Transient Electromawetic Imaginq Of A Basin Margin Underneath The Savannah River Site, Aiken, South Canolina

An experimental transient electromagnetic (TEM) survey was performed at<br>the Savannah River Site (SRS) near Alken, South Carolina, to assess the<br>viability of a subsurface imaging method. The goal of the survey was to<br>improve resolution of mapping the margln of a buried Triassic basin partially<br>underlying SRS. A high data density survey was conducted utiiizing multiple<br>large loop transmitters with overlapping profiles of measurements from each<br>transmitter. Any attempt to image subsurface structure requires such data density.<br>A gap in interpretation tools exists in all electrical methods between<br>layered earth inversion and numerical modeling of two-dimensional (20) or<br>three-dimensional (3D) structures. Recent TEM research has focused on<br>development of methods to image subsurface geoelectric structure without<br>resorting to numerical modeling. The method described here constructs a<br>subsurface geoeiectric image by a novel means of stacking TEM data in both<br>time and space.<br>The stacking procedure is based on knowledge of where currents are<br>flowing in the subsurface of an approximate background model. Weights for<br>stacking of the TEM data into estimates of anomalous current flow on a<br>subsurface grid are calculated from these known current flow patterns as a<br>function of time, measurement position and subsurface grid element position<br>using Biot-Savart’s Law.<br>Thus, for the SRS survey, a 2D structure is imaged built on knowledge<br>of the fleids of an approximate one-dimensional (1D) model. The basin margin<br>under SRS is successfully resolved by the imaging method. The imaging<br>result provldes better resolution than obtained by splicing together layered<br>earth inversion models.<br>Various pros and cons of this imaging method are discussed to indicate<br>future directions for TEM imaging.