Geology Reference
In-Depth Information
Figure 8.11
Results of a detailed EM31 resistivity survey, plotted as a
shaded-relief image. The aim of the survey was to locate buried pipes and
cables in a redevelopment site. Fewer than half the objects actually located
had been previously known to the developers.
conductor, then effectively only a half of the Slingram anomaly would be
produced; that is, a steeply dipping body would have a zero response when
a horizontal receiver coil was immediately above it and the anomaly would
be anti-symmetric. Fixed source systems often measure dip-angles or their
tangents, which are the ratios of the vertical to the horizontal fields.
8.3.2 Turam
In the now almost obsolete
Turam
(Swedish, 'two coil') method, the field
produced by a fixed extended source was recorded at two receiving coils
separated by a distance of the order of 10 m. Anomalies were assessed by
calculating
reduced ratios
equal to the actual ratios of the signal amplitudes
through the two coils divided by the
normal
ratios that would have been
observed over non-conductive terrain. The phase differences between the
