Transient electromagnetic imaging of a basin margin underneath the Savannah River Plant, Aiken, South Carolina

摘要

An experimental transient electromagnetic (TEM) survey was performed at the Savannah River Plant (SRP) near Aiken, South Carolina, to assess the viability of a subsurface imaging method. The goal of the survey was improve resolution of mapping the margin of a buried Triassic basin partially underlying SRP. A high data density survey was conducted utilizing multiple large loop transmitters with overlapping profiles of measurements from each transmitter. Any attempt to image subsurface structure requires such data density. A gap in interpretation tolls exists in all electrical methods between layered earth inversion and numerical modeling of two-dimensional (2D) or three-dimensional (3D) structures. Recent TEM research has focused on development of methods to image subsurface geoelectric structure without resorting to numerical modeling. The method described here constructs a subsurface geoelectric image by a novel means of stacking TEM data in both time and space. The stacking procedure is based on knowledge of where currents are flowing in the subsurface of an approximate background model. Weights for stacking of the TEM data into estimates of anomalous current flow on a subsurface grid are calculated from these known current flow patterns as a function of time, measurement position and subsurface grid element position using Biot-Savart's Law. Thus for the SRP survey, a 2D structure is imaged built on knowledge of the fields of an approximate one-dimensional (1D) model. The basin margin under SRP is successfully resolved by the imaging method. The imaging result provides better resolution than obtained by splicing together layered earth inversion models. Various pros and cons of this imaging method are discussed to indicate future directions for TEM imaging. 10 refs., 6 figs. (ERA citation 16:001703)