64 results about "Controlled source electro-magnetic" patented technology

A method for identifying features in the Earth's subsurface below a body of water using transient controlled source electromagnetic measurements includes acquiring a plurality of transient controlled source electromagnetic measurements. Each measurement represents a different value of an acquisition parameter. Each measurement is indexed with respect to a time at which an electric measuring current source is switched. The plurality of measurements is processed in a seismic trace display format, in which each trace corresponds to the measurement acquired for a value of the acquisition parameter. A subsurface feature is identified from the processed measurements.

Method for rapid inversion of data from a controlled-source electromagnetic survey of a subterranean region. Selected (51) common-receiver or common-source gathers of the data are reformed into composite gathers (52) by summing their data. Each composite gather is forward modeled (in the inversion process) with multiple active source locations (53). Computer time is reduced in proportion to the ratio of the total number of composite gathers to the total number of original common-receiver or common-source gathers. The data may be phase encoded to prevent data cancellation. Methods for mitigating loss of far offset information by data overlap in the summing process are disclosed.

Method for denoising a receiver signal from a controlled source electromagnetic survey. A discrete wavelet transform is performed on the signal, and the resulting detail coefficients are truncated using a selected threshold value, which may be zero. Further levels of decomposition may be performed on the approximation coefficients from the preceding level. After the final level of decomposition, a denoised signal is restructured by performing the inverse wavelet transformation on the last set of approximation coefficients combined with the thresholded detail coefficients accumulated from all levels of decomposition.

A method for suppressing measurement system signature, or artifacts, that arise when controlled source electromagnetic survey data are inverted to obtain a resistivity image of a subsurface region. The method involves identifying regions (47) where the image has low or rapidly varying sensitivity to data acquired by a given receiver, typically regions close to and under the given receiver. Then, in the iterative inversion process where a resistivity model is updated to minimize an objective function, the model update is modified (48) to reduce the impact of such low sensitivity regions on the update.

Joint processing of seismic and controlled source electromagnetic (CSEM) surface data is performed by using a common rock physics model which relates reservoir properties (such as porosity, lithology, saturation, and shaliness) to surface seismic AVO (or AVA) data. This allows one to determine how perturbations in the reservoir properties affect surface data. This can be carried out by systematically changing the reservoir properties and examining the effect on the synthetic data. This allows the hydrocarbon type of a reservoir to be established, e.g. oil or gas, as well as the saturation level of the hydrocarbon in the reservoir, which is useful for determining whether the reservoir has a non-commercial, low hydrocarbon saturation or a commercial, high hydrocarbon saturation.

A method and apparatus of constructing a signal for a controlled source electromagnetic survey is described. In one embodiment, a method is described that includes determining a first waveform and a second waveform, the first waveform and second waveform related to a combined frequency spectrum and bandwidth associated with a geophysical survey line. Then, a signal is constructed by sequencing the first waveform with the second waveform. This signal may be utilized in a transmitter, which may be pulled by a vessel along the geophysical survey line.

A method for measuring magnetotelluric response of the Earth includes measuring transient controlled source electromagnetic response of the subsurface below a body of water over a plurality of actuations of an electromagnetic transmitter. The transient response measurements are stacked. The stacked transient responses are subtracted from measurements of total electromagnetic Earth response over a time period including the plurality of transient response measurements to generate the magnetotelluric response. A method for determining a component of electric field response to a time varying electromagnetic field induced in the Earth's subsurface, includes measuring magnetic field gradient in at least two orthogonal directions in response to the induced electromagnetic field and determining an electric field response in a direction normal to the magnetic field gradient measurements.

A method of analysing results from an electromagnetic survey of an area that is thought or known to contain a subterranean resistive or conductive body within a background strata configuration is described. The method comprises providing a set of electromagnetic field data obtained using at least one electromagnetic receiver and at least one electromagnetic source for a range of source-receiver separations, e.g. providing conventional controlled-source electromagnetic survey data. A subset of the electromagnetic field data is identified that comprises data obtained for source-receiver separations greater than a selected threshold offset. The threshold offset is chosen so that data beyond this offset are characteristic of magnetotelluric data. Thus the subset of data is then processed in accordance with a first technique to obtain information on the background strata configuration. Other electromagnetic field data obtained for source-receiver separations less than the threshold offset may then be processed in accordance with a second technique to obtain information on any subterranean resistive or conductive body within the background strata configuration.

Method for generating a three-dimensional resistivity data volume for a subsurface region from an initial resistivity model and measured electromagnetic field data from an electromagnetic survey of the region, where the initial resistivity model is preferably obtained by performing multiple ID inversions of the measured data [100]. The resulting resistivity depth profiles are then registered at proper 3D positions [102]. The 3D electromagnetic response is simulated [106] assuming the resistivity structure is given by the initial resistivity model. The measured electromagnetic field data volume is scaled by the simulated results [108] and the ratios are registered at proper 3D positions [110] producing a ratio data volume [112]. A 3D resistivity volume is then generated by multiplying the initial resistivity volume by the ratio data volume (or some function of it), location-by location [114]. A related method emphasizes deeper resistive anomalies over masking effects of shallow anomalies.

A method of analysing controlled source electromagnetic (CSEM) survey data to determine probability density functions (PDFs) for values of an electromagnetic parameter at locations in a subterranean region of interest is provided. Structural features in the subterranean strata are identified, e.g. from seismic survey data. An initial PDF for values of the electromagnetic parameter is then assigned to each feature. Models lo specifying values for the parameter in each structural feature are generated by sampling the PDFs. A subset of the models are deemed acceptable based on an acceptance criterion. The PDF for each feature is modified based on values for the parameter in the subset of accepted models to generate replacement PDFs for each feature. The process may be iterated a number of times to generate final PDFs for values of the electromagnetic parameter in the structural features identified in the subterranean region of interest.

Noise compensation in controlled source electromagnetics (CSEM) comprises measuring time-varying magnetic gradients of the marine environment subjected to CSEM. From the measured magnetic gradients, oceanographic electric and magnetic field noise is determined and used for noise compensation of CSEM measurements of electric and magnetic fields. Selection of magnetic gradient measurement provides improved measurement of oceanographic magnetic noise as other electromagnetic noise sources produce negligible magnetic gradients in the marine environment. Electric field noise is then predicted from the magnetic measurements.

Waveforms for controlled source electromagnetic surveying. The waveforms have frequency spectra that include three or more frequencies spaced at substantially equal intervals on a logarithmic frequency scale and spanning a bandwidth of about one decade or more, at least three of which frequencies have approximately equal corresponding amplitudes.

A method for acquiring transient electromagnetic survey signals includes applying a transient electric current to an electromagnetic transmitter disposed above a portion of the Earth's subsurface to be surveyed. Electromagnetic signals are detected at spaced apart locations above the portion of the subsurface in response to an electromagnetic field induced in the Earth's subsurface by the applying transient current. Electromagnetic signals are detected at least one position proximate a position of the electromagnetic transmitter such that the subsurface transient response is substantially always identifiable therefrom.

A waveform design method is presented for controlled source electromagnetic surveying. A desired source spectrum (82) is specified based on a desired resistivity depth image resolution (81) with spectral amplitudes determined by expected noise levels (83). Techniques are disclosed for designing a source waveform to match the desired source spectrum. When better resolution is desired at a target zone, this leads to a required clustering of frequency components. A modulated waveform can be used to provide this clustering of frequency components.

A method for measuring a resistivity of a subsurface formation that includes transmitting continuously a signal at a first fundamental frequency at full power for a first period of time within a single window of time causing electromagnetic energy to propagate in the subsurface formations, transmitting continuously the signal at a second fundamental frequency at full power for a second period of time within the single window of time causing electromagnetic energy to propagate in the subsurface formations, measuring variations in the electromagnetic energy propagated through the subsurface formations at receivers at the first and the second fundamental frequencies, and determining the resistivity of the subsurface formations using the measurements of the variations in electromagnetic energy at the receivers.

Method for separating responses of multiple transmitters m a controlled source electromagnetic survey by using mutually orthogonal transmitter waveforms and transforming the combined response to the frequency domain (144) The mutual orthogonality can be based disjoint frequency spectra or on phase encoding of a common waveform element (FIG. 14)

A method for mapping a property of a subsurface reservoir includes determining a value of at least one reservoir property from measurements obtained from a well drilled through the reservoir. A relationship is determined between the at least one property of the reservoir and at least one seismic attribute and at least one electromagnetic survey attribute at a geodetic position of the well. A value of the at least one reservoir property is determined at at least one other geodetic position from a value of the at least one seismic attribute, a value of the at least one electromagnetic survey attribute at the at least one other geodetic position, and from the determined relationship.

A method for mapping a property of a subsurface reservoir includes determining a value of at least one reservoir property from measurements obtained from a well drilled through the reservoir. A relationship is determined between the at least one property of the reservoir and at least one seismic attribute and at least one electromagnetic survey attribute at a geodetic position of the well. A value of the at least one reservoir property is determined at at least one other geodetic position from a value of the at least one seismic attribute, a value of the at least one electromagnetic survey attribute at the at least one other geodetic position, and from the determined relationship.