New! Resistivity Mapping System
No Metal Electrodes Required
Fast Resistivity Measurements Without Probes
Single Person Operation
Low Power Consumption
Fast Data Acquisition
Optimized for Use in Highly Resistive Areas
(permafrost, deserts, sand, snow, resistive
geology, even roads and pavement)
The new OhmMapper is a capacitively-coupled resistivity meter that
measures the electrical properties of rock and soil without cumbersome
galvanic electrodes used in traditional resistivity surveys. A simple
coaxial-cable array with transmitter and receiver sections is pulled
along the ground either by a single person or attached to a small
all-terrain vehicle. Thus, data collection is many times faster than
systems using conventional DC resistivity.
Create Continuous Depth Sections Easily
Multiple passes with the OhmMapper, at different transmitter-receiver
spacings, permit electrical sounding done at a fraction of the time of
expanding-spread resistivity or electromagnetic methods. Data collection
is continuous so the near surface is finely sampled, providing high
quality data even in areas with complex geology.
Superior Mapping Technology
The OhmMapper's DataMapper console provides graphical display of both
position and data. View the last five profiles or scroll a window
through the entire data set - right in the field.
OhmMapper TR data collected over granite with surface
weathering. Inversion done with RES2DINV program from MH lake.
Quality Results in Difficult Areas
The OhmMapper operates on ice, frozen ground, rock outcrops, or paved
roads where standard DC resistivity cannot be used. There are no stakes
to put in the ground.
Fast Survey Operations
Operators can collect resistivity data as fast as they can walk, or
quickly collect data using a snow mobile, ATV, or other tow vehicle
using the OhmMapper’s towable resistivity array.
Accurate, High-Resolution Target Detection:
The OhmMapper provides high-resolution resistivity information for a
wide variety of applications because of its high data acquisition rate.
A compact unit and simple operation allows surveying by a single
Operators can see what has been surveyed, any gaps in the survey,and
where they are on the survey grid using the OhmMapper’s real-time grid
The OhmMapper allows real-time acquisition of position information
through a GPS interface.
Detect geologic faults
Detect fracture zones Image clay layers and aquitards
Find buried stream channels
Locate likely aquifer structures
Trace salt-water intrusions
Measure depth to bedrock
Detect mineralized zones
Find sand and gravel beds
Define limits of clay and marble deposits.
Define structural geology of potential mines
Measure integrity of levees
Detect permafrost and ice lenses
Detect seepage from dams
Measure resistivity for power-line grounding
Detect voids under roads and building sites
Detect leakage plumes from landfills
Map environmental contaminant plumes
Two separate pseudosections made from OhmMapper data with
First section done in N-S direction and second section done in
Note close matching in overlapping areas.
General Description and Theory of Operation
General Description and Theory of Operation
The OhmMapper TR1 is a capacitively-coupled resistivity system
designed to measure ubsurface resistivity in areas where exploration
using a traditional galvanically coupled (DC) resistivity system is
impractical, slow, and expensive. The OhmMapper consists of an
ungrounded dipole transmitter, receiver, and a data logger. An AC
current is coupled into the earth by the transmitter and measured at the
receiver. This measured voltage is proportional to the resistivity of
the earth between the dipoles. Apparent resistivity is calculated using
the appropriate geometric factor for the capacitively-coupled antenna
The OhmMapper is designed to be pulled along the ground as a streamer,
providing a nearly continuous apparent resistivity profile. This design
increases the resolving power and productivity of the system relative to
traditional DC resistivity systems. Data is logged using the DataMapper
Console. The OhmMapper receiver is connected to one of the console’s
serial ports for data acquisition via a fiber-optic interface. Data are
graphically displayed in real time on the console screen. At a sampling
rate of two times per second the OhmMapper TR1 has a total storage
capacity of approximately 24 hours of data acquisition.
DataMap software is used to download, edit, and process OhmMapper data
and export it in standard format for inversion using commercial DC
resistivity interpretation tools.
Survey Types: OhmMapper surveys are based on the in-line
(axial), dipole-dipole arrangement for resistivity measurements.
There are three ways to view OhmMapper data depending on the method of
data collection. The simplest method is to keep a single, constant
distance between the transmitter and receiver (N-spacing) as the
operator walks a multi-line survey grid. This provides a
"constant-depth" resistivity contour map of the survey site.
A slightly more involved technique can be used to give a 2-D
depth section, which provides a view of how resistivity varies with
depth along a profile. This involves resurveying a single profile line
with several different transmitter-receiver spacings.
By combining the two methods above, and collecting data from
multiple profile lines with multiple transmitter-receiver spacings, a
3-D data set can be collected for processing in an appropriate 3-D
imaging software package.
Depth of Investigation: The depth to which OhmMapper data can be
reliably interpreted depends on the dipole length and the distance from
the transmitter dipole to the receiver dipole. The practical distance at
which the receiver can detect the transmitter depends on the resistivity
of the earth. Typical depths of investigation are 10 to 20 meters. Skin
depth effects on EM measurements often determine the practical limit of
the depth of investigation in highly-conductive areas. The approximation
of skin depth, in meters, is 500 times sq. rt. (rho/f) where rho =
resistivity and f = frequency. For example, in 10 Ohm-meter earth the
skin depth is 12 meters.
Here’s How it Works
A transmitter electrifies two coaxial cables (transmitter dipole)
with an AC current. Current is thus coupled to the earth through the
capacitance of the cable. A matched receiver, automatically tuned to the
transmitter frequency, measures the associated voltage picked up on the
receiver’s dipole cables. The receiver then transmits a voltage
measurement, normalized to current, to the logging console. After
downloading to a computer the DataMapper software calculates apparent
resistivity by applying a geometric correction factor suitable for the
array being used.
Constant-current, capacitively coupled, dipole-dipole resistivity.
From < 3 to > 100,000 Ohm-meters.
Selectable data logging rate up to 2 times per second.
2 Mbytes of non-volatile RAM.
Metronome, signal amplitude, error alarm.
320 by 200 graphic LCD display, daylight visible with selectable outputs
Data Display: 5 line profiles of resistivity.
All system setup functions.
All survey functions: survey profile number and direction,
station or GPS number, test line number.
Survey monitor functions.
Console: 1.6 kg.
Transmitter with batteries: 3.2 kg.
Receiver with batteries: 3.2 kg.
Battery Pack/Harness: 1.6 kg.
Depressor Weight: 3.2 kg.
Dipole Cables: 85 g/meter.
approx. 16.5 kHz.
Output Power: Up to 2 Watts.
Output Current Maximum: 16 mA.
Output Current Minimum: 0.125 mA.
Cable Lengths: 5m standard (x4),others optional.
Input Impedance: >5 M Ohm.
Measured Voltage Accuracy: Better than 3%.
Input Voltage Range: 0-2 V RMS.
Power Line Rejections: > 100 dB.
15 x 8 x 28 cm.
Battery: 8 x 13 x 20 cm.
Transmitter/Receiver - 2 x 6
VDC (12 VDC).
2. Console - 28 V DC.
3. Internal battery backup for
clock and non volatile RAM.
-25 C to +50 C.
Note: At less than -10 C the LCD screen must be kept warm.
Resolution of 0.1 drift < 1 second/day
Options and Spares:
(specify lm, 2.5m, 5m, or 10m).
Connector termination kit.
Replacement spares kit.
Dipole-Dipole Inversion Software.