Diverse Applications Show Power of OhmMapper

Improved Void Detection

Cavity detection has historically been one of the toughest geophysical problems, the equivalent of trying to prove there's no needle in the haystack. With the OhmMapper, high-volume, repeatable data collection over shallow void-like targets is easy. The vertical depth section (Figure 8), created from OhmMapper data, clearly shows the mapped image of a known cavity, the cave in the photograph opposite.

Electrical pseudo-section with square showing the anomaly associated with the cave. Surveyed by WREDCO Geosurveys, Spooner, WI. Released by permission of Barr Engineering Co. Resistivity shown in Ohm-Meters

Growing Industries: Resistivity and Precision Agriculture

The science of precision agriculture uses exacting measurements to improve field operations and information on tillage, applications, planting, weeds, insect and disease infestations, cultivation, and irrigation. Resistivity surveying is one of these new tools being used to lead the way to higher agricultural yields. In the UK, resistivity is commonly used to delineate the boundaries of different soil types.

In a recent experiment undertaken by Martin deSaire of the U.S. Department of Agriculture, a clearly defined OhmMapper soil-depth profile allowed a comparison of soil depth to corn yield. The depth section below, created with RES2DINV Resistivity Inversion Software available from Geometrics, shows conductive soil overlaying undulating resistive rock. Borehole testing to confirm bedrock depth showed that areas with the deepest topsoil matched areas of highest yield.

Archaeology: Mapping a Buried Roman Amphitheater

Teaching: Clay Lens Clarity

The OhmMapper Resistivity Meter makes a great tool for evaluating archaeological sites. The figure opposite shows an OhmMapper plan-view map of a suspected Roman amphitheater. The map shows circular resistive features (the terrace of the ancient amphitheater) surrounding a more conductive area (the central arena). The survey was conducted by pulling the OhmMapper along several parallel lines, then plotting the data over the entire site to see the underlying structures. Data presented courtesy of English Heritage, Archaeometry Branch.

Differences in electrical resistivity detected by an OhmMapper survey clearly show the boundaries of this Roman Amphitheatre.

Clay lenses are hard to image, but Bucknell University professor Dr. Fred Day-Lewis and students used the OhmMapper TR1 to survey the site represented in the figure below in a single afternoon. The known geology has a resistive rock base, covered by four meters of conductive clay, covered in turn by approximately three meters of moderately resistive, loose sandy soil. The clay lens is the least-resistive body between three and seven meters.

3D Resistivity Surveys after Lunch

Before the OhmMapper TRN, three-dimensional (3D) resistivity surveys were rare. These measurements have been prohibitive because of the enormous amount of time, money, and effort needed to acquire data using electrodes. But today that's all changed. Now, a survey including a series of ten 300-meter profiles, each consisting of data sampled at one-meter intervals and four different n-spaces ― which would have taken several days with an electrode-based system ― can be done in a single afternoon.

A volumetric image of data collected by the OhmMapper is shown in the figure opposite The survey consists of four 300-meter lines separated by 20 meters, with data sampled at approximately one meter intervals. Data were first inverted to true resistivity and depth using RES3DINV Resistivity Inversion Software, then displayed using Slicer/Dicer three-dimensional software

High data collection rates make 3-D resistivity surveys now practical. These data were collected in a single afternoon.

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