ELECTRICAL CONDUCTIVITY IMAGING USING CONTROLLED SOURCE ELECTROMAGNETICS FOR SUBSURFACE CHARACTERIZATION

Controlled Source Audio-Frequency Magnetotellurics (CSAMT) is a frequency domain electromagnetic (EM) sounding technique. CSAMT typically uses a grounded horizontal electric dipole approximately one to two kilometers in length as a source. Measurements of electric and magnetic field components are made at stations located ideally at least four skin depths away from the transmitter to approximate plane wave characteristics of the source. Data are acquired in a broad band frequency range that are sampled logarithmically from 0.1 Hz to 10 kHz.

The usefulness of CSAMT soundings is to detect and map resistivity contrasts in the top two to three km of the Earth’s surface. Higher frequency data can be used to image shallow features and lower frequency data are sensitive to deeper structures. Some practical applications that CSAMT soundings have been used for include: mapping ground water resources; mineral/precious metals exploration; geothermal reservoir mapping and monitoring; petroleum exploration; and geotechnical investigations.

We have a 3D CSAMT data set consisting of phase and amplitude measurements of the Ex and Hy components of the electric and magnetic fields respectively. The survey area is approximately 3 X 5 km. Receiver stations are situated 50 meters apart along a total of 13 lines with 8 lines bearing approximately N60E and the remainder of the lines oriented orthogonal to these 8 lines. We use an unconstrained Gauss-Newton method with positivity to invert the data. Inversion results will consist of conductivity versus depth profiles beneath each receiver station. These 1D profiles will be combined into a 3D subsurface conductivity image. We will include our interpretation of the subsurface conductivity structure and quantify the uncertainties associated with this interpretation.