3D geological mapping using ambient electromagnetic interferometry

Abstract

The present invention discloses a system and method for generating high-resolution 3D maps of the subsurface using ambient electromagnetic (EM) signals captured by multiple EM apparatuses distributed across a target exploration area. Each portable nodal unit, deployed on ground or water surfaces, records weak natural EM signals with high-sensitivity magnetometers and dipole electrodes. Using interferometric and non-iterative processing methods, the recorded signals are converted into a detailed 3D data cloud that reveals subsurface features, including bedding structures, faults, folds, and lithological variations. Unique EM signatures, automatically assigned during analysis, allow the system to (a) delineate distinct geological units and (b) trace these units across different locations within the mapped volume. The present invention provides a novel method to transform natural EM signals into precise data sets for high-definition 3D geological mapping and advanced exploration and research applications.

Description

3D GEOLOGICAL MAPPING USINGAMBIENT ELECTROMAGNETIC INTERFEROMETRYTechnical Field of the Invention

[0001] The present invention relates to a geophysical mapping system and method that utilizes ambient electromagnetic (EM) field measurements to generate high-resolution 3D maps of subsurface geological features.Background of the Invention

[0002] Geological maps are traditionally created through field-based surveys, where ground features and rock exposures are observed and documented. However, due to natural obstructions and surface coverage, geological data collected in the field are often incomplete. Gaps in this data are typically bridged using geomorphological analysis, satellite remote sensing, and interpretation of airborne or satellite geophysical maps. While drillholes and tunnels can provide rock samples and structural insights from below the surface, geophysical surveys remain the most comprehensive method for acquiring subsurface geological data. Advances in geophysical sensor sensitivity, survey methodologies, and analytical techniques have significantly improved the quality of geological models. Nevertheless, many three-dimensional (3D) geological visualizations remain hypothetical, as geophysical anomalies often appear pixelated, overly smooth, or highly generalized, limiting their precision in defining subsurface structures. Only two-dimensional (2D) and 3D seismic reflection profiles can provide high-resolution images of these structural features at depth.

[0003] EM surveys play a crucial role in geophysical exploration, providing data about the underlying geology. Due to the range of natural EM signal strengths, active EM survey methods, where energy is artificially introduced and the resulting ground response is measured, are commonly used. Passive EM methods, which rely on detecting naturally occurring EM signals, face challenges due to weak signal strength relative to background noise. These methods emphasize improving sensor sensitivity, reducing noise, and advancing data collection and processing techniques.

[0004] Modern EM instruments often include multiple sensors, simultaneously measuring electric and magnetic fields in various orientations to record natural waves that define geophysical anomalies and maximize data accuracy. Instruments equipped with three orthogonal magnetic sensors can capture the natural magnetic field, and high-precision magnetometers provide detailed measurements of magnetic field strength. Electric field sensors, which may be deployed horizontally or vertically, detect geoelectric properties through capacitive or galvanic coupling. The advent of miniaturized sensors, enhanced processing power, and digital communication capabilities has made EM instruments more sensitive, portable, and accessible for geophysical applications.

[0005] Passive EM methods, specifically electrotellurics (ET) and magnetotellurics (MT), use specialized configurations to study subsurface features. ET employs only electrodes, while MT combines both electrodes and magnetometers to capture variations in subsurface conductivity. Related techniques, such as audio-magnetotellurics (AMT), controlled-source audio-magnetotellurics (CSAMT), and long-offset transient EM (LOTEM), complement MT in exploring different subsurface depths. These EM technologies can be deployed on land, underwater, or in airborne surveys for versatile geological mapping.

[0006] Geophysical inversion processes convert field data into models that reveal subsurface properties, aiding in the interpretation of structures and geological features. Advances in computing have made inversion a standard technique for transforming EM data into anomaly maps. Yet, despite improved anomaly resolution and consistency, inversion techniques still struggle to clearly define intricate geological structures. Anomalies often appear generalized, with gradual transitions that mask finer details, making high-resolution reflection seismics the preferred method for detailed subsurface mapping. This limitation underscores a technological gap, as current EM methods lack the precision to map faint subsurface features accurately.

[0007] Moreover, there are only few prior art documents that relate to using interferometry on electromagnetics. For instance, International Patent Cooperation Treaty (PCT) Application No. PCT / US2013 / 075117 discloses an EM interferometry system used for subsurface imaging within a borehole environment. This system involves multiple EM field sensors placed at different positions within a downhole environment and utilizes cross-correlation between sensors to create a “virtual source” response, followed by an inversion process for deriving a subsurface model. Moreover, it employs Green’s functions for processing in both time and frequency domains, enabling subsurface property modeling through iterative inversions.

[0008] Furthermore, International PCT Application No. PCT / US2013 / 041691 describes a method of subsurface monitoring using controlled-source EM signals transmitted via transmitters arranged at specific angles with respect to a target structure. The system positions phase-locked receivers on the surface to detect and record transmitted signals. It also introduces phase-shifting techniques to steer transmissions and gather coherent data for interferometric imaging, aiming for subsurface imaging without inversion with seismic data.Summary of the Invention

[0009] The present invention addresses the mentioned technological gaps, focusing on the use of weak natural or ambient EM signals that current technologies struggle to utilize effectively for geological mapping and geophysical anomaly imaging.

[0010] The present invention discloses an apparatus and a method for generating high-resolution 3D geological maps using weak natural EM signals, without relying on iterative model fitting. This invention leverages high-sensitivity EM apparatus to detect geological anomalies with greater precision, offering an alternative to conventional seismic and electromagnetic techniques for subsurface mapping.

[0011] An embodiment of the present invention relates to a method for generating a three-dimensional (3D) subsurface map using ambient electromagnetic (EM) signals, comprising: distributing a plurality of EM apparatuses across an exploration area, wherein each EM apparatus is aligned with the true north and the vertical axis when deployed on the ground surface; collecting ambient EM signal from each location of the EM apparatus over a specified time period; processing the collected data using an interferometric method to generate interferometric data and depth information data; organizing the interferometric data into a regularized 3D grid using non-iterative method, wherein the data is structured into a voxel representation; and generating a subsurface 3D map from the voxel representation.

[0012] In another embodiment, wherein the interferometric method comprises: providing pairs of spatially separated sensors within the plurality of EM apparatuses; cross-correlating the collected ambient EM signal from each pair of spatially sensors to generate interferometric data along the lines separating the sensors; and applying a frequency-depth lookup table to translate the interferometric data into depth information data.

[0013] In another embodiment, the subsurface 3D map is color-coded to correspond with existing geological or geophysical data attributes.

[0014] In yet another embodiment, the present invention discloses a system for generating a 3D subsurface map using ambient EM signals, comprising: a plurality of EM apparatuses, wherein each EM apparatus comprises: a plurality of magnetometers aligned along three orthogonal axes to measure magnetic field components; at least two dipole electrodes positioned perpendicularly to measure electric potential in two horizontal axes; wherein the plurality of magnetometers and the at least two dipole electrodes are configured to measure and record the ambient EM signals within the exploration area; a first set of preamplifier and filter connected to the plurality of magnetometers; a second set of preamplifier and filter connected to the at least two dipole electrodes; a processing unit configured to record and process the measured ambient EM signals; and a power source comprising a battery with recharging options.

[0015] In another embodiment, the system further comprising a central processing unit, wherein the central processing unit processes the data measured by the plurality of magnetometers and the at least two dipole electrodes in each EM apparatus using an interferometric method to generate interferometric data and depth information data and wherein the interferometric data is organized into a regularized 3D grid using non-iterative method, wherein the data is structured into a voxel representation.

[0016] In another embodiment, the processing unit comprises Global Positioning System (GPS) or Global Navigation Satellite System (GNSS) capabilities for time synchronization, geographic coordinate acquisition, and elevation data.

[0017] In another embodiment, the processing unit further comprises communication options.Description of the Drawings

[0018] shows a diagram illustrating the minimum configuration of an embodiment of the EM apparatus of the subject invention.

[0019] shows a schematic diagram of an embodiment of the EM apparatus of the subject invention.

[0020] illustrates the field deployment of an embodiment of the EM apparatuses of the subject invention, demonstrating how data from these apparatuses generates Green’s function in the intervening regions to map underlying geological features.

[0021] shows an illustration of electric sensors and magnetic sensors paired during the interferometric calculations.

[0022] provides a data processing flowchart showing the use of non-iterative methods in the processing sequence.

[0023] illustrates the survey results from four sites in northern Luzon, Philippines, each surveyed separately over a period of approximately four hours.

[0024] provides detailed imaging of Site 1 from.

[0025] illustrates survey results depicting EM images of various sections of Boracay Island, in northern Panay, Philippines, where the geological composition primarily consists of near-horizontal reefal limestone characterized by karst terrain.

[0026] FIGS. 8A and 8B shows yet another example of EM imaging results of various sections of Boracay Island in northern Panay, Philippines.

[0027] FIGS. 9A and 9B shows geological maps for a proposed irrigation dam site in Tapaz, Capiz Province, Philippines, underlain by mildly folded volcaniclastic, conglomerate, and wacke layers.

[0028] FIGS. 10A, 10B, and 10C display detailed EM data illustrating the geological structure underlain by mildly folded volcaniclastic, conglomerate, and wacke layers.Detailed Description of the Invention

[0029] It must be noted that the preferred embodiments illustrated in the drawings and detailed description below do not limit the disclosure. Instead, they provide a foundation for those skilled in the art to identify alternative forms, equivalents, and other modifications that fall within the scope of the appended claims.

[0030] An embodiment of the present invention discloses an apparatus for generating a three-dimensional (3D) subsurface map using ambient electromagnetic (EM) signals comprising: a plurality of high-sensitivity triaxial magnetometers(130)aligned along three orthogonal axes (Hx, Hy, Hz)(125); at least two dipole electrodes(140)to measure electric potential in two horizontal axes (Ex, Ey)(120); a first set of preamplifier and filter(135)connected to the plurality of magnetometers; a second set of preamplifier and filter(145)connected to the at least two dipole electrodes; a single-board computer (SBC)(110)for recording data, featuring ample storage, a high-rate analog-to-digital (AD) converter, and a multi-sensor interface for data collection; and a battery power(115)with a solar panel(150)or AC recharging capabilities for extended surveys.

[0031] In an embodiment of the present invention, the SBC has a Global Positioning System (GPS)(105)that can manage GPS / GNSS synchronization, collect geographical and elevation data, and provide various communication options (e.g., LAN, USB, Bluetooth, WiFi). Data is stored on removable media (USB, SD cards).

[0032] In another embodiment of the present invention, particularly in field deployment, multiple EM apparatuses(100)work collaboratively to capture data across a larger spatial area, with sensor pairings allowing for denser data points than single units alone. As illustrated in, a network of eight (8) apparatuses(200)can generate data across twenty-eight (28) additional locations(205), increasing the signal-to-noise ratio (SNR) and enhancing the imaging of deeper geological zones. Survey duration varies by depth target: for features around 500m deep, data collection should span at least four hours, while imaging beyond 1km requires a minimum of 24 hours.

[0033] The invention also supports versatile deployment, including on water surface, underwater or aerial setups. The EM recorders can be anchored, floated, or positioned via drones. Short, insulated conductors may serve as perpendicular field antennas for sensing electric components in floating or submarine environments.

[0034] A distinctive feature of the present invention is its non-iterative approach to EM data processing. Rather than using conventional model-fitting algorithms, the system applies deterministic interferometry to convert raw EM data into identifiable anomalies. By preserving faint natural EM signals, the method enhances mapping accuracy for subtle subsurface features. Interferometry uses paired data inputs, whether from two sites or from different sensors at a single location.

[0035] The present invention capitalizes on the ubiquitous nature of natural EM signals, which, when captured from various sensor pairings, reveal geological signatures.illustrates potential sensor pairings, enabling flexible mapping strategies for diverse geological conditions. While traditional magnetotellurics (MT) uses Ex-Hy and Ey-Hx pairings, this invention also allows for pairing electric sensors (e.g., Ex-Ey)(300), magnetic sensors (e.g., Hy-Hx)(305), or combining horizontal and vertical sensors (e.g., Ex-Hz). Anomalies can even be mapped using identical-direction sensors (e.g., Ex-Ex), though the theoretical basis for all sensor pairings is not yet fully understood.

[0036] Another innovation of the present invention is the use of an empirical frequency-depth lookup table, replacing the iterative depth-categorization approach in conventional EM inversion. This table aligns specific frequencies, ranging from 0.025 Hz to 1,200 Hz, with depths up to ~2km, derived from publicly available electromagnetic skin depth models. While a simple 1D model is employed, this method achieves high vertical resolution in mapping results.

[0037] Following interferometric analysis, data is organized into a regularized 3D grid using a non-iterative nearest-neighbor method, creating a voxel-based 3D map that can be color-coded to align with existing geological or geophysical data. These maps can be qualitatively compared to ground-truth information and interpreted for geological continuity and structural features. The system’s output is “self-interpreting,” eliminating the need for manual layer tracing often required in seismic analysis.

[0038] The flexibility in sensor pair selection and application of standard mathematical transformations (e.g., derivatives or ratios as in MT) provides an array of options for generating geophysical anomaly maps. By comparing with known resistivity or chargeability maps, users can simulate similar 3D maps using targeted EM anomaly patterns. For instance, using VLF-measured resistivity forms, the invention produced 3D resistivity maps for selected sites(FIGS. 6, 7, and 9). Empirical metrics, such as “metal factor,” can also be developed to identify potential ore-bearing sites within the mapped 3D volumes.

[0039] The generated 3D EM maps exhibit repeatability and consistency across tests. For example, in, surveyed in four sites, each approximately 50x50 meters, EM data accurately mapped geological layers observed in 60m-deep vertical shafts. These layers, disrupted by faults carrying groundwater, are reflected in the 3D models. The ~60m tectonic uplift is visible in the offset between matching layers, with isovalue lines in the 3D model aligning closely with VLF resistivity anomalies. As illustrated in, sub-horizontal anomalies that correspond to conglomerate layers, as well as vertical faults identified from excavated shafts located at the center of each site. A notable dark layer(505)appears consistently around 100 meters depth across all sites, including the two elevated sites on a hill to the right, where faulting has caused a lateral shift. The background chart incorporates Google Earth terrain and imagery to provide geographic context for the surveyed areas. Faults appear as vertically smeared patterns marked by “strings of pearls,” a characteristic signature in EM imaging of linear structures(FIGS. 5 and 6).

[0040] In, three panels are illustrated: the Very Low Frequency (VLF) resistivity profile on the left (where low resistivity areas are indicated in blue and high resistivity areas in brown)(600), the electromagnetic (EM) image of the geological structure on the right(610), and a combined overlay of the two images in the center(605). The effect of the fault is visibly apparent in each panel, influencing the structural form depicted in both the resistivity and EM images.

[0041] In another survey result as illustrated in, the top panel(700)displays an EM image of the geological structure, highlighting limestone beds that are interspersed with a reverse fault. The middle panel(705)presents a 3D resistivity map, where low resistivity is represented in blue(715)and high resistivity in red(720). This map is derived from the 2D panels indicated by red labels and is partially exposed to illustrate the form of the 2D resistivity anomalies. The bottom panel(710)features a Google Earth image of the site for geographical context. In a more detailed illustration,FIGS. 8A and 8B, shows the width of the image which spans approximately 750 meters from left to right, showcasing geology characterized by near-horizontal reefal limestone and karst terrain.displays sets of 2D apparent resistivity profiles generated using VLF data, where low resistivity is represented in blue and high resistivity in red. Each profile panel has a thickness of approximately 100 meters.presents 3D EM data formatted to closely resemble the apparent resistivity profiles shown in.

[0042] In another survey result in Tapaz, Capiz Province, Philippines,features an overlay of surface geological data plotted on a colored terrain background, complemented by 3D EM data rendered to illustrate the subsurface geological structure.displays 3D electromagnetic data that further elucidates the subsurface geological structure, alongside two charts of 100-meter vertical apparent resistivity profiles collected from proposed dam abutment locations. The Panay River flows between the two panels, providing geographical context. FIGS. 10A, 10B, and 10C provides a detailed illustration of EM data of FIGS. 9A and 9B.displays the folded sedimentary beds featuring small synclines, along with Very Low Frequency (VLF) resistivity profiles. The presence of these synclines is corroborated by the observed dips of the beds in the surface geology.presents the same image as the top panel, but with the VLF profile blanked out for a clearer visualization of the geological structures.shows a 110-meter vertical profile of apparent resistivity, where low resistivity is represented in blue and high resistivity in red.

[0043] The present invention provides a robust and adaptable solution for creating high-resolution 3D geological maps using ambient EM signals, offering significant potential for geological research and exploration.Preferred Embodiments

[0044] The preferred hardware embodiment of the present invention consists of a portable, battery-powered nodal unit equipped with GNSS capabilities for time synchronization and positioning. Each unit includes at least two electric dipole electrodes and three magnetic sensors arranged orthogonally. Commercially available single-board computers (SBCs) with a sampling rate of 1kHz at 24-bit resolution are suitable for this configuration, and they can accommodate removable USB or SD storage media for data storage. For data transfer, the system supports multiple connectivity options, such as LAN, USB, Bluetooth, or WiFi.provides an example of this apparatus configuration.

[0045] For field deployment, multiple units are strategically positioned on the ground or water surface, with sensors aligned to true north for consistency. The eight-sensor layout illustrated inserves as an effective setup to detect and map electromagnetic (EM) anomalies under the central region of the sensor network. To capture the desired subsurface features, data recording should span at least four hours for targets a few hundred meters deep. For more extensive targets reaching depths of several kilometers, data collection over several days is recommended to enhance resolution and image clarity.

[0046] Data from all sensors are processed centrally, with inputs analyzed simultaneously. This method avoids stochastic or iterative approaches, using only deterministic processes to preserve the faint high-frequency components of natural EM signals, which are essential for capturing subtle subsurface anomalies. The main processing technique employed is time-domain interferometry, which utilizes paired sensor inputs from one or separate sites, as shown in. This method filters out incoherent noise and isolates the signals common to each sensor pair. Provided there is no significant discontinuity between sensor positions, the resulting signal represents the Green’s function of the region between paired sensors.

[0047] To achieve vertical imaging without iterative depth estimation, the invention employs a predefined frequency-depth lookup table that provides frequency-dependent skin depth values. This table, which can be prepared, validated, and calibrated as needed, represents the vertical 1D skin depth formula that distributes the Green’s function into depth-distributed values. Applied across all paired sensors, the lookup table generates a data point cloud containing estimated Green’s function values for all the sites and all the calculated depths. The resulting EM data point cloud is then processed through a non-iterative nearest-neighbor gridding approach, which produces a 3D EM anomaly map. This map can be further refined using external geological or geophysical data, which aids in selecting sensor pairs and mathematical formulas for anomaly calculation. The vertical scale of the map, controlled by the frequency-depth lookup table, can also be adjusted with reference to external datasets.

[0048] The process outlined here generates a high-definition 3D grid with unique numerical signatures for each geological unit, enabling self-interpretation of the data. This allows specific features, such as outcrops, map units, anomalies, or stratigraphic layers, to be extrapolated from known locations and tracked across the 3D volume to other occurrences. Additionally, the invention provides flexibility for creating various 3D maps that represent other qualitative geological features. The choice of features mapped is customizable, based on the selected sensor pairs and mathematical operations applied.

[0049] Finally, by conducting repeated measurements in the same area, this method allows for temporal monitoring of subsurface changes. This capability is invaluable for studying dynamic geological processes, as it enables consistent observation and mapping of evolving subsurface conditions over time.

Claims

A method for generating a three-dimensional (3D) subsurface map using ambient electromagnetic (EM) signals, comprising:distributing a plurality of EM apparatuses across an exploration area, wherein each EM apparatus is aligned with the true north and the vertical axis when deployed on the ground surface;collecting ambient EM signal from each location of the EM apparatus over a specified time period;processing the collected data using an interferometric method to generate interferometric data and depth information data;organizing the interferometric data into a regularized 3D grid using non-iterative method, wherein the data is structured into a voxel representation; andgenerating a subsurface 3D map from the voxel representation.The method of Claim 1, wherein the interferometric method comprises:providing pairs of spatially separated sensors within the plurality of EM apparatuses;cross-correlating the collected ambient EM signal from each pair of spatially sensors to generate interferometric data along the lines separating the sensors; andapplying a frequency-depth lookup table to translate the interferometric data into depth information data.The method of Claim 1, wherein the subsurface 3D map is color-coded to correspond with existing geological or geophysical data attributes.A system for generating a 3D subsurface map using ambient EM signals, comprising:a plurality of EM apparatuses, wherein each EM apparatus comprises:a plurality of magnetometers aligned along three orthogonal axes to measure magnetic field components;at least two dipole electrodes positioned perpendicularly to measure electric potential in two horizontal axes;wherein the plurality of magnetometers and the at least two dipole electrodes are configured to measure and record the ambient EM signals within an exploration area;a first set of preamplifier and filter connected to the plurality of magnetometers;a second set of preamplifier and filter connected to the at least two dipole electrodes;a processing unit configured to record and process the measured ambient EM signals; anda power source comprising a battery with recharging options.The system of Claim 4, wherein the processing unit comprises Global Positioning System (GPS) or Global Navigation Satellite System (GNSS) capabilities for time synchronization, geographic coordinate acquisition, and elevation data.The system of Claim 4, wherein the processing unit further comprises communication options.

Images