Approximate calculations of the transient electromagnetic response from buried conductors in conductive half-space note
The transient electromagnetic (TEM) response from a conductive plate buried in a conductive half-space and energized by a large-loop transmitter is investigated in a heuristic manner. The vortex and galvanic components are each calculated directly in the time domain using an approximate procedure which ignores the electromagnetic coupling present in the complete solution. In modeling the vortex and galvanic current flows, the plate is replaced with a single-turn wire loop of appropriate parameters and a distribution of current dipoles, respectively. The results of calculations of the transient magnetic field at the surface of the earth are presented for a few selected cases of practical interest.
The relative importance of the vortex and galvanic components varies with the half-space resistivity. The vortex component dominates if the half-space is resistive, in which case free-space algorithms suffice for numerical modeling. Furthermore the measured responses give much useful information about the target, and large depths of exploration should be achieved. As the half-space resistivity decreases, a significant half-space response is observed, caused by currents induced in the half-space itself. This response can be very large. Spatial variations in it caused by relatively small changes in resistivity, i.e., geologic noise, obscure the response from deep targets making them difficult to detect. The effect of the half-space is also to delay, distort, and reduce the vortex component in comparison with the free-space response.
The behavior of the galvanic component is determined by the half-space current flow. The presence of this component explains the large enhancement of over-ail target response seen at early times over relatively resistive ground and the departure from an exponential decay seen over more conductive ground, again with respect to responses predicted by free-space modeling. In more conductive ground the galvanic component completely dominates the vortex component, resulting in the loss of useful diagnostic information. Although target location and depth can still be determined, target shape and orientation are poorly defined. Because of galvanic current saturation good conductors are difficult to distinguish from poor ones.