Real-time resistivity sounding using a hand-held broadband electromagnetic sensor

Common situations encountered in geophysical investigations involving environmental and engineering problems call for simple and fast onsite interpretation of the survey data so results can be available in a timely manner for the projects. We have developed a method of interpreting broadband electromagnetic induction (EMI) data using a hand-held electromagnetic (EM) sensor based on a layered-earth model. In conductive environments, a hand-held EM sensor yields sufficient information for shallow resistivity soundings. The result is a conductivity-depth section based on the frequency-sounding principle. Tests using simulated data show that the method can reasonably recover earth conductivity structures with up to several layers. We have field tested the method using data obtained with a GEM-2 broadband EMI sensor. The conductivity-depth section converted from the field data reasonably reflects the known variations in earth conductivity. Because the current GEM-2 is band-limited to 48 kHz, the method works mainly in conductive environments. Sample GEM-2 and GEM-3 data collected over shallow seawater produced a bathymetric section with a precision of a few centimeters. Application of the method in resistive environments requires a broader bandwidth.