GEOMETRICS Geometrics OhmMapper TR1

Resistivity Mapping Twenty Times Faster

Geometrics' new OhmMapper is a high productivity, electrical resistivity mapping system. Because the instrument's transmitter and receiver are capacitively coupled to the ground, there is no need for metal probes or porous pot electrodes. The OhmMapper is pulled along the surface allowing rapid measurement by a single person at a walking pace, or by a towing vehicle.

The OhmMapper employs the well-known dipole-dipole configuration. The system consists of a transmitter, receiver, matching antennae, fiber optic isolator, and data logging console. The dipole lengths and Tx-Rx separation ("a" spacing) can be varied in order to provide apparent resistivity readings from multiple depths. Depth sections can be constructed from apparent resistivity readings using industry standard software.

Physical layout of OhmMapper (16879 bytes)

The capacitive electrode design eliminates traditional problems with high contact resistance. Measurements can even be made on roads, rock outcrops, snow, ice, and frozen ground. And, since reading are taken twice per second while traversing, smaller, more subtle features can be detected with great efficiency. A single operator with an OhmMapper can collect, at minimum, 5 times more data than a crew of 2 with a traditional DC resistivity instrument.

Ohmmapper xsect.jpg (9217 bytes) Here's How It Works: Thin, lightweight transmitting wire, electrified with a 17 KhZ signal, is dragged along   the ground, inducing current flow in the earth. A matched receiver, carefully tuned to the transmitter frequency, measures the associated voltage. A computer in the receiving unit converts the voltage to apparent resisitivity by applying a geometric correction factor suitable for the array being used.

Data logging and storage is done using Geometrics DataMapper technology. The OhmMapper uses the same logging console as the Geometrics G-858 cesium magnetometer. A G-858 magnetometer can be easily upgraded to become a dual purpose MagMapper OhmMapper at reasonable cost. All the mapping and positioning capabilities of the G-858 and the MagMap software, such as editing line and segment position, correcting orientation and direction are also available for the OhmMapper TR1.

Pseudosections made with both the OhmMapper and with traditional DC resistivity measurement are very similar. The OhmMapper generally shows more detail however, since measurements are taken every few centimeters, whereas the conventional resistivity method generally limited to a few meters between probes. Where standard galvanic resistivity data may require a 2 hour period for collection, the same area can be measured in only 10 minutes using the OhmMapper.

OhmMapper apparent resistivity (unfiltered).
The upper data set is from the OhmMapper and took 10 minutes to obtain. The bottom data set was obtained using traditional DC measurements and took 2 hours to obtain.
The galvanic measurements were obtained courtesy of Jim Rezowalli of JR Associates

Click for full pic of OhmMapper data (27126 bytes)

For more info contact Red Dog Scientific Services , authorized representative for South / Southern Africa