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Geology topics

Walter L. Anderson

Publications and source records attributed to Walter L. Anderson.

At least 19 recordsLinked to original sources
USGS1987

Effect of transmitter turn-off time on transient soundings

A general procedure for computing the effect of non-zero turn-off time on the transient electromagnetic response is presented which can be applied to forward and inverse calculation methods for any transmitter-receiver configuration. We consider in detail the case of a large transmitter loop which has a receiver coil located at the center of the loop (central induction or in-loop array). For a linear turn-off ramp of width t 0 , the voltage response is shown to be the voltage due to an ideal step turn-off averaged over windows of width t 0 . Thus the effect is similar to that obtained by using averaging windows in the receiver. In general when time zero is taken to be the end of the ramp, the apparent resistivity increases for a homogeneous half-space over a limited time range. For time zero taken to be the start of the ramp the apparent resistivity is affected in the opposite direction. The effect of the ramp increases with increasing t 0 and first-layer resistivity, is largest during the intermediate stage, and decreases with increasing time. It is shown that for a ramp turn-off, there is no effect in the early and late stages. For two-layered models with a resistive first layer ( ρ 1 > ρ 2 ), the apparent resistivity is increased in the intermediate stage. When the first layer is more conductive than the second layer ( ρ 1 < ρ 2 ) and the layer thickness is comparable or greater than the loop radius, similar results are obtained; however, when the layer is thin compared to the loop radius the apparent resistivity is initially decreased and then increases as time increases. Examples are presented which illustrate the strong influence of the geoelectrical section on the turn-off effect. Neglecting the turn-off ramp will affect data interpretation as shown by field examples; the influence is the greatest on near-surface layer parameters.

Geoexploration↗
USGS1979

Programs EMCUPL and SCHCOPL: computation of electromagnetic coupling on a layered halfspace with complex conductivities

A number of efficient numerical computer algorithms are incorporated into a general program called EMCUPL, which calculates the electromagnetic (EM) coupling between two straight wires on the surface of a multilayered half space. Each layer has an isotropic conductivity which may be either real or complex. A second computer program, called SCHCOPL, is described which calculates the coupling for the special case of a Schlumberger or Wenner array also on a multilayered half space. Comparison with other programs shows that EMCUPL is at least as accurate, more generally applicable, and computationally more efficient FORTRAN listings of all subprograms and example calculations are given in the Appendix.

Open-File Report↗