Electromagnetic sources used in geophysical surveying generally produce unfocused regions of subsurface excitation. Unfocused excitations result both from the diffusive nature of low frequency electromagnetic wave propagation in the earth and from the use of relatively simple source configurations in the surveys. Unfocused excitations are a severe disadvantage when a target-oriented survey is needed, such as when it is desirable to probe the electrical properties of a prospective formation in a location where a complex geologic background may be present.
To eliminate the disadvantages of unfocused surveys, a method of source focusing is disclosed in U.S. patent application Ser. No. 09/656,191, filed Sep. 6, 2000 (hereinafter referred to as “Srnka”). The method, referred to as the remote reservoir resistivity mapping method (“R3M”) uses a spatially extended grounded electromagnetic source array that is positioned on the earth's surface, or near the seafloor, to focus electromagnetic energy on the subsurface target. The source array consists of two concentric electrode rings whose radii are optimally determined from theoretical modeling to focus the subsurface excitation at the approximate depth of the subsurface target. A limitation of the Srnka method, however, is that for typical reservoir depths, which range from 500 meters to greater than 5000 meters, the total length of electrode wire that is required in the concentric electrode rings ranges from about 23.5 km to greater than 235 km.
These extreme lengths of electrode wire raise numerous operational challenges. For example, difficulties in handling and deploying the electrode wires, and problems in ensuring accurate positioning of the wires in the desired electrode ring locations may be involved, and there is the need to avoid obstacles in the area of the desired ring locations, and the requirement that the electrodes be uniformly grounded along their lengths. Other problems influence the economics of the survey, including the large cost of the electrode wire, some of which is likely to be damaged or lost in each survey, and the large labor and logistics costs associated with mobilization, demobilization, deployment, and recovery of the wire.
For these reasons, Srnka also disclosed an alternative method that reduces the required length of electrode wire by substituting a set of equally spaced radial electric bipoles, each having a length equal to the difference in the lengths of the radii of the two concentric rings. FIG. 1 depicts both the R3M concentric ring method and this radial bipole alternative. Concentric electrode rings 4 and 5 lie on surface 1 of earth 2 and have radii a and b, respectively, and a center 7 generally above reservoir 3. In the radial bipole alternative method these concentric rings are replaced by radial electrode bipole sources 6. Each bipole source 6 has a length L=b−a. Srnka discloses that a minimum of six radial bipole sources 6 is preferable. A signal equivalent to the signal that a receiver array would receive from the concentric ring source array is produced mathematically by summing the signals received by the receiver array from each of the bipole sources 6 (For convenience, FIG. 1 does not depict the location of the receiver array). This alternative method, which allows the simulation of a concentric ring source array, is referred to as the virtual concentric ring source method, or as the radial bipole method, and substantially reduces the required length of electrode wire as compared to the R3M physical concentric ring source method.
The virtual concentric ring source method nevertheless uses relatively long lengths of electrode wire and inherently retains many of the operational challenges of the R3M physical concentric ring source method. In addition, both methods have other inherent disadvantages, in particular for certain aspects of the subsurface resistivity-imaging problem. Specifically, both methods produce maximum responses near a pre-specified subsurface excitation focus depth (in other words, at the approximate depth of the target), but provide little differential sensitivity to features within an appreciable vertical range of that depth. More specifically, the ability to image separately features above or below the target a distance of less than five percent of the target's depth below the surface is limited. This limitation greatly hinders the imaging of stacked reservoirs. In addition, amplifier saturation effects can influence data quality and hinder analysis of such data. A third issue is that both methods produce primarily vertical subsurface currents in the region of the target. These currents are not adequate for imaging reservoir electrical macro-anisotropy (vertical electrical isotropy), such as exists in many reservoirs due to the presence of shale interbeds. Imaging this anisotropy is essential for estimating reservoir net and gross volumes, and for determining hydrocarbon pore volumes.
Accordingly, a method is desired which retains the advantageous electromagnetic responses of the R3M physical concentric ring source and radial bipole methods, but which reduces or eliminates the disadvantages of those methods. The present invention satisfies that desire.