Remote reservoir resistivity mapping

Abstract

A method for surface estimation of reservoir properties, wherein location of and average earth resistivities above, below, and horizontally adjacent to the subsurface geologic formation are first determined using geological and geophysical data in the vicinity of the subsurface geologic formation. Then dimensions and probing frequency for an electromagnetic source are determined to substantially maximize transmitted vertical and horizontal electric currents at the subsurface geologic formation, using the location and the average earth resistivities. Next, the electromagnetic source is activated at or near surface, approximately centered above the subsurface geologic formation and a plurality of components of electromagnetic response is measured with a receiver array. Geometrical and electrical parameter constraints are determined, using the geological and geophysical data. Finally, the electromagnetic response is processed using the geometrical and electrical parameter constraints to produce inverted vertical and horizontal resistivity depth images. Optionally, the inverted resistivity depth images may be combined with the geological and geophysical data to estimate the reservoir fluid and shaliness properties.

Description

This application
1/ρ_horiz=(1/ρ_ss)×ntg+(1/ρ_sh)×(1−ntg)  (2)

Equations (1) and (2) contain three unknown averaged reservoir parameters: ρ_ss, ρ_sh, and ntg. Estimates of ρ_sh within the reservoir interval, derived independently from the facies model or available facies data, are used next to derive and map the two remaining unknown values ρ_ss and ntg over the spatial extent of the reservoir. Reservoir fluid type, hydrocarbon pore volume or water saturation are then derived from the mapped ρ_ss value within the area of the seismically defined reservoir. Statistical methods including Monte Carlo inversions may also be used for deriving hydrocarbon pore volume, net-to-gross, water saturation, and other reservoir properties from the ρ_vert and ρ_horiz inversion measurements. The derivation uses the facies model of rock properties distributions combined with Archie's Equations for the electrical resistivity of a porous rock containing fluid in the pore spaces, relative to ρ_ss values within the same geologic unit outside of the reservoir.

The invention described above is designed to provide an order of magnitude improvement in subsurface vertical electromagnetic resolution over current technology.

FIG. 9 is a flowchart that illustrates a preferred embodiment of the method of the invention for surface estimation of reservoir properties of a subsurface geologic formation, as just described. First, at step 900, location of the subsurface geologic formation is determined, using geological and geophysical data in the vicinity of the subsurface geologic formation. Next, at step 902, average earth resistivities above, below, and horizontally adjacent to the subsurface geologic formation is determined, using geological and geophysical data in the vicinity of the subsurface geologic formation. Next, at step 904, dimensions for a high-current multi-mode electromagnetic source are determined to substantially maximize transmitted vertical and horizontal electric currents at the subsurface geologic formation, using the location and the average earth resistivities. Preferably, the dimensions are calculated by numerically solving the uninsulated buried low-frequency electromagnetic antenna problem, as described previously. Next, at step 906, probing frequency for a high-current multi-mode electromagnetic source is determined to substantially maximize transmitted vertical and horizontal electric currents at the subsurface geologic formation, using the location and the average earth resistivities. Again, the probing frequency preferably is calculated by numerically solving the uninsulated buried low-frequency electromagnetic antenna problem, as described previously. Alternatively, iterated 3-D modeling calculations of the subsurface geologic formation's electromagnetic response may be used to verify the dimensions and probing frequency of the high-current multi-mode electromagnetic source in steps 904 and 906. Next, at step 908, the electromagnetic source is activated at or near the surface, approximately centered above the subsurface geologic formation. Next, at step 910, a plurality of components of electromagnetic response are measured with a receiver array. Preferably, when the array of receivers 10 is positioned on land, two orthogonal horizontal electric fields, two orthogonal horizontal magnetic fields, and a vertical magnetic field are measured. Alternatively, when the array of receivers 10 is positioned offshore, an additional vertical electric field is measured. Next, at step 912, geometrical and electrical parameter constraints are determined, using the geological and geophysical data. Next, at step 914, the electromagnetic response is processed using the geometrical and electrical parameter constraints to produce inverted vertical and horizontal resistivity depth images. Preferably, the components of the electromagnetic response are processed using full 3-D wave-equation methods, as described previously. 1-D inversion of the electromagnetic response is used to verify the average earth resistivities above, below, and horizontally adjacent to the subsurface geologic formation, as determined in step 902. Finally at step 916, the inverted resistivity depth images are combined with the geological and geophysical data to estimate the reservoir properties. Details of the preferred method of inversion are described later in conjunction with the following example.

The following example illustrates the application of the invention for onshore (land) hydrocarbon reservoir resistivity mapping. After 3-D seismic data in the survey area are acquired, interpreted, and converted to the depth domain, the prospective reservoir is identified (depth d and extent l). Knowledge of the earth's electrical resistivity for the survey area, averaged over intervals of 0.10×d, from the earth's surface to three times the reservoir depth (3×d) and five times the reservoir extent (5×l), is gathered using existing electromagnetic survey data and well logs, or is estimated using geologic basin analogs. The diameters of the grounded electrodes are calculated by numerically solving the uninsulated buried low-frequency electromagnetic antenna problem as discussed above, or by iterated 3D electromagnetic modeling, using the reservoir depth and vertically averaged layered-earth resistivities as inputs. The diameter of the optional insulated loop electrode is determined using standard methods known in the art.

FIG. 6 shows land source and receiver configurations for a target reservoir 3 identified seismically at d=1000 meters depth to top of reservoir, having an average lateral extent (radius) 1/2=1250 meters. Eight (8) partially grounded radial electrodes 11 and connected terminating electrodes 12, as described also in FIG. 2, are deployed in a radial array in conjunction with an insulated loop source 6. The geometrical center of the grounded electrode array (intersection of their 8 radius lines) and the center of the insulated loop are positioned at the surface of the earth 1 vertically above the center of the reservoir target. The grounded electrodes are positioned symmetrically around the circumference of the source array, each separated by an angle of 45±1 degrees from the adjacent electrode as measured from the center of the source array. The source dimensions are a=1500 meters, b=6000 meters, γ=90 meters, and c=1000 meters. The value γ is determined from the calculation of vertical current leakage from a continuously grounded bipole antenna of length L, using the method described above to numerically solve the uninsulated buried low-frequency electromagnetic antenna problem. This shows that most of the current leaves the grounded wire within a distance≦L/5 at each end of the antenna. The grounded terminating electrodes 12 each have a length of 30 meters. The grounded array and the insulated loop are not moved during the survey. Alternatively, if the number of power sources/controllers 7 is limited, or if survey logistics or terrain difficulties make simultaneous use of the eight radial grounded electrode positions impractical or too costly, the eight radial partially grounded electrode positions are occupied sequentially in groups of one or more positions, in any sequential order.

A preferred procedure is to obtain substantially optimal parameter values to substantially maximize the electric field at the reservoir depth. However, as an alternative procedure, a sub-optimal aspect ratio b/a could be used to reduce electrode cost, installation effort, and survey permitting. For instance, an aspect ratio b/a=4 could be used. Use of this value for b/a would result in a 24.5% reduction in vertical electric field at the reservoir target, as shown in FIG. 5a, and a corresponding reduction in the electromagnetic responses of the reservoir to the grounded electrode excitation as measured at the surface receiver array 10.

Assume a vertically averaged resistivity of the earth of value ρ_e=1 Ohm-m. Then the central operating frequency of the grounded electrode array is derived from d/δ=9/4 and d=2250 meters, or f=0.050 Hz. The output bandwidth of the grounded electrode sources is 0.005≦f≦5.0 Hz. Using the analysis of B. R. Spies (1989, op. cit.), the central operating frequency of the insulated loop source is set by d/δ=1, or f=0.253 Hz. The output bandwidth of the insulated loop source is 0.025≦f≦25 Hz.

Nine power sources/controllers 7 are placed at the surface of the earth 1. Each source/controller is powered by connection to a municipal power grid, if available, or is powered by one or more generators in the field survey area. Each source/controller is nominally rated at 300 kVa, with outputs of 120 VAC and 2500 A (rms). One source/controller is situated at any position along the circumference of the insulated loop source 6, and is connected by a coaxial power cable 9 at the surface of the earth to the insulated loop source. The remaining eight power sources/controllers 7 are placed within a distance of L/10 of the midpoints of the partially grounded electrodes, as shown in FIG. 2. These eight power sources/controllers are connected to the grounded electrodes by means of coaxial or single-conductor power cables 8. The satellite Global Positioning System (GPS) signal is used to monitor and synchronize the phases of all the sources. Alternatively, if the number of power sources/controllers 7 is limited, or if survey logistics or terrain difficulties make simultaneous use of the eight radial grounded electrode positions impractical or too costly, one or more power sources/controllers may be used to energize separately the eight partially grounded and the insulated loop source, in any sequential order.

The partially grounded electrodes 11 and the terminating electrodes 12 each consist of three uninsulated size 4/0 multi-strand copper wires. The grounded wires that comprise the grounded portion of each partially grounded electrode element 11 are buried in parallel within the top 1.0 meter of the earth's surface by means of manual digging or standard mechanical cable-laying devices. The ungrounded portions of each of the radial partially grounded electrodes 11 consist of three uninsulated size 4/0 multi-strand copper wires that are connected to the uninsulated buried electrode wires comprising the grounded portions. The ungrounded portions of each radial electrode are laid on the surface of the earth. Electrical contact of the grounded radial electrodes and the terminating electrodes is maintained with the earth by periodically wetting the buried electrode areas with water, as needed according to local ground moisture conditions. The loop source 6 consists of one single-conductor multi-strand insulated size 4/0 copper wire. Power connection cables 8 and 9 are electrically rated according to U.S. NEMA (National Electrical Manufacturing Association) codes and standards to carry the current delivered to the grounded electrodes 11,12 and to the insulated loop 6, respectively.

Electromagnetic receivers 10, such as Electromagnetic Instruments, Inc. (EMI) type MT-24/NS™ or equivalent, are positioned over the surface of the earth 1 within a radial distance r=(x²+y²)^1/2=5000 meters from the center of the array, but not within 25 meters of any grounded electrode 11, 12 or the insulated loop 6, to minimize source-generated noise and saturation of the receiver signals. The receivers are positioned on a uniform grid as shown in FIG. 6, with a lateral spacing of x=y=100 meters, within a radius of 2000 meters from the center of the array, and on a uniform grid with a lateral spacing of x=y=300 meters from a radius of 2500 meters to a radius of 5000 meters from the center of the array. Each five-channel receiver measures two components (x and y directions) of the horizontal electric field, two components (x and y directions) of the horizontal magnetic field, and one component (z direction) of the vertical magnetic field. The receivers are modified by standard industry methods including feedback stabilization so that the phase accuracy of the magnetic field induction sensors (EMI type BF-4™ or equivalent) is greater than or equal to 0.10 degrees in the full frequency range of the survey (0.005≦f≦25 Hz).

The five-component receivers are deployed simultaneously in large groups (16 or more) within the survey area, with as many receiver groups deployed as possible and practical for the local conditions of the survey (e.g. terrain difficulties, logistical support). Data are gathered for each receiver group by a central processing unit (EMI type FAM/CSU™ or equivalent). Differential GPS geodetic methods are used to measure the positions (x, y, z) of all receivers to within 0.1 meters accuracy. The GPS signal is also used for phase synchronization (timing) of all receiver data.

The receiver data are collected in three ways. First, the receiver data are collected as time records with all sources 7 turned off, to record zero excitation currents. These data are collected over a length of time that is sufficient to record raw stacked magnetotelluric data having three-sigma errors less than or equal to 5% over the frequency range 0.0025≦f≦25 Hz. Typically, collection of this data will take 1-10 days, depending upon local conditions and the logistics of receiver deployment. This first set of receiver data is magnetotelluric data. Second, the insulated loop source is energized using a standard electromagnetic industry bipolar square wave current from its attached power source/controller 7, as shown in FIG. 7. In this preferred embodiment of the method of the present invention, current pulse on-time, T₁ equals current waveform off-time T₂, that is, T₁=T₂, but this is not a limitation of the method. Other source current waveforms may also be used for the insulated loop source current, including sinusoidal waveform combinations and pseudo-random sequences as well known to one skilled in the art, provided the insulated loop source frequency range is as stated. The receiver responses are collected using time-domain measurements acquired during the current waveform off-time, T₂ in FIG. 7. The duration of the current pulse on-time, T₁ in FIG. 7 (and hence also the off-time T₂), is set at three values, 0.01, 1.0, and 10.0 seconds. Sufficient repetitions (typically 50 to 1000) of the loop source current are made at each on-time value so that the raw stacked data time series data have three-sigma errors less than or equal to 1% over the frequency range 0.025≦f≦25 Hz. This second set of receiver data is vertical magnetic dipole data. Third, the insulated loop source is turned off (zero current) and the eight partially grounded electrodes 11, 12 are simultaneously energized in phase. Alternatively, if the number of power sources/controllers is limited, the partially grounded electrodes are energized separately in groups of one or more, in any sequential order. Each of the eight power sources/controllers 7 produces a standard electromagnetic industry bipolar square wave current pulse, as shown in FIG. 7, with the duration of the current pulse on-time, T₁ in FIG. 7, (and hence T₂) set at three values, 0.05, 5.0, and 50.0 seconds. Other source current waveforms may also be used for the grounded source current, including sinusoidal waveform combinations and pseudo-random sequences as well known to one skilled in the art, provided the grounded electrode source frequency range is as stated. Sufficient repetitions (typically 50 to 1000) of the grounded electrodes' source currents are made at each on-time value so that the raw stacked data time series data have three-sigma errors less than or equal to 1% over the frequency range 0.005≦f≦5 Hz. This third set of receiver data is grounded radial electrode data.

The three sets of receiver data are processed in the following way. After noise suppression using standard industry methods as described above, the second set of vertical magnetic dipole data and the third set of grounded radial electrode measurements are converted to the complex frequency-wavenumber domain using standard industry 2-D Fourier and Radon transform techniques. The first set of magnetotelluric data and the second set of vertical magnetic dipole data are merged together in the frequency-wavenumber domain, for each electromagnetic tensor component of the data. The merged magnetotelluric and vertical magnetic dipole data sets are inverted, and the grounded radial electrode data set is inverted separately. Then the merged magnetotelluric and vertical magnetic dipole data and the grounded radial electrode data are inverted jointly, as discussed in D. Jupp and K. Vozoff, Geophys. Prospecting, v. 25, 460-470, 1977. The magnetotelluric data, the vertical magnetic dipole data, and the grounded radial electrode data are also inverted separately. All data inversions use the 3-D frequency-domain finite-difference fully nonlinear methods of G. A. Newman and D. L Alumbaugh (1996, 1997, op. cit.), modified to allow for the geometries of the grounded radial electrode and the insulated loop source current arrays. Depth and parameter value constraints are enforced during the inversion, using sharp-boundary methods (G. Hoversten et al, 1998, op. cit.) and integral resistance and conductance bounds within the update region of the nonlinear inversion 3-D mesh that contains the reservoir target, combined with minimum-gradient support techniques (O. Portniaguine and M. Zhdanov, 1998, op. cit.). The nonlinear inversion update region is centered on the target reservoir, and extends 100 meters above and below he reservoir and 200 meters laterally from each reservoir edge.

The starting model for both the merged magnetotelluric and vertical magnetic dipole data inversion and the grounded radial electrode data inversion is an interpreted seismic depth model in which the mechanical properties (primarily the interval acoustic impedances) are replaced with resistivity estimates. The resistivity estimates may come from electromagnetic survey data, well logs, empirical relations to seismic parameters, or geologic basin analogs, as described above. The inversions are performed by means of a digital electronic computer of the massively parallel processor (MPP) type, or alternatively using a network of electronic digital computers that mimic an MPP computer. After the separate magnetotelluric, vertical magnetic dipole, and grounded radial electrode data inversions are completed, the magnetotelluric—vertical magnetic dipole and grounded radial electrode data are inverted jointly. The five respective 3-D depth cubes of inverted resistivity (magnetotelluric, vertical magnetic dipole, grounded radial electrode, merged magnetotelluric—vertical magnetic dipole, and merged magnetotelluric—vertical magnetic dipole—grounded radial electrode) are compared, and the ratios of their resistivity values are formed at each depth location using 3D visualization methods. Finally, values of ρ_ss, ρ_sh, and ntg are derived for the reservoir interval using the methods described above, and are mapped. These mapped values are interpreted in conjunction with the 3-D seismic data and its attributes.

FIG. 8 shows the complex magnitude of the calculated radial component E_r=(E_x²+E_y²)^1/2 of the surface electric field response from the example target reservoir described above, due to excitation by the grounded electrode array. The example reservoir is assumed to have a vertical thickness of 20 meters and a vertically averaged resistivity of 100 Ohm-m. The electromagnetic response was calculated using the SYSTEM 3-D integral equation computer code developed at the University of Utah's Consortium for Electromagnetic Modeling and Inversion. This electric field component response is normalized to the uniform earth (halfspace) response, and is shown on FIG. 8 as a function of radial distance from the center of the array and of the source frequency, along the x=0 (or y=0) axis. Most of the normalized E_r response is contained within r≦1300 meters, and has a maximum value of approximately 33% at r=0 at the lowest survey frequency (f=0.005 Hz). The large normalized E_r value at r=1500 meters is a local effect of the inner radial electrode.

The benefits provided by this invention include at least the following two. The first benefit is cost and cycle-time reduction in hydrocarbon exploration, development, and production activities, including reducing exploration drill-well risk, improving discovered-undeveloped reservoir delineation and assessment, and improving reservoir monitoring and depletion. The second benefit is improved business capture of new exploration ventures and field commercializations by offering unique, proprietary reservoir properties estimation technology.

It should be understood that the invention is not to be unduly limited to the foregoing which has been set forth for illustrative purposes. Various modifications and alternatives will be apparent to those skilled in the art without departing from the true scope of the invention, as defined in the following claims.

Claims

1. A method for surface estimation of a resistivity depth image of a subsurface geologic formation, comprising the steps of:

determining the location of and at least one average earth resistivity for the vicinity of the subsurface geologic formation using geological and geophysical data from the vicinity of the subsurface geologic formation;

determining dimensions and probing frequency for an electromagnetic source to substantially maximize transmitted vertical and horizontal electric currents at the subsurface geologic formation using the location and the at least one average earth resistivity;

activating the electromagnetic source at or near the surface of the earth, approximately centered above the subsurface geologic formation;

measuring a plurality of components of electromagnetic response with a receiver array;

determining one or more geometrical and electrical parameter constraints, using the geological and geophysical data; and

processing the electromagnetic response using the geometrical and electrical parameter constraints to produce the resistivity depth image.

2. The method of claim 1, further comprising the step of:

combining the resistivity depth image with the geological and geophysical data to estimate one or more properties of the subsurface geological formation.

3. The method of claim 1, wherein the step of determining dimensions and probing frequency is accomplished by numerically solving the uninsulated buried low-frequency electromagnetic antenna problem.

4. The method of claim 1, wherein the electromagnetic source comprises

two continuously grounded circular electrodes positioned in concentric circles.

5. The method of claim 4, wherein each circular electrode comprises one or more electrically uninsulated conductors.

6. The method of claim 4, further comprising:

a third circular electrode positioned concentric with the two circular electrodes.

7. The method of claim 6, wherein the third circular electrode comprises one or more electrically insulated conductors.

8. The method of claim 1, wherein the electromagnetic source comprises six or more grounded linear radial electrodes of equal lengths placed along radii separated by equal angles, whose radial projections intersect at a common central point.

9. The method of claim 8, wherein the radial electrodes are continuously grounded along their entire length.

10. The method of claim 8, wherein the radial electrodes are continuously grounded only within a distance less than one half of the length of the radial electrode from each end.

11. The method of claim 1, wherein the subsurface geologic formation is located onshore.

12. The method of claim 1, wherein the subsurface geologic formation is located offshore and the surface of the earth is the seafloor.

13. The method of claim 1, wherein the receiver array is positioned on a grid.

14. The method of claim 1, wherein the receiver array is positioned as a linear array.

15. The method of claim 1, wherein the receiver array is positioned as a swath array.

16. The method of claim 1, wherein the step of processing the electromagnetic response further comprises:

verifying the at least one average earth resistivity using the plurality of components of electromagnetic response measured with the receiver array.

17. The method of claim 1, wherein the step of processing the electromagnetic response further comprises:

applying 3-D wave-equation data processing to the electromagnetic response.

18. The method of claim 1, wherein the step of processing the electromagnetic response further comprises data noise suppression, source deconvolution, and model-guided inversion.

19. The method of claim 7, wherein the steps of activating the electromagnetic source and measuring the plurality of components of electromagnetic response further comprises:

measuring a first electromagnetic response without activating the electromagnetic source;

measuring a second electromagnetic response while activating only the third circular electrode; and

measuring a third electromagnetic response while activating only the two continuously grounded circular electrodes.

20. The method of claim 19, wherein the step of processing the electromagnetic response further comprises:

merging the first and second electromagnetic responses to produce a fourth electromagnetic response;

inverting the fourth electromagnetic response; and

inverting jointly the third and fourth electromagnetic responses.

21. The method of claim 20, wherein the step of processing the electromagnetic response further comprises at least one step chosen from:

inverting the first electromagnetic response;

inverting the second electromagnetic response; and

inverting the third electromagnetic response.

22. The method of claim 1, wherein the resistivity depth image comprises at least one depth image component chosen from an inverted vertical resistivity depth image, an inverted horizontal resistivity depth image and an inverted three-dimensional resistivity depth image.

23. The method of claim 1, wherein the dimensions and probing frequency are verified using iterated 3-D modeling.

24. The method of claim 8, further comprising continuously grounded linear terminating electrodes connected substantially orthogonally at each end of the grounded radial electrodes.

25. The method of claim 24, wherein the length of the terminating electrodes is less than or equal to one tenth of the length of the radial electrodes.

26. The method of claim 1, wherein the electromagnetic source comprises a sub-optimal configuration.

27. The method of claim 11, wherein the plurality of components of electromagnetic response comprise:

two orthogonal horizontal electric fields;

two orthogonal horizontal magnetic fields; and

a vertical magnetic field.

28. The method of claim 27, wherein the plurality of components of electromagnetic response further comprises a vertical electric field.

29. The method of claim 12, wherein the plurality of components of electromagnetic response comprise:

two orthogonal horizontal electric fields;

two orthogonal horizontal magnetic fields;

and a vertical electric field.

30. The method of claim 29, wherein the plurality of components of electromagnetic response further comprise a vertical magnetic field.

31. A method for surface estimation of an inverted resistivity depth image of a subsurface geologic formation, comprising the steps of:

determining the location of and average earth resistivity above, below, and horizontally adjacent to the subsurface geologic formation using geological and geophysical data from the vicinity of the subsurface geologic formation;

determining dimensions and probing frequency for an electromagnetic source to substantially maximize transmitted vertical and horizontal electric currents at the subsurface geologic formation using the location and the at least one average earth resistivity, said source comprising six or more grounded linear radial electrodes of equal lengths placed along radii separated by equal angles whose radial projections intersect at a common central point, continuously grounded linear terminating electrodes connected substantially orthogonally at each end of the grounded radial electrodes;

activating the electromagnetic source at or near the surface of the earth, approximately centered above the subsurface geologic formation;

measuring a plurality of components of electromagnetic response with a receiver array;

determining one or more geometrical and electrical parameter constraints, using the geological and geophysical data; and

processing the electromagnetic response using the geometrical and electrical parameter constraints to produce the inverted resistivity depth image.

32. A method for surface estimation of one or more properties of a subsurface geologic formation, comprising the steps of:

determining the location of and at least one average earth resistivity for the vicinity of the subsurface geologic formation using geological and geophysical data from the vicinity of the subsurface geologic formation;

determining dimensions and probing frequency for an electromagnetic source to substantially maximize transmitted vertical and horizontal electric currents at the subsurface geologic formation using the location and the at least one average earth resistivity, said source comprising six or more grounded linear radial electrodes of equal lengths placed along radii separated by equal angles whose radial projections intersect at a common central point;

activating the electromagnetic source at or near the surface of the earth, approximately centered above the subsurface geologic formation;

measuring a plurality of components of electromagnetic response with a receiver array;

determining one or more geometrical and electrical parameter constraints, using the geological and geophysical data;

processing the electromagnetic response using the geometrical and electrical parameter constraints to produce one or more inverted resistivity depth images of the subsurface geologic formation; and

combining the inverted resistivity depth images with the geological and geophysical data to estimate the properties.

33. A method for surface estimation of one or more properties of a subsurface geologic formation, comprising the steps of:

determining the location of and at least one average earth resistivity for the vicinity of the subsurface geologic formation;

determining dimensions and probing frequency for an electromagnetic source to substantially maximize transmitted vertical electric currents at the subsurface geologic formation using the location and the at least one average earth resistivity;

activating the electromagnetic source at or near the surface of the earth, approximately centered above the subsurface geologic formation;

measuring at least a vertical electromagnetic response with a receiver array;

determining one or more geometrical and electrical parameter constraints, using geological and geophysical data from the vicinity of the subsurface geologic formation;

processing the electromagnetic response using the geometrical and electrical parameter constraints to estimate the one or more properties.

34. A method for designing a focused electromagnetic source for geophysical prospecting of a subsurface geologic formation comprising the steps of:

determining the location of and at least one average earth resistivity for the vicinity of the subsurface geologic formation using geological and geophysical data from the vicinity of the subsurface geologic formation; and

determining dimensions and probing frequency for said source to substantially maximize transmitted vertical and horizontal electric currents at the subsurface geologic formation using the location and the at least one average earth resistivity.

35. The method of claim 34, wherein the step of determining dimensions and probing frequency is accomplished by numerically solving the uninsulated buried low-frequency electromagnetic antenna problem.

36. The method of claim 34, wherein the dimensions and probing frequency are verified using iterated 3-D modeling.

37. The method of claim 34, wherein the electromagnetic source comprises two continuously grounded circular electrodes positioned in concentric circles.

38. The method of claim 37, wherein each circular electrode comprises one or more electrically uninsulated conductors.

39. The method of claim 37, further comprising a third circular electrode positioned concentric with the two circular electrodes.

40. The method of claim 39, wherein the third circular electrode comprises one or more electrically insulated conductors.

41. The method of claim 34, wherein the electromagnetic source comprises six or more grounded linear radial electrodes of equal lengths placed along radii separated by substantially equal angles, whose radial projections intersect at a common central point.

42. The method of claim 41, further comprising continuously grounded linear terminating electrodes connected substantially orthogonally at each end of the grounded radial electrodes.

43. The method of claim 42, wherein the length of the terminating electrodes is less than or equal to one-tenth of the length of the radial electrodes.

44. The method of claim 41, wherein the radial electrodes are continuously grounded along their entire length.

45. The method of claim 41, wherein the radial electrodes are continuously grounded only within a distance less than one half of the length of the radial electrode from each end.

46. The method of claim 34, wherein the electromagnetic source comprises a sub-optimal configuration.