Geometrics Ohmmapper

Overview

The Ohmmapper is a capacitively-coupled resistivity meter that uses the capacitance of an antenna, a coaxial cable, to couple an AC signal, approximately 16.5kHz, into the ground. The conductors in the cable act as one plate of a capacitor and the earth acts as the other plate with the insulating sheath as the capacitor’s insulator.

The amount of current put into the ground is determined by the length of the cable: a longer cable has a greater capacitance and therefore will put more current into the ground. Since an AC signal can pass through a capacitor, the transmitter current in the transmitter cable will pass into the ground. The capacitance of the receiver cable is similarly charged, allowing the receiver electronics to measure the AC voltage at the transmitter frequency. This provides an AC equivalent of a traditional DC resistivity measurement.

A resistivity meter possesses two current “electrodes” or in this case transmitter cables that force a current into the soil. This current doesn’t follow a direct path; it spreads out, curves around, and comes back together at the other “current electrode.” Therefore, some of the current will move left, some right, and some straight into the ground. Because the soil has resistance and there is current flowing through the soil, there is measurable resistance if we know the voltage.

Two electrodes called the potential “electrodes” or in this case the receiver and its associated cables measure the voltage in the soil. Knowing the position of the “current electrodes” and the “potential electrodes” allows the ohmmapper to calculate a resistivity value. This value is an average over the measured space and called the “apparent resistivity.” Collecting apparent resistivity values as the ohmmapper moves over the surface allows the user to map areas of higher and lower resistivities to identify different soil types and subsurface anomalies.

Configuration and Considerations

To gather good data, careful considerations must be made. It is important to set up and accurately mark the survey before collecting data. Always finish the survey setup and recheck the survey markings before starting. Usually, the operator makes either a square or rectangle parallel to the North-South axis and marks lines in a manner which are easily understood. Simple but accurate survey marking should prevent confusion and error.

In addition to easy-to-follow markings, the operator should pull the ohmmapper at a consistent speed to prevent tracking errors. A mid checkpoint may help prevent a large variation in speed that could cause such an error. It is better to take your time performing the survey than rushing through it only to have to recollect a part of a survey.

Given the length and logistics of using the ohmmapper, a minimum of two people is required to run the survey but more is ideal. The operator may need help pulling, guiding, and turning the instrument, especially if the dipole spacings are large. Further, he or she may need help maintaining accurate field notes.

Typical Ohmmapper Packing List

1 x OhmMapper/MagMapper Console
1 x Console Interface Cable with Tow Bar
1 x Transmitter
1 x Fiberglass Rod With Optical Coupler
2 x 2.5 meter cables
4 x 5 meter cables
3 x 10 meter cables
5 x Cable Termination Plugs With Metal Spring Clips
1 x Battery Charger for Transmitter and Receiver Batteries With AC Cord
1 x Battery Charger for 24V Console Batteries With AC Cord
1 x Weight
2 x 24V Belt Battery Packs for Console
8 x 6V Batteries for Transmitter and Receiver
3 x Large Rubber Skid Plates
3 x Small Rubber Skid Plates
1 x Software USB
1 x Allen wrench
2 x Gray Reinforced Plastic Carrying Cases
1 x Red Shipping Tote 28 x 21 x 16, 60lbs
1 x Cardboard Shipping Boxes 34 x 17 x 15, 80lbs

Ohmmapper Specifications

Operating Principle: Constant-current capacitively-coupled, dipole-dipole resistivity
Operating Range: 1 – 100,000 Ω m
Cycle Rate: 2 Hz
Data Storage: 2 Mbytes of nonvolatile RAM
Audio Output: Metronome, signal amplitude, disconnect
Visual Output: 320 x 200 LCD
Data Display: 5 line-profiles of resistivity
Output: ASCII columnar
Clock Resolution: 0.1 sec; drift <1 sec/day
Transmitter Frequency: <18 kHz
Transmitter Output Current: Variable from 16 mA to 0.25 mA
Dipole lengths: 5, 10, 15, 20 m; longer lengths optional