as the survey device is continuously moving through the borehole). In particular, the non-idealized shape of the borehole interior and / or the presence of mud, fluid, or other debris, result in difficulties using electrodes, or other physical members, to establish reliable electrical contact with the formation for the purpose of applying the excitation signal and measuring the resulting response signal.

[0079] An alternative approach to performing a survey of a subsurface environment involves electromagnetic induction (EMI). For example, in a borehole EMI-based survey, the electrical properties of a formation are determined by the measurement of a response signal in a receiver component (e.g., an induced voltage in a wire coil) as the result of energizing a transmitter component with a known excitation signal. The induced voltage has a contribution from currents induced in the formation (known as “eddy currents”), allowing the derivation of properties of the formation material such as the electrical conductivity.

[0080] EMI is a non-invasive and non-contact survey technique, and can therefore be conducted in boreholes with nearly all types of construction. Electrical conductivity (and its inverse, resistivity) is influenced by lithology, minerology, porosity, permeability, saturation, and concentration of dissolved ions within the groundwater. Borehole EMI may be used to obtain a bulk (i.e., formation plus fluid) measurement of one or more geological parameters, such as apparent magnetic susceptibility, and / or to identify relative changes in the same.

[0081] EMI can work in the presence of a plastic casing or in open hole applications where there is any combination of fluid, plastic and air inside or outside the casing. Provided that the materials have non-zero electrical conductivity the distribution of complete current carrying pathways provides a means to determine measurements of geological parameters.

[0082] Further, EMI surveying enables the collection of depth-dependent measurements of the geological parameters of a borehole while the survey device remains at a physical distance from the interior walls of the borehole. Further, EMI surveying can be completed above or below the fluid level inside a borehole, or other subsurface environment (where as galvanic methods will not work in air).

[0083] Fig.1a illustrates a schematic diagram of a conventional survey device 10 configured to perform borehole EMI. Exemplary survey device 10 comprises a board 12 including a transmitter coil 14, and a receiver coil 16 separated by a predetermined fixed distance. A bucking coil 18 is located axially between the transmitter coil 14 and receiver coil 16. The board 12 is connected to a console 20 including a signal generator 24, reference signal converter 28, response signal converter 26, and a DSP circuit 22.

[0084] Fig.1b illustrates a schematic diagram of the electromagnetic signals produced by a transmitter coil 14, and received by a receiver coil 16, of a conventional borehole EMI survey device, such as device 10 of Fig.1a. Rock formation 11 represents a formation of a borehole (e.g., where the formation 11 surrounds the borehole) in which the survey device 10 is deployed. The transmitter coil 14 is energized by application ofan excitation signal (e.g., an electrical current or voltage signal) generated by an excitation source (e.g., signal generator 24). The application of the excitation signal to the transmitter coil 14 generates an electromagnetic signal in the form of a time-varying electromagnetic field. The generated field electromagnetically induces an electrical current (eddy current) within the formation 11. The result is an electromagnetic signal induced in the receiver coil 16 that may be considered to have a primary component Hp (from the transmitter coil 14) and a secondary component Hs (from the currents in formation 11).

[0085] M12, M13and M23represent the mutual inductance between the transmitter coil 14 and the rock formation 11 (M12), the transmitter coil 14 and the receiver coil 16 (M13), and the receiver coil 16 and the rock formation 11 (M23). As shown in Fig.1b, the receiver coil 16 measures an induced response signal from an electromagnetic field, which, once determined, may be used to characterise the external complex impedance of the formation 11. The response has a primary component Hp and a secondary component Hs (with corresponding inductances M13and M23), where the characteristics of the formation 11 are a function of only the secondary component Hs. It will be appreciated that Fig.1b, and illustrated components Hp and Hs, are abstractions to illustrate the effect, at the receiver coil 16, of the EM forces acting on charge carriers that can circulate in a material from the free space EM field generated by the excitation source (i.e., transmitter coil 14).

[0086] Conventional approaches to borehole EMI address this issue by using one or more additional coils (e.g., the bucking coil 18 of device 10 omitted from Fig.1b) to estimate the primary component Hp in the response signal, and to allow removal of the primary component from the response signal induced in the receiver coil 16. For example, the bucking coil 18 of device 10 is configured to provide a reference signal to converter 28, and receiver coil 16 provides the electromagnetically induced response signal to converter 26. DSP circuit 22 receives digitized reference signal and response signals, as generated by converters 28 and 26, and determines the digital response signal by normalizing the response signal with the reference signal. Use of the bucking coil 18 therefore allows for the direct removal of the contribution of the primarycomponent from the response signal induced in the receiver coil 16. This enables the DSP circuit 22 to determine the electrical properties of the formation 11 by processing the residual response signal which contains only the secondary component.

[0087] The need to eliminate the primary component contribution in the response signal leads to conventional electromagnetic (EM) surveying approaches, such as borehole EMI, being constrained in the frequency domain. Specifically, conventional approaches to borehole EMI use an excitation signal containing a single frequency, or a small number of discrete pre-selected frequencies. This is because these conventional techniques rely on the use of hardware elements (e.g., bucking coil) to determine and isolate the contribution of the secondary component in order to obtain an indication of the electromagnetic properties of the formation.

[0088] More specifically, it is a challenge to recover the extremely small differences in EM signal amplitude and / or phase offset resulting from material properties (i.e., the conductivity distribution) as compared to the determined variations in the free space EM signal that are linked to the measurement device itself (i.e., “self-response”).

[0089] A drawback of the conventional approaches to borehole EMI is that the required hardware components must be calibrated for each individual frequency of the excitation signal. As a result, if the frequency is varied, even slightly, then the ability to eliminate or reduce the primary component within the response signal will be compromised. For example, a bucking coil will not be able to accurately estimate the primary component when the frequency of the excitation signal varies over a wide range, since the bucking coil must be re-tuned to match the receiver coil for proper operation. This tuning cannot be done in real-time thereby preventing conventional borehole EMI-based survey devices from operating effectively with a frequency varying excitation. Further, as conventional approaches conduct the EM survey at a singular pre-selected frequency they are often vulnerable to noise, particularly at higher frequency values. Accordingly, it is desired to develop devices and methods that address one or more of these problems, or other problems, or that at least provide a useful alternative.Overview

[0090] Disclosed herein are methods and devices that generally relate to performing electromagnetic (EM) surveying or sensing of a material in an environment. An exemplary device comprises one or more transmitter coils configured to, in response to an applied excitation signal, generate an electromagnetic field having components that are distributed in time and space with a dependence on a set of material specific parameters. The devices also include one or more receiver coils configured to have a response signal induced by the generated electromagnetic field. A control circuit is configured to generate and apply the excitation signal to the transmitter coil(s) and to measure electromagnetic data (e.g., impedance values) from the induced response signal for the purpose of recovering values and / or a distribution of the material specific parameters in the environment around the coils. The electromagnetic data measured from the induced response signal represents the material as a function of at least the frequency of the excitation signal.

[0091] Such devices can be used for, but are not limited to, a unique form of induction logging where the EMI is performed over a selectable, and dynamically variable, spectral range (i.e., frequency band). In some embodiments, a step interval of the frequency band of the excitation signal is set to generate discrete frequencies that are closely spaced and substantially span the frequency band. In some embodiments, the value of the step interval of the frequency band is set such as to oversample the frequency of the excitation signal within the frequency band (e.g. based on a bandwidth or a selected frequency of the band). This advantageously increases the amount of electromagnetic data collected, thereby allowing for an improved signal-to-noise ratio at least at higher frequencies which are typically prone to the effects of noise. Further, by selecting the frequency band to extend over a relatively broad range the proposed techniques advantageously generate an increased amount of electromagnetic data compared to a conventional survey, thereby resulting in an improved ability to analyze the distribution(s) of material parameters at varying depths and distances from the borehole into the formation, and to enable a more detailed analysis of the EM response behaviour of the material across the broad range of frequencies.

[0092] The embodiments described herein relate to specific devices and methods for performing an EM survey of a subsurface environment such as for example the interior of a borehole. The borehole has one or more formations comprising respective materials characterized by values of the one or more material specific parameters.

[0093] Fig.2 illustrates a borehole survey device 100, including transmitter and receiver coil pair 104, and a control circuit 102 configured to control the operation of the coil pair 104 to perform electromagnetic induction (EMI) on a formation 11 of a borehole 101 (not shown) into which the survey device 100 is deployed. The transmitter and receiver components of the coil pair 104 are configured as transmitter 104a and receiver 104b wire coils respectively. EMI occurs by application of an excitation signal to the transmitter coil 104a causing the coil 104a to produce an electromagnetic field to induce an electrical current in the formation 11. As a result, a response signal is induced in the receiver coil which has a contribution from the transmitter 104a and from the currents in formation 11 (referred to as primary and secondary components respectively).

[0094] In some embodiments, the device 100 comprises one or more transmitter coils and one or more receiver coils that are collectively configured to perform EMI on a material (e.g., formation 11) within the subsurface environment (e.g., interior of borehole 101) in response to the application of an excitation signal to the one or more transmitter coils. The number of transmitter and / or receiver coils may be selectable or configurable for the surveying application of the device 100. While transmitter and receiver coils 104a 104b are shown as a pair with a co-axial orientation in Fig.2, the number and configurations of receiver and transmitter coils may be varied to suit different environments or constraints (e.g., to conform to a length of the device 100 or the dimensions of the interior of the borehole 101).

[0095] In some embodiments, relative measurements between two or more receiver coils 104b may provide information that can be used to recover the material specific parameters in which measurements are made.

[0096] The control circuit 102 operates the transmitter and receiver coil pair 104 to: i) generate the excitation signal at a frequency that varies over time and incrementally over a frequency band; ii) apply the excitation signal to the one or more transmitter coils (e.g., coil 104a); iii) process a response signal induced within the one or more receiver coils (e.g., coil 104b) to generate electromagnetic data representing the formation 11 as a function of at least the frequency of the excitation signal; and iv) process the electromagnetic data to determine one or more geological parameters of the formation 11.

[0097] In some embodiments, the electromagnetic data is impedance data including relative impedance data. However, it will be appreciated that other electromagnetic parameters, such as voltage, may instead be measured and processed by the device 100 to achieve the same results.

[0098] The proposed technology advantageously provides a means for performing EM surveying using a single device that is able to be dynamically configured to characterize formations of a wide variety of materials, in varying subsurface environments. This results in reduced costs and time spent conducting borehole surveying operations, at least by eliminating the need to manufacture, configure, and deploy multiple surveying devices across for example multiple boreholes on a mine site. Frequency scan

[0099] The excitation signal is generated with a frequency that varies incrementally. That is, unlike conventional approaches, the control circuit 102 generates the excitation signal over a set of discrete frequencies that are selected to substantially span the frequency band. This may be achieved, for example, by generating the excitation signal at each of a set of output frequencies that are very closely spaced within the band (i.e., such as to substantially span and densely sample the frequency band). The set of output frequencies are determined by incrementally increasing the frequency value of the continuously generated excitation signal within the frequency band. This is referred to as a frequency scan operation herein.

[0100] In some embodiments, the device generates impedance or other electromagnetic data rapidly over a large number of discrete frequencies. For example, relative impedance (i.e., real and quadrature components) may be recovered for hundreds of frequencies per measurement point at typical logging speeds.

[0101] For example, the control circuit may be configured with a selected starting frequency, a frequency step size (i.e., interval), and a number of steps by which to advance the scan from the selected starting frequency. Selection of the step interval to a value significantly less than the bandwidth (e.g., < 100 Hz for a 1 kHz to 100 kHz band) results in a large number of closely spaced frequencies that collectively span the frequency band. For example, the step interval of the discrete frequencies may be limited by the highest frequency component (i.e., the maximum frequency value) of the frequency band (e.g., set to several orders of magnitude below this value).

[0102] In some embodiments, the frequency band extends from 10 Hz to 1 MHz. In other embodiments, the frequency range spans a smaller sub-range, such as preferably a 3 kHz to 80 kHz range or a 5 kHz to 80 kHz range. In some embodiments, the maximum frequency value of the frequency band is greater than 1MHz.

[0103] By generating the excitation signal over a range of frequencies substantially spanning the frequency band, data collected from the corresponding response signal at any time instant during the survey provides information about the dynamic response of the formation 11 in at least the frequency domain. This is advantageous over conventional borehole EMI approaches in which the excitation signal has a fixed frequency over time, or a small number of discrete frequencies, which prevents or limits an accurate determination of the spectral response of the surveyed formation 11.

[0104] In some embodiments, the set of discrete frequencies for which the device generates impedance data, or other electromagnetic data, are selected to densely sample the frequency band, or a portion thereof. That is, the set of discrete frequencies are selected to represent an oversampling of the frequency band relative to a bandwidth of the frequency band, or to a selected frequency value of the frequency band.

[0105] For example, the value of the step interval may be set to a significantly smaller value compared to the bandwidth of the frequency band, or to the maximum frequency value, such as to oversample the frequency of the excitation signal within the range of the frequency band. In some embodiments, the value of the step interval is dynamically adjusted depending on the current frequency value of the excitation signal (e.g., to use a smaller increment for higher frequencies where a greater degree of frequency oversampling is desired). This advantageously increases the amount of electromagnetic data collected, therefore allowing for a better signal-to-noise ratio, at the higher frequencies which are typically prone to the effects of noise.

[0106] The excitation signal is a time-varying signal. For example, the excitation signal may be a sinusoidal voltage signal transmitted from the control circuit 102 to the transmitter coil 104a. The frequency of the sinusoidal excitation signal is varied incrementally as the signal is constantly outputted according to the frequency scan operations described herein. In some embodiments, the frequency band exists within a range of 1 kHz to 100 kHz. That is, the frequency band has a minimum frequency value at or above 1 kHz and a maximum frequency value at or below 100 kHz. In other embodiments, the frequency band exists within a wider interval with a lower minimum frequency value (e.g., 10 Hz) and a larger maximum frequency value (e.g., 1 MHz). This provides a significantly larger bandwidth than electrode-based surveying approaches for which the excitation signal frequency typically resides within 0.001 Hz to 20 kHz. Time-frequency characteristics

[0107] The EM survey is performed by a survey device 100 deployed into an interior section of the borehole 101. In some embodiments, the survey is conducted by performing EMI with the survey device 100 held in a fixed position within the borehole 101 (e.g., a position adjacent to the formation 11).

[0108] In other embodiments, the survey device 100 is moved through the interior of the borehole 101 (e.g., from a collar position of the borehole 101 to an end position of the borehole), either periodically or continuously, as the EMI is performed. In suchembodiments, the control circuit 102 is configured to repeat the generation and application of the excitation signal, and the processing of the response signal, over time as the device 100 is deployed through the borehole 101 during the survey. Processing the impedance data comprises capturing temporal variations of the generated impedance values as the survey device 100 moves through the borehole and for the frequencies of the excitation signal.

[0109] For example, the control circuit 102 may be configured to generate an impedance map representing the formation 11 as a function of both the time of the survey and the frequency of the excitation signal. In some embodiments, a depth- frequency impedance map is generated from impedance data generated at one or more depths of the device within the borehole (e.g., by matching the impedance data generated at a plurality of time instants of a survey performed by device 100 and corresponding depths of the device 100 within the borehole 101).

[0110] Other representations of the impedance data or other electromagnetic data generated by the survey may also be used to model the dynamic behaviour of the formation 11 as an electromagnetic signature or “fingerprint” in the time-frequency, and / or spatio-frequency, domains. For example, the set of impedance data, or other electromagnetic data, may be transformed into corresponding geological data indicating values of one or more geological parameters (e.g., electrical and / or magnetic permittivity, conductivity, permeability, and / or susceptibility) at a particular survey depth, where the geological data may be subsequently used to identify the formation (e.g., by performing pattern classification on features extracted from the geological signature(s)). In some embodiments, the geological data is generated using machine learning (ML), classification and / or pattern recognition techniques, where the impedance data, or other electromagnetic data, is used as input to a parameter model (e.g., a geological formation model trained on a priori data). Processing the response signal

[0111] The response signal induced in the receiver coil 104b includes an EM contribution from the component associated with the currents circulating in theformation 11 (the “secondary component”), and a contribution from the transmitter coil 104a (the “primary component”). The control circuit 102 is configured to process the response signal to generate impedance data or other electromagnetic data and to generate material parameter values that account for the effect of the primary component of the response signal within that data.

[0112] The control circuit 102 is thereby able to advantageously generate the set of impedance data to represent the characteristics of the formation 11 dynamically over a range of excitation frequencies, and without adjusting the control circuit 102 to each specific discrete frequency, to perform the EMI-based analysis. Further, the control circuit 102 operates to determine impedance data that is associated with the secondary component of the response signal (i.e., representing only the signal generated as a result of induction in the formation 11), without measuring the primary component using a bucking coil. The resulting impedance, or electromagnetic, measurement produced contains the EM response of the formation 11 plus that of the device 100 conducting the measurement (the “self-response”).

[0113] As a result, the control circuit 102 generates the impedance data, or other electromagnetic data, by processing a digital representation of the response signal induced in the receiver coil 104b based on one or more calibration operations. In some embodiments, the calibration operation(s) correct for deviation of the values of the measured electromagnetic data (e.g., relative impedances) from the true values representative of the material. Sources of this deviation may include: i) the contribution of the primary component of the transmitter coil 104a in the response signal 103b; and ii) measurement error intrinsic to the components of the device 100 (e.g., the impedance converter, as described below).

[0114] In some embodiments, a first calibration operation is performed to configure the control circuit 102 to generate relative impedance data with values in a measurement range of interest (i.e., a range that is appropriate to the frequency band and the material). For example, the first calibration operation may be conducted during a validation survey (i.e., at a time prior to the live survey of borehole 101).

[0115] The first calibration operation configures the control circuit 102 to determine a reference, or baseline, of the sensitivity and / or resolution of the EMI process for the frequency band and the formation 11. The first calibration operation may be performed using a priori known impedance data for the formation 11, or a similar material or material type, as determined from a validation survey. This enables the survey device 100 to produce accurate relative impedance, or other electromagnetic, data during a live survey on the formation 11 using any excitation signal with a frequency in the selected band. That is, as a result of the first calibration operation, the control circuit is sensitized to obtain impedance, or other electromagnetic, data in the range of values that are appropriate for the formation 11 and for the frequency band. This is referred to herein as “baseline calibration”.

[0116] In some embodiments, a second calibration operation is performed to configure the control circuit 102 to generate relative impedance, or other electromagnetic data, that accounts for the self-response of the device 100 relative to the EMI. This “self-response calibration” operation takes into account any stray capacitive reactance, inductive reactance or resistance introduced by the system (including control circuit 102 and / or coils 104a, 104b).

[0117] In some embodiments, the self-response is calculated based on a pre- programmed or default value. In other embodiments, the second calibration operation is conducted during a validation survey (i.e., at a time prior to the live survey of borehole 101), either the same validation survey as the first calibration operation or otherwise. The second calibration operation configures the control circuit 102 to determine a contribution to the impedance data, or other electromagnetic data, values from the primary component by conducting EMI through the frequency band on a validation formation for which the true impedance (electromagnetic) data values are known. This enables the survey device 100 to dynamically calculate the system impedance during a live survey on the formation 11 using the difference between the measured and validation values, for any excitation signal with a frequency in the selected band. The system impedance contribution, either calculated or pre-programmed, may then beremoved from the measured impedance (electromagnetic) data to be left with relative impedance related to the formation 11.

[0118] In some embodiments, one or more calibration operations are performed as post-processing operations on data generated by the control circuit 102 (e.g., in response to a survey conducted with a control circuit 102 configured in a default or uncalibrated state, or in a pre-calibrated state). In some embodiments, the post- processing operations are performed by one or more processing devices of the control circuit 102. Alternatively, or in addition, the post-processing operations are performed by a computing system on the electromagnetic data received from the control circuit 102.

[0119] The post-processing operations may include operations to correct or mitigate the effects of temperature on the values of the electromagnetic data generated by the control circuit 102. For example, the one or more processing devices may utilize temperature profile data that describes a known linear relationship between values of the electromagnetic and / or geological parameters and temperature of the material of formation 11 to adjust the electromagnetic data.

[0120] The post-processing operations may also include operations to correct or mitigate the effects of self-response on the values of the electromagnetic data generated by the control circuit 102. For example, the one or more processing devices may be configured to determine a value of the system impedance contribution, as described above, following the generation of the electromagnetic data by the control circuit 102, and to remove the system impedance contribution from the electromagnetic data values.

[0121] In the some embodiments, the control circuit 102 comprises at least an impedance converter 110 and a microcontroller 120 in communication with, and configured to control, the impedance converter 110. The impedance converter 110 is adapted to drive the transmitter and receiver coil pair 104 by generating the excitation signal as an output and receiving the response signal as an input. The impedanceconverter 110 is configured to generate, based on a digital response signal, spectral response values over the set of output frequencies of the band of the excitation signal. For example, determining the spectral response values may involve applying a discrete Fourier transform (DFT) to the digital response signal.

[0122] In some embodiments, the control circuit 102 generates the impedance data as a set of complex impedance vectors, each vector having real and imaginary impedance values for a corresponding excitation signal frequency value at a sample time instant of the survey. Values of the impedance vectors are generated, by the microcontroller 120, from the magnitude and phase values of the spectral response values. The calibration process enables the calculation of magnitude and phase values of the spectral response (e.g., the DFT coefficients of the digital response signal), correcting for self-response and temperature effects, and the subsequent use of these values to estimate a residual voltage phase offset to represent the formation dynamically over the range of excitation frequencies. Characterizing a formation

[0123] Devices, apparatuses and systems may be configured to perform a method to characterize a formation 11 of a borehole 101 by conducting an electromagnetic survey of the borehole 101 using the borehole electromagnetic induction (EMI) methods described herein. In some embodiments, the electromagnetic survey generates data associated with the formation 11 of the borehole 101, such as for example impedance data of the formation 11. The relative impedance data can be transformed to recover various geological parameters such as the conductivity and shape of the formation 11 (e.g., by generating a 3D distribution of electrical conductivity).

[0124] One or more processing devices are configured to process the data associated with the formation 11 of the borehole 101 as generated by the electromagnetic survey. For example, the impedance data may be transformed into a depth-impedance map describing respective sets of complex impedance values for frequency values across the frequency band at time instants of the survey. The depth-impedance map may be processed to determine corresponding data of one or more geological parameters, suchas electrical and / or magnetic resistivity, permittivity, conductivity, permeability or susceptibility.

[0125] For example, measured parameters in the depth frequency domain may be mapped as measurements at multiple depths and multiple frequencies. That is the depth-frequency domain can be populated with complex components of filtered and processed impedance values. Other maps may be based on real components of filtered and processed impedance values (with the primary component removed), and / or amplitudes or phases of filtered and processed impedance values.

[0126] The one or more processing devices are configured to determine and / or characterize a lithology of the formation based on the processing of the data associated with the formation. For example, the formation may be characterized as comprising a particular type of rock such as sandstone, clay-bearing rock, or pyrite-bearing rock, or a particular mineral, such as coal or ore. The characterization may be based on the processing of the impedance values, and / or the corresponding geological parameter values, such as for example average values of the resistivity, conductivity, permittivity, and / or susceptibility determined over the formation 11. For example, a measure of apparent resistivity may provide an indication of the composition of the formation 11 (e.g., to distinguish sulphides, graphitic materials, high CEC clays and very salty water etc).

[0127] The proposed approaches for borehole surveying are advantageous in that the survey device 100 does not require the use of a bucking coil, or a similarly configured physical induction component, to directly measure the effect of the primary component on the response signal induced within the receiver coil 104b, during a live survey. Further, the impedance of the formation can be modelled using excitation signals with frequencies that vary dynamically over a selected frequency band in real-time during the survey.

[0128] The ability to vary the frequency of the excitation signal substantially over the whole frequency band enables a complete analysis of the formation in the frequencydomain. This provides improved resolution compared to conventional borehole EMI survey devices, which are constrained by an inability to accurately eliminate the primary component from the response signal when the frequency of the excitation signal is changed from a pre-selected value (e.g., due to an imperfect matching of the receiver and bucking coils). Survey device

[0129] Fig.2 illustrates an exemplary borehole survey device 100 comprising a transmitter and receiver coil pair 104 and a control circuit 102. Coil pair 104 comprises a transmitter coil 104a and a receiver coil 104b each electrically connected to the control circuit 102. Transmitter coil 104a is configured to receive an excitation signal 103a from an output terminal 102a of the control circuit 102. The application of the excitation signal 103a to the transmitter coil 104a by the control circuit 102 energizes the transmitter coil 104a to perform EMI on a formation 11 of a borehole 101 (not shown) located adjacent to the coil pair 104.

[0130] Receiver coil 104b is configured to provide a response signal 103b to the control circuit 102 in response to experiencing induction by one or more electromagnetic signals resulting from the EMI. The transmitter and receiver coil pair 104 are configured to perform EMI as illustrated by Fig.1b. The excitation signal 103a applied to energize the transmitter coil 104a, and the corresponding response signal 103b induced in the receiver coil 104b, are time-varying electromagnetic signals such as voltage or current signals. When energized, the transmitter coil 104a creates a four dimensional force distribution that compels any mobile charge (e.g. in a continuous circuit like the receiver coil 104b and the formation 11) to move. For example, a voltage is induced in the receiver coil 104b by time varying magnetic flux through the coil 104b.

[0131] In some implementations, the transmitter coil 104a and receiver coil 104b are multi-turn wire coils aligned axially within a board or enclosure. In other implementations, the relative location and / or orientation of the transmitter 104a andreceiver 104b coils within the device 100 may be varied according to the application such as to enhance the response signal (e.g., by maximizing a rate of change of flux density along the device axis) during the EM induction process.

[0132] It is a characteristic of the operation of the device 100 that a sinusoidal excitation signal is passed through the transmitter coil(s) 104a at a large number of discrete frequencies thereby performing EMI in an area around the formation 11. Control circuit 102 measures electromagnetic data values (e.g., relative impedance values) at each frequency across the ends of the receiver coil(s) 104b. The electromagnetic data values are associated with the induced EM response (i.e., voltage) in the receiver coil(s) 104b, which is dependent on the frequency of the excitation signal (as controlled by the control circuit 102). The measured values (e.g., real and imaginary components of relative impedance values) are processed by the control circuit 102 based on one or more calibration procedures, to estimate a distribution of the parameters of formation 11 in the area.

[0133] The control circuit 102 comprises an impedance converter system 110 having a frequency generator 112, an analog-to-digital converter (ADC) circuit 113 and a digital-to-analog converter (DAC) circuit 115. The frequency generator 112 is configured to enable the generation of excitation signal 103a with a known frequency, as generated via a frequency scan operation with a user-defined start frequency, end frequency, and number of increment points in the scan. In some implementations, the excitation signal 103a is generated at a subhertz resolution. In some implementations, the frequency generator 112 is configured with a “wait time” between frequencies to ensure that effects from a previous scan do not bleed into the measurement for the next frequency.

[0134] ADC circuit 113 and DAC circuit 115 are respectively connected to output pin 108 and input pin 109 of the impedance converter 110. The impedance converter 110 further includes a spectral analysis module 114 configured to receive signal inputs from the frequency generator 112 and ADC circuit 115 and to output spectral response values to register block 116. In some embodiments, the impedance converter 110further includes one or more sensors 118 to monitor the operation of the impedance converter 110, such as for example a temperature sensor.

[0135] Control circuit 102 includes a microcontroller 120 in communication with the impedance converter 110 via a communications interface 111. The microcontroller 120 includes at least one processor and a memory system, and is configured to control the impedance converter 110 to perform digital signal processing operations associated with generating the excitation signal 103a, applying the excitation signal 103a to the transmitter coil 104a, receiving the response signal 103b from receiver coil 104b, and performing digital signal processing on the response signal 103b.

[0136] In some embodiments, the control circuit 102 includes an analog front end element (AFE) 106 connected to the output pin 108 and input pin 109 of the impedance converter 110. AFE 106 is configured to receive the excitation signal generated by the impedance converter 110 (i.e., by the frequency generator 112 and DAC circuit 113) at an output condition circuit 105. The output condition circuit 105 of the AFE 106 performs pre-processing of the excitation signal prior to the application of the excitation signal 103a to the transmitter coil 104a.

[0137] For example, the output condition circuit 105 may include a power amplifier system to boost the amplitude of the analog excitation signal 103a output by the impedance converter 110. The AFE 106 further includes an input condition circuit 107 configured to receive the analog response signal 103b from the receiver coil 104b and to pre-process the response signal 103b prior to transmission to the impedance converter 110. In such embodiments, the AFE 106 provides a direct interface to the transmitter and receiver coil pair 104 for delivering the excitation signal 103a to the transmitter coil 104a and receiving the response signal 103b from the receiver coil 104a.

[0138] In other embodiments, the control circuit 102 does not include the AFE 106, and the impedance converter 110 is connected to the transmitter 104a and receiver 104b coils via the output pin 108 and input pin 109 respectively.

[0139] In some embodiments, the survey device 100 includes power supply (not shown) configured to provide electrical power to at least the microcontroller 120 and / or impedance converter 110. For example, the power supply may be a battery configured to supply a DC voltage (e.g., 5V) to the control circuit 102. In other embodiments, the control circuit 102 is configured to receive power from a source external to the survey device 100, such as for example via a wireline connection, or similar, used to deploy the survey device 100 into the borehole 101.

[0140] In some embodiments, the survey device 100 is configured to communicate with a remote computing system, such as a bench management system (BMS) configured to receive, store, and process data associated with one or more boreholes. The microcontroller 120 communicates with the BMS via an intermediate communications network, such as the Internet, or another wide area network such as a Global System for Mobile Communications (GSM) network enabling the BMS to be physically separated from the survey device 100.

[0141] Survey device 100 generally operates as a EM sensing device that creates EM fields (at transmitter 104a) and measures an induced response (at the receiver 104b). The specific configuration of the device 100 and its components is dependent on the application (e.g., borehole EMI as described herein), however the device 100 provides advantages that are common to all applications and implementations including: (i) enabling an efficient and low construction of the survey device (e.g., due to the ability to use off-the-self components); (ii) an ability to scale the components to meet size and weight requirements (e.g., for a small and / or light device); (iii) ability to perform sensing operations over very closely spaced discrete frequencies that are dynamically varied (“scanned”) in a selected band; (iv) allowing flexibility in the design of the frequency scan for application specific optimization of the device; and (v) rapid acquisition of material parameters for the scan frequencies. Example converter circuit

[0142] In some examples, the impedance converter 110 is implemented using an Analog Device AD5933 circuit as available from Analog Devices being a 1 MSPS, 12-Bit Impedance Converter, Network Analyzer (see [1]). In other examples, the impedance converter 110, or a functionally equivalent circuit, may be implemented by any device that measures relative impedance accurately in the receiver coil 104b from the EMI. That is, the device 100 may be configured to operate with any circuit that performs accurate measurements of a relative impedance representation of the receiver coil 104b by “converting” a response signal, related to the induced voltage at a given excitation frequency.

[0143] In some examples, the frequency generator 112 is implemented as a Direct Digital Synthesizer (DDS) core that provides a digital output signal at a specified frequency. The excitation signal 103a is generated by setting a selected frequency of the DDS core via an incremental frequency scan operation performed over a user defined frequency band and frequency increment. The scan is programmable to a sub- hertz resolution (e.g., <0.1 Hz). DAC circuit 113 operates on the DDS core output signal to produce an analog output signal in the form of a sinusoidal voltage signal between the output pin 108 and input pin 109 of the impedance converter 110. The peak-to-peak value of the output sinusoidal signal may be programmed within a range. The AD5933 system ( is configured to electrically connect the output pin 108 and input pin 109 to the AFE 106 (if implemented), or to the coil pair 104 (see [1]).

[0144] The response signal 103b as received by the impedance converter 110 (i.e., at the input pin 109) is sampled by the ADC circuit 115. The ADC circuit 115 includes an ADC configured to perform sampling with a 12-bit resolution and a 1 MSPS throughput, a current-to-voltage amplifier, an antialiasing filter, and a programmable gain amplifier (PGA) configured to allow the user to adjust the output of the current-to- voltage amplifier. The digitized response signal values are processed by an on-board DSP engine of the spectral analysis module 114 configured to apply a DFT operation to the signal values. The DFT operation returns spectral response values having a real and an imaginary data-word at each output frequency. The magnitude and relative phase of the impedance at each frequency point along the scan is calculated (as adjusted according to the calibration).

[0145] The microcontroller 120 is configured to drive the impedance converter 110 via a I2C compliant serial bus interface 111. The microcontroller 120 issues instructions to the impedance converter 110, via the interface 111, from the execution of one or more control programs configured to perform an EM survey of the borehole 101. The microcontroller 120 is configured to read the contents of one or more registers of the register block 116 to obtain the real and imaginary coefficients of the DFT values, and to process the values to generate impedance data (i.e., complex impedance vectors) at frequencies of the excitation signal.

[0146] Many other embodiments of the control circuit 102 may exist in which there are variations in the hardware implementation and configuration of the impedance converter 110, and / or the microcontroller 120 configured to execute the borehole EM surveying methods described herein. The form factor and / or configuration of the control circuit 102 may be determined at least in part by the physical and / or operational requirements of the EM surveying process (e.g., the borehole 101 that is to be logged).

[0147] Fig.3a illustrates a schematic diagram of an embodiment of the EM survey device 100. The control circuit 102 includes impedance converter 110, microcontroller 120, and optional AFE unit 106. The transmitter 104a and receiver 104b coils are contained within an enclosure 104c, which may be implemented as an elongated portion of resilient material to protect the coils 104a, 104b during the deployment of the device 100 into a borehole.

[0148] In some implementations, the enclosure 104c comprises a universal sealed high strength tube (FRP) with a multi O-Ring sealing system at either end. In some implementations, the form and structure of the enclosure 104c are selected to stabilize the device 100 and / or reduce its temperature (i.e., heating that accumulates during transmission), which may include the use of thermal insulation materials between elements of the device 100 that retain heat and those that are needed for accurate measurement.

[0149] Figs.3b and 3c illustrate the deployment of the survey device 100 of Fig.2 to survey a borehole 101. Fig.3b depicts a measurement device (MD) 300 configured to collect measurements of the borehole 101 using the EM survey device 100. In such embodiments, the EM survey device 100 is configured as a geological sensing module or component that is operated by the MD 300 to provide a geological parameter data and / or impedance data in relation to the formation / strata 11 surrounding the borehole 101.

[0150] For example, values of relative impedance are related to the dB / dt (expressed as voltage amplitude and phase) measured through the receiver coil 104b or apparent resistivity which can then be inverted to recover a conductivity distribution. Alternatively, the apparent resistivity can be used as a stand-alone outcome that is interpreted to infer geology and / or water chemistry about the formation 11.

[0151] The MD 300 further includes one or more measurement instruments 302a, and one or more additional geological sensors 302b (e.g., electrical or electronic sensors) collectively configured to provide geological parameter surveying additionally to the EM survey device 100. For example, measurement instruments 302a may include a calliper set configured to physically engage with the interior walls of the borehole 101 to measure a diameter of the borehole 101, and the one or more geological sensors 302b may include a gamma sensor configured to detect gamma radiation through the scintillation of light produced by the interaction of the gamma rays with a scintillator crystal material.

[0152] The components of the MD 300 may be arranged in interconnected sections enabling the modular attachment and detachment of the components in accordance with a desired function of the MD 300. For example, measurement instrument module 302a may be removed from the MD 300 if measurement of the borehole diameter is not desired for a logging operation. Other components of the MD 300 include: a deployment connector (not shown) enabling the MD 300 to be lowered into the borehole 101 via a wireline 319; and a housing 339 that encapsulates the components. The housing 339 is composed of a resilient material, such as a metal or hard plastic, toprovide protection to the internal modules of the MD 300, including the survey device 100, during movements of the MD 300 within the borehole 101 (e.g., as the MD 300 is moved through the borehole 101 during the logging process).

[0153] In some embodiments, the MD 300 may include other exemplary sensors such as a temperature sensor, a water sensor, a deviation sensor that can sense pitch, roll and heading, or any other number of varying modules in addition to the EM survey device 100. For example, MD 300 may include a pressure sensor configured to provide data enabling the location of the pressure head above the MD 300. For a vertical borehole this will accurately provide depth below water level in a well. In some embodiments, the MD 300 includes multiple temperature sensors configured to measure temperature gradients enabling the correction or mitigation of temperature effects in the electromagnetic data generated by the survey device 100.

[0154] Additionally, in some embodiments the MD 300 is provided with centralizers 304 at, or adjacent to, proximate and distal ends of the MD 300 that are positioned about the housing 339. The centralizers 304 provide stability to the MD 300 within the borehole 101 which can further assist the functionality of the survey device 100, the measurement instruments 302a and / or the additional geological sensors 302b.

[0155] The MD 300 includes a controller 330 configured as an embedded system with a processor and memory (not shown) implemented, for example, as an integrated microcontroller with a RISC architecture. The survey device 100, measurement instruments 302a, and additional geological sensors 302b may be configured as peripheral devices providing data to, and receiving control data from, the controller 330.

[0156] In some embodiments, the controller 330 includes one or more operational modules configured to store data including, at least, the measurement values generated by the measurement components 100, 302a, 302b of the MD 300. In some embodiments, the controller 330 is configured to store additional data including: position data; and optionally depth offset, correction and / or adjustment data generatedby, or provided to, the MD 300. The depth values and / or depth offset values may be provided to the MD 300 by an external logging apparatus, and / or generated on-device by the MD 300.

[0157] In some embodiments, the MD 300 controls the operation of the survey device 100 via an exchange of data and control signals between the controller 330 and the microcontroller 120 of the control circuit 102. For example, the microcontroller 120 may be configured to receive and process instructions from the controller 330 to cause the device 100 to commence or cease an EM survey of the borehole 101. The instructions received from the controller 330 may specify a set of survey parameters to be used by the device 100 to perform the EM survey. The survey parameters may include a survey rate or period defining the time between successive samples of the survey, excitation parameters defining the excitation signal 103a to be generated by the survey device 100 at each survey sample time, and an indication of one or more geological parameters to be determined by the survey device 100.

[0158] The microcontroller 120 is configured to transmit to the controller 330 one or more of: impedance data generated by the EM survey (e.g., values of complex impedance vectors); and data indicating one or more geological parameters determined from the impedance data generated by the EM survey (e.g., values of electrical resistivity, electrical permittivity, electrical conductivity, magnetic permeability, and / or magnetic susceptibility). In some embodiments, the microcontroller 120 is configured to receive additional data from the controller 330 to facilitate the processing of the impedance data. For example, the microcontroller 120 may receive from the controller 330 depth data indicating relative depths of the MD 300, and therefore the device 100, within the borehole 101 at one or more time instants during the survey.

[0159] It will be appreciated that in other embodiments the survey device 100 is configured to perform an electromagnetic survey of the borehole 101 as a standalone measurement device. In such embodiments, the survey device 100 is deployed into the borehole 101, and subsequently operated to generate impedance and correspondinggeological parameter data without integrating the device 100 with, or attaching the device 100 to, another measurement or surveying device.

[0160] With reference to Fig.3b, operation of the survey device 100 by the MD 300 causes the transmitter and receiver coil pair 104 to perform EMI on the formation 11 located adjacent to the MD 300. Transmitter coil 104a is energized by the application of excitation signal 103a generated by the control circuit 102. In response, the transmitter coil 104a generates an electromagnetic signal to induce a current within the formation 11.

[0161] The receiver coil 104b is induced with a response signal 103b containing respective primary component Hp, and secondary component Hs which may be measured to characterise the complex impedance of the formation 11. The response signal 103b received at the control circuit 102 from the receiver coil 104b is specific to the frequency characteristic of the excitation signal 103a. The incremental variation of the frequency of the excitation signal 103a enables the survey device 100 to generate impedance data indicating complex impedance values of the formation 11 across the frequency band at a given time instant of the survey.

[0162] In Fig.3b the survey device 100 is in a fixed position within the borehole 101. In a typical electromagnetic survey, the MD 300, and therefore the survey device 100, is moved through the borehole 101 enabling measurements to be obtained for varying depths of the device 100 within the borehole 101.

[0163] Fig.3c illustrates the movement of the MD 300, and therefore the survey device 100, through the borehole 101 during an exemplary EM survey process. The as drilled borehole 101 extends into the ground at a collar position A and terminates at an end or “toe” position A’. The EM survey involves moving the device 100 along an axial path within the borehole 101, and in a direction either heading into the borehole (i.e., from A towards A’, as depicted) and / or inversely heading out of the borehole 101 (i.e., from A’ towards A).

[0164] The EM survey involves the generation of data periodically at sample time instants as the device 100 is moved within the borehole 101. For example, the survey data may be collected at T time instants ^^^,… ^^்at which the device 100 has corresponding depths, relative to the collar position A, of ^^^,…, ^^். At each time ^^^and corresponding depth value ^^^the EM survey produces a set of impedance datavalues {^^^^, ^^^ଶ, … ,where each ^^^^ is a complex vector representing theimpedance measured for excitation frequency ^^^within the band. By collating the setsof impedance data values over each sample time ^^^ ∈ {^^^, … ^^்} (and depth ^^^ ∈{^^^, … ^^்}) the survey device 100 generates a time-frequency (or depth-frequency)impedance map characterizing the formation 11 (i.e., comprising values{^^^^, ^^^ଶ, … , ^^^^},…{^^்^, ^^்ଶ, … , ^^்^}, where m is the number of frequenciesincremented over in generating the excitation signal 103a). Method for a borehole EMI based survey

[0165] Fig.4 illustrates a flow diagram of a method 400 for performing an electromagnetic (EM) survey of the borehole 101 using the EM survey device 100. At step 401, a device configuration process is performed to initialize the device 100 in preparation for conducting a survey of the borehole 101. In some embodiments, device configuration involves: i) configuration of the frequency scanning operation; and ii) optionally, one or more calibration operations.

[0166] In some embodiments, the one or more calibration operations configure the control circuit 102 to generate electromagnetic data with values within a measurement range appropriate to the frequency band and the material. In some embodiments, the calibration operations may further configure the control circuit to generate electromagnetic data that accounts for the self-response of the device relative to the EMI. In other embodiments, one or more calibration operations are performed to post- process the electromagnetic data to correct or mitigate the self-response and / or other effects (e.g., temperature effects). The post-processing may be performed by the processing device(s) of the control circuit 102 and / or a computing system configured toreceive electromagnetic data generated the (calibrated or uncalibrated) control circuit 102. Frequency scanning

[0167] Fig.5 illustrates a flow diagram of a sub-method 500 for performing the frequency scan setup operation. Setup involves configuring the microcontroller 120 with one or more parameters that enable the control circuit 102 to generate excitation signals in the scan operation. In examples using the AD5933 circuit (see [1]) as the impedance converter system 110, the frequency scan setup involves the programming of parameters including: a start frequency ^^^௧^^௧, a frequency increment ^^^^^, and a number of increments ^^^^^to be performed over the scan.

[0168] At step 502, a frequency band is first determined for the generation of the excitation signal 103a. The band is defined by a minimum frequency ^^^^^and maximum frequency ^^^^௫as [^^^^^, ^^^^௫].In some embodiments, a user of the survey device 100 selects the band from a set of selectable predetermined bands. In other embodiments, the user may customize the band to set the values of ^^^^^and ^^^^௫to any arbitrary positive real number within the operational limits of the impedance converter 110. In some embodiments, the frequency band is selected to extend at least from 10 Hz to 100 KHz, for example as the [10Hz,100kHz] band. In some embodiments, the frequency band has minimum frequency ^^^^^is equal to or greater than 10Hz and maximum frequency ^^^^௫is equal to or less than 100 KHz. In some embodiments, the maximum frequency ^^^^௫is set as equal to or less than a larger value, such as for example 1 MHz.

[0169] At step 504, the one or more parameters defining the frequency scans to be conducted by the impedance converter 110 are determined based on the selected frequency band. The start frequency ^^^௧^^௧is determined as the ^^^^^value of the frequency band.

[0170] The increment frequency ^^^^^and number of increments ^^^^^are determinedrelative to the bandwidth (^^^ ^௫ − ^^^^^) of the selected band [^^^^^, ^^^^௫] and / or themaximum frequency ^^^^௫to facilitate generation of the excitation signal 103a with a set of frequencies that substantially and densely sample the band. In some embodiments, this is achieved by setting a frequency increment ^^^^^value relative to the bandwidth, and / or maximum frequency ^^^^௫, relative to a value of ^^^^^selected to be above a minimum number and below a maximum increment number supported by the converter 110. For example, in some embodiments ^^^^^is determined as the bandwidth / ^^^^^(i.e., ^^^^^≤ 200Hz for use of the [10,100kHz] band with ^^^^^= 500). This ensures that the impedance converter 110 facilitates the generation of the excitation signal 103a, at each sample time instant of the survey, by incrementally advancing through the frequency domain of the signal with sufficiently high resolution relative to the bandwidth (i.e., to densely sample the frequency band).

[0171] At step 506, the impedance converter 110 is programmed with the one or more parameters specifying the frequency scan. For example, in some configurations the start frequency ^^^௧^^௧and frequency increment ^^^^^may be set by programming corresponding 24-bit words to on-board registers of register block 116. The ^^^^^value is programmed as a 9-bit word in register block 116. A start frequency code and a frequency increment code are loaded into start frequency and frequency increment registers based on the determined ^^^௧^^௧and ^^^^^values and the frequency of the clock signal (see [1]). EMI via a frequency varying excitation signal

[0172] With reference to Fig.4, the control circuit 102 is configured to perform method 400 by executing at least steps 402 to 406 to: (i) generate the excitation signal 103a by performing the programmed frequency scan operation to vary the frequency of the excitation signal 103a over the frequency band (at step 402); (ii) apply the excitation signal to the transmitter coil 104a (at step 404); and (iii) process the response signal 103b received from the receiver coil 104b to generate impedance data representing the formation 11 of borehole 101 as a function of at least the frequency of the excitation signal 103a (at step 406). In some embodiments, the control circuit 102 isfurther configured to (iv) process the impedance data to determine one or more geological parameters of the formation 11.

[0173] Fig.6a illustrates a flow diagram of a method performed by the control circuit 102 for generating the excitation signal 103a (i.e., at step 402 of Fig.4), according to an example embodiment. Fig 6b illustrates a schematic diagram of an example frequency generator 112 of the impedance converter 110 according to one embodiment. Frequency generator 112 includes a 27-bit phase accumulator DDS core 112a that provides a digital output signal at a selected frequency ^^ (see [1]). The DDS core 112a operates on a clock signal provided externally by the control circuit 102 via an external clock pin CLK 112b of the impedance converter 110. In some configurations, the impedance converter 110 provides an internal clock with a default maximum frequency (e.g., 16.776 MHz) by means of an on-chip oscillator 112c. In some embodiments, the clock signal is additionally provided to the ADC circuit 115.

[0174] At step 602, the microcontroller 120 issues a start frequency scan command to the impedance converter 110 (e.g., using a control register). At step 604, the impedance converter 110 generates a digital excitation signal by operating the frequency generator 112 at an output frequency (current selected frequency) within the frequency band.

[0175] At step 606, the digital excitation signal with the current output frequency ^^ is converted to an analog signal via the DAC circuit 113. Fig 6c illustrates a schematic diagram of an example DAC circuit 113 of the impedance converter 110 according to one embodiment (as described in [1]). The output signal of the frequency generator 112 is passed into a DAC module 113a and a programmable gain stage to generate a voltage signal, at output pin 108, as the output (analog) excitation signal in a pre-specified peak-to-peak amplitude range (as controlled by ^^^^^^and the value of resistor 113b). The peak-to-peak output excitation voltage is selected by setting one or more bits in the control register of the circuit 110 and is made available at the output pin 108.

[0176] Referring to Fig.6a, at step 608 the analog excitation signal 103a is output from the DAC circuit for application to the transmitter coil 104a. The transmitter coil104a is energized with the excitation signal 103a at the current frequency ^^ resulting in generation of the impedance data. As described below, generation of the impedance data involves the impedance converter: measuring, using the ADC circuit 115, a corresponding digital response signal from the response signal induced in the receiver coil 104b; and processing the digital response signal to determine spectral response values at the current output frequency ^^.

[0177] At step 609, the impedance converter 110 checks whether the frequency scan is complete. If the frequency scan has completed over all frequency points (i.e., discrete output frequencies of the frequency band), the impedance converter 110 indicates completion of the scan to the microcontroller 120 (i.e., at step 612).

[0178] Otherwise, if the frequency scan is not complete at step 609, then, at step 610, the impedance converter 110 incrementally increases the output frequency within the frequency band, and re-invokes the frequency generator 112 to perform the frequency scan at the next (incremented) output frequency (i.e., by repeating steps 604 to 608). The next output frequency of the digital excitation signal is determined by the frequency generator 112 based on the current output frequency ^^ and the frequency increment ^^^^^. The impedance converter 110 loads the current output frequency ^^ and increment ^^^^^from values stored in one or more registers of register block 116. A newoutput frequency ^^^ ∈ [^^^^^, ^^^^௫] is determined by incrementing the current outputfrequency ^^ with the ^^^^^value. Steps 604 to 609 are then repeated with the new output frequency to perform the next iteration through the frequency scan.

[0179] With reference to Fig.4, at step 404 the time and frequency varying excitation signal 103a generated by the frequency scan operation is applied to the transmitter coil 104a and a corresponding response signal 103b is induced in the receiver coil 104b. In some embodiments, the excitation signal 103a is first passed to the AFE 106 for processing by the output condition circuit 105 prior to application to the transmitter coil 104a (as illustrated in Fig.2). In other embodiments, the excitation signal 103a is applied to the transmitted coil 104a directly from output pin 108.Generating impedance data

[0180] At step 406, the control circuit 102 processes the response signal 103b to generate impedance data representing the formation 11 as a function of at least the frequency of the excitation signal 103a.

[0181] Fig.7a illustrates a flow diagram of a method performed by the control circuit 102 for generating the impedance data, according to an example embodiment. At step 702, the control circuit 102 receives the response signal 103b from receiver coil 104b as an analog signal. Response signal 103b is induced into receiver coil 104b as a result of EMI performed with the excitation signal 103a at a particular point in the frequency scan operation (i.e., excitation signal 103a has a particular output frequency ^^).

[0182] At step 704, the analog response signal 103b is processed to generate a corresponding digital response signal. In some embodiments, the analog response signal 130b is received at the AFE 106 where input condition circuit 107 performs one or more pre-processing operations (e.g., magnitude scaling). In other embodiments, the response signal 103b is received directly by the impedance converter 110 at the input pin 109. The impedance converter 110 measures, using the ADC circuit 115, a corresponding digital response signal from the response signal 103b induced in the receiver coil 104b.

[0183] Fig.7b illustrates a schematic diagram of an example ADC circuit 115 of the impedance converter 110 according to one embodiment (as described in [1]). The ADC circuit 115 comprises a current-to-voltage amplifier 720, connected to a programmable gain amplifier (PGA) 724, antialiasing filter 726, and an ADC module 728. The first stage current-to-voltage amplifier 720 is configured such that a voltage present at the input pin 109 is a virtual ground with a DC value set at VDD / 2. The response signal 103b results in a voltage signal at the output of the current-to-voltage amplifier 720. The gain of the current-to voltage amplifier 720 is determined by a user-selectable feedback resistor 721 connected between RFB pin 723 and the input pin 109. Thefeedback resistance value R of resistors 723, 727, 729, and the value 5 × ^^ of resistor730, are selected such that, in conjunction with the selected gain of the PGA 724 (e.g.,either × 1 or × 5), the amplitude of the signal is maintained within the linear range ofthe ADC module 728 (0 V to ^^^^).

[0184] The PGA 724 enables the adjustment of the output of the current-to-voltage amplifier 720 by a factor of 5 or 1, which is configurable via a status bit in a control register of the impedance converter 110. The signal is then passed through low-pass filter 726 and presented to the input of the 12-bit, 1 MSPS ADC 728.

[0185] Referring to Fig.7a, at step 706 the impedance converter 110 processes the digital response signal data, as output from the ADC circuit 115, to determine spectral response values corresponding to the current frequency of the scan. In some embodiments, determining the spectral response values involves applying a discrete Fourier transform (DFT) to the digital response signal.

[0186] Fig.7c illustrates a schematic diagram of an example spectral analysis module 114 of the impedance converter 110 according to one embodiment (as described in [1]). The spectral analysis module 114 receives input data including: a number of samples of the digital response signal from the ADC circuit 115; and corresponding cosine and sine components of the excitation signal provided by the frequency generator 112 (e.g., DDS core) at the current output frequency. Windowing module 740 processes sets of digital response signal samples and applies windowing and pre-processing operations before outputting the processed samples to a DSP core 742. The DSP core 742 performs a DFT on the windowed data of the digital response signal samples. For example, a 1024-point DFT is calculated over a corresponding number of digital response signal samples at the current frequency ^^ of the excitation signal 103a according to: (cos(^^) − ^^^^^^^^(^^))where ^^(^^) ^^^^ the power in the signal at the current frequency ^^, ^^(^^) is the output of the ADC circuit 115 (i.e., the nth digital response signal sample of the set), and ^^^^^^(^^)and ^^^^^^(^^) are the corresponding excitation signal component values provided by the frequency generator 112 at the frequency ^^. The result is stored in two output registers 744, 746 of register block 116 representing the real and imaginary components of the raw spectral response at frequency ^^.

[0187] The microcontroller 120 is configured to control the impedance converter 110 to repeat the steps associated with outputting the digital excitation signal, and the measurement and processing of the corresponding digital response signal, iteratively over the frequency scan (i.e., as the excitation frequency is varied over the set of output frequencies of the frequency band).

[0188] In some embodiments, the control circuit 102 varies the frequency of the digital excitation signal as performed during the frequency scan operation (e.g., at steps 604 to 610), synchronously with the measurement and processing of the digital response signal (e.g., at steps 702 to 706).

[0189] For example, the impedance converter 110 may be configured to indicate the completion of the frequency measurement for each frequency via a bit in a status register of the register block 116 (as described in [1]). Incrementing to the next frequency is performed in response to a control command (e.g., an increment frequency command) received from the microcontroller 120. The microcontroller 120 issues the increment frequency command following the reading of the real and imaginary data of the spectral response values stored in the register block 116 resulting from processing the digital response signal associated with the current excitation signal (i.e., resulting from the digital excitation signal at the current output frequency).

[0190] That is, by reading the processed spectral response values before issuing an increment frequency command to the impedance converter 110 to move to the next frequency increment, the microcontroller 120 controls the frequency variation in the digital excitation signal synchronously with the processing of the spectral response data.

[0191] The control circuit 102 processes the raw spectral response values to determine a corresponding complex impedance vectors for the formation 11. Referring to Fig.7a, at step 708 the microcontroller 120 is configured to: receive the real and imaginary components of the spectral response values, as generated at the frequency ^^, from the impedance converter 110 via a data bus. For example, the microcontroller 120 reads the contents of the two output registers of register block 116 via the interface 111. The microcontroller 120 determines a complex impedance value, comprising a vector of a real and an imaginary component, by calculating a magnitude and a phase of the spectral response values for each of the one or more output frequencies.

[0192] In the described examples, the microcontroller 120 is configured to process the real and imaginary parts of the of the raw spectral response values (i.e., the DFT coefficients generated at each given frequency ^^) to produce the magnitude |^^| and phase ^^(^^) of the complex impedance ^^. For a spectral response ^^(^^) at frequency ^^ of the scan, the spectral response magnitude is given by |^^(^^)| = √(^^^^{^^(^^)}ଶ + ^^^^{^^(^^)}ଶ)and the spectral response phase is given by ^^(^^(^^)) = ^^^^^^ି^(^^^^{^^(^^)} / ^^^^{^^(^^)})where ^^^^{^^(^^)} and ^^^^{^^(^^)} denote the real and imaginary parts respectively of the raw spectral response values (e.g., the DFT coefficients in the described embodiments). Example calibration

[0193] The processing of the response signal 103b by the control circuit 102 is based on a baseline calibration of the control circuit 102 to generate impedance data with values within a measurement range appropriate to the frequency band and the formation.

[0194] In the described examples, the baseline calibration operation is specific to the impedance converter 110 to enable the microcontroller 120 to convert the spectral magnitude |^^(^^)| and phasevalues to a corresponding magnitude and phase of the impedance of formation 11, and to subsequently permit determination of the complex impedance vector Z.

[0195] As described in [1], a baseline calibration operation of the control circuit 102 comprises determining (1) a gain factor ^^ to apply to the magnitude of the spectral response |^^(^^)|, and (2) a phase offset Δ to apply to the phase ^^(^^(^^)) of the complex spectral response value ^^(^^).

[0196] In some embodiments, the gain factor ^^ and phase offset Δ are determined by an external computing device with one or more processors configured to operate on spectral response values provided by the device 100. The gain factor ^^ and phase offset Δ are subsequently programmed into the microcontroller 120 prior to the live survey of method 400.

[0197] In some embodiments, initial values of the gain factor ^^ and the phase offset Δ are determined by conducting validation measurements on a reference formation with a corresponding set of known complex impedance vectors for each output frequency over the frequency band. Validation measurements are obtained during a validation EM survey performed with the device 100 on the reference formation prior to the (live) EM survey of method 400.

[0198] During the validation EM survey, device 100 produces a set of validationspectral response magnitudes and phases (^^௩(^^), ^^௩(^^(^^))) which are associated withknown impedances ^^^^^௪^of the reference formation at the given scan frequency. The relationship between the magnitude of the known impedance |^^^^^௪^| and the corresponding magnitude of the validation spectral response values |^^௩(^^)| is given by

[0199] This enables the gain factor ^^ to be determined as a function of the frequency of the excitation signal ^^. The |^^௩(^^)| value used to calculate the gain factor may be a value obtaining by averaging, collating, or applying any other statistical measure of a series of validation spectral magnitude values associated with the reference formation at the excitation frequency ^^. Conducting the validation EM survey for a known impedance range therefore enables the impedance converter 110 to be calibrated to predetermine the gain factor ^^ before live survey measurements take place. Knowledge of the validation impedance values ^^^^^௪^for the frequency range of interest may be determined from a time-frequency impedance map, or similar, of the reference formation.

[0200] In some embodiments, determining the gain factor ^^ is dependent on the configuration of the ADC circuit 115. For example, the excitation signal voltage range, the current-to-voltage gain resistor, and the PGA gain may be configured to enable the current-to-voltage amplifier 720 and the PGA 724 to collectively operate in a linear region, and to avoid producing an output voltage that would saturate the ADC module 728 (see [1]).

[0201] The phase offset Δ value accounts for a phase shift introduced to the digital excitation signal as it passes through the internal amplifiers on the transmit and receive side of the impedance converter 110 and to the coil pair 104 connected between the output pin 108 and input pin 109. The phase offset value Δ has i) a transmitter error component Δ௧^^^^representing the contribution of the primary component of the transmitter coil 104a to the phase ^^(^^(^^)) of the response signal induced in the receiver coil 104b, and ii) a system error component Δ^௬^௧^^representing the error in the phase that is due entirely to the internal components of the impedance converter 110.

[0202] For an impedance converter 110 implemented using the AD5933 circuit (see [1]), the system error component Δ^௬^௧^^is calculated by placing a resistor across the output pin 108 and input pin 109 and calculating the phase for each output frequency inthe frequency band. The transmitter error component Δ௧^^^^is calculated from the phase values ^^௩(^^(^^)) determined from the validation EM survey conducted on the reference formation (see [1]).

[0203] The phase offset value Δ is determined by summing the system error component Δ^௬^௧^^and the transmitter error component Δ௧^^^^. The magnitude and phase of the impedance of the formation 11 are determined by applying the gain ^^ and the phase offset Δ to the corresponding measured spectral magnitude and phase values as

[0204] The impedance phase ^^(^^)may be expressed in degrees. In some embodiments where the arctangent function is used the phase angle is returned in radians and a conversion process is performed to convert from radians to degrees.

[0205] At step 710, in response to the microcontroller 120 determining the magnitude (|Z|) and the phase angle of the impedance (^^(Z), in radians), as in step 708 above, the microcontroller 120 determines the complex impedance vector (^^) including real (resistive, ^^^^^^) and imaginary (reactive, ^^^^^^) components by vector projection. The real component is given byand the imaginary component is given byTime-frequency impedance data

[0206] With reference to Fig.4, the control circuit 102 is configured to repeat steps 402 to 406 iteratively as the device 100 is deployed through the borehole 101 during the survey. Impedance data, as generated at step 406, is recorded by the control circuit 102 at the one or more time instants^^்of the survey process. For example, the impedance data may be a set of frequency dependent complex vectors determined ateach survey time denoted ^^ = {^^^^, ^^^ଶ, … , ^^^^},…{^^்^, ^^்ଶ, … , ^^்^}. At step 408,the impedance data Z is processed by one or more processors to determine one or more geological parameters of the formation 11. In some embodiments, the one or more geological parameters comprise at least one of: an electrical resistivity; an electrical permittivity; a magnetic susceptibility; a magnetic permeability and an electrical conductivity, of the formation.

[0207] In some embodiments, the processing of the impedance data Z is performed by the microcontroller 120 of the control circuit 102. For example, the microcontroller 120 may be configured to process each individual impedance vector value ^^^^, and / or afrequency dependent set of impedance vectors {^^^^, ^^^ଶ, … , ^^^^} in real-time during thesurvey to produce the corresponding values of the one or more geological parameters. Alternatively, the microcontroller 120 may be configured to process the set of impedance data Z in response to the completion of the movement of the survey device 100 within the borehole 101. Specifically, the microcontroller 120 buffers thefrequency dependent set of impedance vectors {^^^^, ^^^ଶ, … , ^^^^} determined at eachsurvey time instant, and derives corresponding values of the one or more geological parameters following completion of the impedance data generation.

[0208] In other embodiments, determination of the one or more geological parameters is performed by one or more processors external to the survey device 100 (i.e., in an “off-device” mode). For example, the microcontroller 120 of the survey device 100 may be configured to transmit the generated impedance data to the BMS, where the BMS is configured to process the received data to generate values of the one or moregeological parameters. In some embodiments, the impedance data is provided to the BMS by microcontroller 120 in real-time during the survey of the borehole 101.

[0209] In some embodiments, the determination of the one or more geological parameters involves determining a representation of the respective parameter(s) over frequency, and over time or depth. Fig.8a illustrates a flow diagram of a method 800 performed by the control circuit 102 for generating geological parameter values in the frequency and time or depth domains from impedance data, according to an example embodiment.

[0210] At step 802, the values of the complex impedance data are collated by the microcontroller 120 over the survey time instants. At step 804, the microcontroller 120 generates an impedance map comprising an ordered representation of the impedance data of the survey. Map ^^௭may be constructed in the time-frequency domain, wherevalue ^^௭(^^, ^^) is a complex impedance vector ^^ generated at the survey time ^^ and thefrequency ^^ of the excitation signal 103a. Alternatively map ^^௭may be constructed inthe depth-frequency domain, where ^^௭(^^, ^^) is a complex impedance vector ^^generated at the device depth d and frequency ^^ of the excitation signal 103a. At step 806, the microcontroller 120 processes the impedance map ^^௭to calculate values of one or more of the geological parameters from which the formation can be characterized at step 808. In other embodiments, steps 806 and 808 are carried out after the complex impedance, or other electromagnetic, data is collated at step 802 (i.e., the one or more geological parameters are determined directly from the complex impedance, or other electromagnetic, data without the prior generation of a map).

[0211] Fig.8b illustrates an example table 820 of the complex impedance values determined from impedance vectors of an impedance map ^^௭, for a formation. In some implementations, the data is filtered and / or averaged (e.g., across a frequency window) and then stacked (e.g., across several depths) to produce the table values. Generation of the impedance map ^^௭, and / or corresponding geological parameter map(s), thereby advantageously provides a representation of the electromagnetic properties or“signature” of the formation 11, such as to capture its dynamic response to electromagnetic stimulation over frequency and time / depth.

[0212] In some embodiments, the microcontroller 120 is configured to generate corresponding parameter maps of the respective geological parameters, such as for example maps ^^ோ, ^^^௨^, and ^^^^^ௗrepresenting the electrical resistivity, magnetic susceptibility and electrical conductivity of the formation 11 in the frequency and time / depth domains. Borehole characterization

[0213] In some embodiments, the formation 11 is characterized based on the generated impedance data and / or the geological parameter data generated by the surveying of the borehole 101 by the survey device 100. Optionally at step 808, the microcontroller 120 determines the characterization of the formation 11 by processing the complex impedance vectors and / or the corresponding values of the geological parameter(s). Alternatively, or in addition, the microcontroller 120 may be configured to transmit the impedance data and / or the geological parameter data to an external computing system, such as the BMS, with one or more processors configured to perform the characterization.

[0214] Fig.9a illustrates a method 900 for characterizing a formation 11 surrounding a borehole 101, according to the proposed technology. At step 902, an EM survey of the borehole 101 is conducted using a survey device 100. The survey device 100 generates data associated with the formation 11 of the borehole 101. For example, the data associated with formation 11 may include one or more depth-frequency maps ^^௭, ^^ோ, ^^^௨^, and ^^^^^ௗof geological parameters of the formation 11 as described above. Processing of the depth-frequency maps ^^௭, ^^ோ, ^^^௨^, and ^^^^^ௗmay include, for example, performing a statistical analysis of the impedance and / or geological parameter values, and / or generating classification scores of the values against predetermined models of rock type (e.g., sandstone and limestone).

[0215] At step 904, the data associated with the formation 11 of the borehole 101 is obtained by one or more processing devices of the system or device performing the characterization (e.g., the survey device 100, the BMS, or another computing system).

[0216] At step 906, the data associated with the formation 11 of the borehole 101 is processed by the one or more processing devices to determine a lithology of the formation 11. In some embodiments, the lithology of the formation 11 is determined based on electromagnetic data such as complex impedance data and / or geological parameter data. For example, a composition of rock within formation 11, or as occurring over a subset of the formation 11, may be determined from the classification scores of the map values against the one or more rock models.

[0217] In some embodiments, the impedance data and / or other electromagnetic data are processed by a computing system to generate one or more visualizations of the formation 11 over the frequency band. The one or more visualizations of the formation 11 may be included, entirely or partially, within a geological parameter log of the borehole 101. For example, the computer system may be configured to receive geological parameter data generated by the microcontroller 120 and to process the geological parameter data with a logging software package such as, for example, WellCADTM.

[0218] Fig.9b illustrates a method 920 performed by the computing system for generating a visualization of a formation surrounding a borehole, according to the proposed technology.

[0219] At step 922, the computing system obtains data associated with the formation 11 of the borehole 101 by conducting an electromagnetic survey of the borehole 101. The survey is conducted using the survey device 100 and the corresponding methods according to the embodiments described herein.

[0220] At step 924, the computing system processes data associated with the formation 11 of the borehole 101 as generated by the electromagnetic survey (e.g.,electromagnetic data and / or geological parameter data) to determine the visualization data of the formation. In some embodiments, the visualization data of the formation 11 includes data that, when rendered on an electronic display, visually indicates at least: a spectral (frequency) representation of the data associated with formation 11 over the frequency band; and a lithology of the formation. In some embodiments, the computer system is configured to render the visualization data by plotting the spectral or other frequency representation of the data associated with the formation 11 against the lithology of the formation 11.

[0221] At step 926, the computing system renders the visualization data on the electronic display. For example, at least one processor of the computing system may be configured to represent values of the visualization data as a collection of interface elements on a corresponding display component of the computing system or an external system (e.g., a touch-sensitive interactive display panel of the system that is configured for operation by a user).

[0222] In some embodiments, the computer system is configured to generate label data representing one or more labels of the visualization, such as for example to indicate one or more names and / or characteristics of the formation 11. This advantageously provides the ability to easily and quickly interpret the data associated with formation 11 by viewing the visualization and / or the associated labels.

[0223] Fig.10a illustrates an example of a standard geological parameter log 1000 using a conventional survey device configured at a single frequency for formation 11.

[0224] Fig.10b illustrates an example of a geological parameter log 1020 derived from electromagnetic data (i.e., complex impedance values in this example) generated by performing an EM survey according to method 400, for formation 11.

[0225] Fig.10c illustrates an example visualization 1030 derived from the same geological parameter data (i.e., complex impedance values) of Fig.10b, where the formation is represented over a depth registered frequency range of 40kHz to 80kHz(plot 1032). Plot 1034 of Fig.10c shows geological parameter data collected at a single frequency (i.e., 70kHz), being a subset of the geological parameter data collected over the full frequency range as visualized in plot 1032. That is, the generation of the geological parameter data over the densely sampled frequency band advantageously allows the generation of corresponding visualization data representing the formation 11 over a large number of frequencies (e.g., plot 1032), thereby providing a more detailed visual representation compared to the use of data collected at only a single frequency (e.g., plot 1034). The geological parameter data is further processed to determine the lithology of the formation, and corresponding visualization data is generated to visually depict the lithology as shown in example plot 1036 (i.e., the boundary between mudstone and siltstone, and sandstone).

[0226] Fig.10d illustrates the visual spectra 1032 of Fig.10c in a scaled view. Fig. 10e illustrates a representation 1050 of various data generated by the computer system to produce the visualizations depicted in Figs.10c and 10d, according to one embodiment. A first panel 1051 illustrates a depth value of the survey device 100 at which the device generates corresponding electromagnetic data. A second panel 1052 illustrates values of a synthetic phase offset (i.e., values without calibration) of the response signal generated by the device 100 at two selected frequencies within the frequency band. A third panel 1053 illustrates values of the device phase offset of the response signal generated by the device 100 (i.e., which incorporates calibrations to the values) at the same selected frequencies of the second panel 1052. A fourth panel 1054 illustrates values of the electrical conductivity parameter as obtained by the one or more processing devices by processing the electromagnetic data generated by the survey device 100 (e.g., including the values of the synthetic and device phase offsets). A fifth panel 1055 illustrates a representation of the phase offset data as a frequency depth map over the 40 kHz to 80 kHz frequency range as generated by the one or more processing devices. A sixth panel 1056 illustrates a lithology of the formation 11 as a function of depth as determined by the one or more processing devices, as also depicted in Fig.10c.

[0227] The one or more visualizations produced from the electromagnetic data generated by survey device 100 advantageously provides a visual interpretation of the formation 11 over the broad frequency range rather than at a single frequency. The resulting visual representation of the geological parameter data provides more detailed information about the characteristics of the formation as obtained in a single surveying operation using the device 100, compared to a surveying operation conducted with a conventional survey device.

[0228] It will be appreciated by persons skilled in the art that numerous variations and / or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. References [1] Analog Devices, Data Sheet AD5933, retrieved 28 June 2023, https: / / www.analog.com / en / products / ad5933.html

Claims

CLAIMS:

1. A device for performing an electromagnetic survey of a subsurface environment, comprising: one or more transmitter coils and one or more receiver coils, the coils collectively configured to perform electromagnetic induction (EMI) on a material of the subsurface environment in response to application of an excitation signal to the one or more transmitter coils; and a control circuit configured to: (i) generate the excitation signal at a frequency that varies over time and incrementally over a frequency band; (ii) apply the excitation signal to the one or more transmitter coils; (iii) process a response signal induced within the one or more receiver coils from the EMI to generate electromagnetic data representing the material as a function of at least the frequency of the excitation signal; and (iv) process the electromagnetic data to determine one or more material specific parameters of the subsurface environment around the coils.

2. The device of claim 1, wherein the control circuit generates the excitation signal over a set of discrete frequencies that substantially span the frequency band.

3. The device of claim 2, wherein the set of discrete frequencies represent an oversampling of the frequency band relative to a bandwidth of the frequency band, or to a selected frequency value of the frequency band.

4. The device of any of claims 2 to 3, wherein a step interval of the discrete frequencies is limited by the highest frequency component of the frequency band.

5. The device of claim 4, wherein the frequency band has a minimum frequency value at or above 10 Hz and a maximum frequency value at or below 1MHz.

6. The device of any of claims 1 to 5, wherein the excitation signal is a sinusoidal voltage signal.

7. The device of any of claims 1 to 6, wherein the subsurface environment comprises a formation surrounding a borehole, the formation comprising the material, and wherein the device is deployed into the borehole during the survey.

8. The device of claim 7, wherein the control circuit is configured to repeat at least steps (i) to (iii) iteratively as the device is deployed through the borehole during the survey.

9. The device of claim 8, wherein processing the electromagnetic data comprises generating a depth-frequency map from the electromagnetic data generated at one or more depths of the device within the borehole.

10. The device of any of claims 1 to 9, wherein the processing of the response signal to generate the electromagnetic data is performed based on one or more calibrations of the control circuit.

11. The device of any of claims 1 to 10, wherein the control circuit generates the electromagnetic data without using a bucking coil to measure a component of the response signal induced by the one or more transmitter coils.

12. The device of any of claims 1 to 11, wherein the electromagnetic data comprises a set of complex relative impedance vectors, each vector having real and imaginary impedance values for a corresponding excitation signal frequency value.

13. The device of claim 12, wherein the control circuit comprises:an impedance converter having a frequency generator, an analog-to-digital converter (ADC), and a digital-to-analog converter (DAC); and a microcontroller in communication with the impedance converter and configured to control the impedance converter to: (i) generate a digital excitation signal by operating the frequency generator at a selected frequency within the frequency band; (ii) convert the digital excitation signal to an analog signal using the DAC; (iii) measure, using the ADC and in response to application of the excitation signal to the one or more transmitter coils, a corresponding digital response signal from the response signal induced in the one or more receiver coil; and (iv) process the digital response signal to determine spectral response values at the frequency, wherein steps (i) to (iv) are repeated iteratively as the selected frequency is incrementally increased within the frequency band.

14. The device of claim 13, wherein the microcontroller controls the impedance converter to vary the frequency of the generated digital excitation signal synchronously with the measurement and processing of the corresponding digital response signal.

15. The device of claim 14, wherein determining the spectral response values involves applying a discrete Fourier transform (DFT) to the digital response signal.

16. The device of claim 15, wherein the microcontroller is configured to: receive one or more spectral response values, as generated at one or more discrete frequencies, from the impedance converter via a data bus; anddetermine the set of complex relative impedance vectors by calculating a magnitude and a phase of the spectral response values.

17. The device of any of claims 13 to 16, wherein the microcontroller is configured to process the set of complex relative impedance vectors to determine the material specific parameters of the subsurface environment.

18. The device of claim 17, wherein the one or more material specific parameters comprise at least one of: an electrical resistivity; a magnetic susceptibility; a magnetic permeability; an electrical permittivity; and an electrical conductivity.

19. A method for performing an electromagnetic (EM) survey of a subsurface environment using an EM survey device, comprising: (i) generating, by a control circuit of the device, an excitation signal at a frequency that varies over time and incrementally over a frequency band; (ii) applying, by the control circuit, the excitation signal to one or more transmitter coils of the device to cause the one or more transmitter coils to perform electromagnetic induction (EMI) on a material of the subsurface environment; (iii) receiving, by the control circuit, a response signal induced within one or more receiver coils of the device from the EMI; (iv) processing, by the control circuit, the response signal to generate electromagnetic data representing the material as a function of at least the frequency of the excitation signal; and (v) processing, by one or more processors, the electromagnetic data to determine one or more material specific parameters of the subsurface environment around the coils.

20. The method of claim 19, wherein the control circuit generates the excitation signal over a set of discrete frequencies that substantially span the frequency band.

21. The device of claim 20, wherein the set of discrete frequencies represent an oversampling of the frequency band relative to a bandwidth of the frequency band, or to a selected frequency value of the frequency band.

22. The method of any of claims 20 to 21, wherein a step interval of the discrete frequencies is limited by the highest frequency component of the frequency band.

23. The method of claim 22, wherein the frequency band has a minimum frequency value at or above 10 Hz and a maximum frequency value at or below 1MHz.

24. The method of any of claims 19 to 23, wherein the excitation signal is a sinusoidal voltage signal.

25. The method of any of claims 19 to 24, wherein the subsurface environment comprises a formation surrounding a borehole, the formation comprising the material, and wherein the device is deployed into the borehole during the survey.

26. The method of claim 25, wherein at least steps (i) to (iv) are repeated iteratively as the survey device is deployed through the borehole during the survey.

27. The method of claim 26, wherein processing the electromagnetic data comprises generating a depth-frequency map from the electromagnetic data generated at one or more depths of the survey device within the borehole.

28. The method of any of claims 19 to 27, wherein the processing of the response signal to generate the electromagnetic data is performed based on one or more calibrations of the control circuit.

29. The method of any of claims 19 to 28, wherein the control circuit generates the electromagnetic data without using a bucking coil to measure a component of the response signal induced by the transmitter coil.

30. The method of any of claims 19 to 29, wherein the electromagnetic data comprises a set of complex impedance vectors, each vector having real and imaginary impedance values for a corresponding excitation signal frequency value.

31. The method claim 30, further comprising: (i) generating a digital excitation signal by operating a frequency generator of the control circuit at a selected frequency within the frequency band; (ii) converting the digital excitation signal to an analog signal using a digital- to-analog converter (DAC) of the control circuit; (iii) measuring, using an analog-to-digital converter (ADC) of the control circuit and in response to application of the excitation signal to the one or more transmitter coils, a corresponding digital response signal from the response signal induced in the one or more receiver coils; and (iv) processing, by a microcontroller of the control circuit, the digital response signal to determine spectral response values at the frequency, wherein steps (i) to (iv) are repeated iteratively as the selected frequency is incrementally increased within the frequency band.

32. The method of claim 31, wherein the microcontroller controls the impedance converter to vary the frequency of the generated digital excitation signal synchronously with the measurement and processing of the corresponding digital response signal.

33. The method of claim 32, wherein determining the spectral response values involves applying a discrete Fourier transform (DFT) to the digital response signal.

34. The method of claim 33, further comprising: receiving, at the microcontroller, one or more spectral response values, as generated at one or more discrete frequencies, from the impedance converter via a data bus; and determining, by the microcontroller, the set of complex impedance vectors by calculating a magnitude and a phase of the spectral response values.

35. The method of any of claims 31 to 34, further comprising processing, by the microcontroller, the set of complex impedance vectors to determine the one or more material specific parameters.

36. The method of claim 35, wherein the one or more material specific parameters comprise at least one of: an electrical resistivity; an electrical permittivity; a magnetic susceptibility; a magnetic permeability; and an electrical conductivity.

37. A method for characterizing a formation surrounding a borehole comprising the steps of: conducting an electromagnetic survey of the borehole using the method of any of claims 19 to 36; processing, by one or more processing devices, data associated with the formation of the borehole as generated by the electromagnetic survey; and determining, by the one or more processing devices, a lithology of the formation based on the processing of the data associated with the formation.

38. The method of claim 37, wherein the processing of the data associated with the formation comprises at least one of: generating classification scores on values of the data associated with the formation against one or more predetermined models of a rocktype; and performing a statistical analysis of the values of the data associated with the formation.

39. The method of claim 38, wherein the data associated with the formation includes values of at least one of: an electrical resistivity; an electrical permittivity; a magnetic susceptibility; a magnetic permeability; and an electrical conductivity, of the formation.

40. A method for characterizing a formation surrounding a borehole, the method comprising: using a survey device to perform electromagnetic induction (EMI) on the formation by: generating, by a control circuit of the survey device, an excitation signal to excite at least one transmitter coil of the survey device, wherein the excitation signal is generated at a frequency that varies over time and incrementally over a frequency band; receiving, by the control circuit from at least one receiver coil of the survey device, a response signal induced in the at least one receiver coil in response to the EMI; and processing, by the control circuit, the response signal to generate electromagnetic data representing the formation as a function of at least the frequency of the excitation signal; obtaining, by one or more processing devices, the electromagnetic data representing the formation; processing, by the one or more processing devices, the electromagnetic data to determine one or more geological parameters of the formation; anddetermining, by the one or more processing devices, a lithology characterizing the formation based on the one or more geological parameters.

41. The method of claim 40, wherein the survey device is a device according to any of claims 2 to 18.

42. A method performed by a computing system for generating a visualization of a formation surrounding a borehole, the method comprising: obtaining data associated with the formation of the borehole by conducting an electromagnetic survey of the borehole using the method of any of claims 24 to 36; processing data associated with the formation of the borehole as generated by the electromagnetic survey to determine visualization data that, when rendered on an electronic display, visually indicates at least: a frequency representation of the data associated with the formation over the frequency band; and a lithology of the formation; and rendering the visualization data on the electronic display.

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