The extraction of underground oil or gas usually also generates large amounts of water, called 'produced water'. If not properly managed, it can cause local soil and water pollution.
In this case study, a GEM-2 handheld conductivity meter was used to quickly identify problem areas. In this case, the conductivity variations were large enough that the operator could explore the areas of greatest interest (as constrained by the local terrain) rather than following a fixed grid. This type of opportunistic approach can be very efficient. The whole survey area covered several square miles, but the immediately mapped area shown here required less than 2 man-days to survey.
This case study confirms that maps of apparent electrical conductivity (EC) are very useful in locating potential soil contamination. In the case of brine contamination, the EC values correlate well with laboratory analyses of both soil conductivity and concentration of chlorides. More generally, any type of contaminant whose EC contrasts with the environment can be delineated.
e GEM-2 'ski' is a hand-held, digital, multi-frequency broadband electromagnetic sensor. It operates in a frequency range of 30 Hz to 93 kHz, and can transmit an arbitrary waveform containing multiple frequencies. The unit is capable of transmitting and receiving any digitally-synthesized waveform by means of the pulse-width modulation technique.
A frequently-asked question is the "Depth of Investigation." This is a very complex question because the answer depends on many factors, particularly on ground conductivity and ambient electromagnetic noise. Based on many analyses and field data, we estimate the GEM-2 should be able to see about 20-30m in resistive areas (>1000ohm-m) and about 10-20m in conductive areas (<100ohm-m). This figure assumes an ambient noise level of 5ppm.
The noise level is generally high in urban areas and low in rural areas. For typical applications, we do not recommend the GEM-2 for depths deeper than 30m.
The GEM-2 ski contains three coils: transmitter, bucking, and receiver coils. For frequency-domain operation, the GEM-2 prompts for a set of desired transmitter frequencies. Built-in software converts this into a digital "bit-stream," which is used to construct the desired transmitter waveform (Figs 1 and 2). This bit-stream represents the instruction on how to generate a complex waveform that contains all frequencies specified by the operator.
Figure 1. A three-frequency transmitter waveform.
Figure 2. First 33 points of Figure 1.
Figure 3. The base period of the bit-stream for the GEM-2 is set to 1/30th of a second for areas having a 60-Hz power. The TX switches at 192 kHz and, therefore, the bit-stream contains 6,400 steps within the period. Through a Fourier transform of the transmitter current waveform above, we obtain a power spectrum of the primary field, which shows each transmitted frequency.
Figure 4. With multiple frequencies, one can determine layered conductivity structure of the earth, conceptually shown below. This is called "frequency sounding" method.
What is EM Locator Technology?
Electromagnetic (EM) Locator technology relies upon the detection of an alternating magnetic field. This magnetic field is created by the flow of current through a conductor, whether a wire, cable, pipe, or other conductive material. Often, additional ‘tracer wires’ are run in parallel to non-conductive pipes such as gas lines expressively for the purpose of facilitation such detection. AC power utilities, as long as current is flowing in the form of power usage, usually radiate a very detectable magnetic field as a side-effect of normal operation. It is also possible to intentionally induce a ‘tracer’ current (by inductive clamp or direct attachment) onto otherwise passive conductors in order to create a detectable field that can be followed.
LineTrac technology is a multi-sensor fashion technology that combines the power and advantage of GPR with the specific target-identifying capabilities of EM location. Linetrac operates in both “Power” (50 or 60 HZ Utility Power) and in arbitrary “Frequency Mode” up to 50KHz, using a customer-supplied transmitter.
We provide two models of EM locator technology. The LineTrac system for our UtilityScan series and the LineTrac XT for our StructureScan Mini XT system.