What Makes A Good ERI System?

One question we hear frequently is “what makes a good ERI system?” The obvious answer to that question is “a system that is good at collecting accurate data quickly.”

But what makes a system good at collecting data? How do you determine those things? By paying attention to the transmitter and receiver of the ERI systems you’re considering. Here’s what to look for in both.

1. The Transmitter

A good ERI system will have a transmitter that strikes a balance between power, portability, and cost.

Power. For your system, you’ll want a DC power supply that injects steady current into the ground between two electrodes. You want this power supply to be strong.
Weight. You have to consider the size and weight of your transmitter along with the power output. A transmitter that takes several people, or a machine, to haul won’t be an efficient or realistic choice—instead, find a system that is portable enough to take with you into the field that will give you enough power for the jobs you’ll be performing.
Cost. Finally, don’t fall into the trap of purchasing the least expensive option without considering what you’re paying for. Often, with transmitters, you’ll be wasting your money if you go with the cheapest thing you can find—it won’t be powerful enough or reliable enough to get you the data you need, and you’ll end up spending more because you’ll have to replace it with something that does work.

2. The Receiver

Even more important than the transmitter you choose is the receiver. A great ERI system will have the following characteristics.

Is the receiver insensitive to cultural noise?

In other words, can a receiver distinguish between the signal transmitted and background noise? A reliable, high-quality receiver will be able to distinguish signals from noise well below 0.1 millivolt in real world situations.

What is considered noise?

Background noise can come from the instrument itself, nearby power lines, natural electrical earth currents such as telluric currents (from lightning storms) and magneto-telluric currents (from solar flare events interacting with the Earth’s ionosphere), or noise coupling to the ground with electronics. The tools used must be able to differentiate between the surrounding noise and what you’re actually measuring.

For example, with a typical 200-watt transmitter, you can expect signals anywhere from several volts to a millivolt or less depending on how far away you’re measuring from the transmitter. The electric field strength decreases according to the Inverse Square law with the distance (1/d2). That means that the measured signal decreases with the inverse square of the distance separating the receiver from the transmitter.

How does one improve on the signal to noise ratio?

There are two ways to do this.

Increase the intensity of the transmitter.
Increase the sensitivity of the receiver.

Given that the transmitter and receiver have a proportional relationship, increasing your signal from one millivolt to two millivolts does not give you a significant increase when you consider the background noise strength (which is how most companies try to improve their sensitivity).

However, by increasing it from 1 mV to 2 mV, you don’t get a large gain—doubling doesn’t get you better data (2 mV is still very small), but there’s a huge cost jump ($20,000-$30,000 more), plus there’s a need for large generators, which are difficult and dangerous to work with.

What should you look for instead? Focus on signal processing. With advanced signal processing, a tiny signal hidden in all incoming natural and manmade noise can be retrieved. This strategy saves money by reducing the need for heavy, expensive transmitters, which also may need an equally heavy and expensive motor generator for power.

3. The Multi-electrode Cable

The multi-electrode cable connects all the electrodes to the instrument and is a very important part of an ERI system. Experience shows that most erroneous data is caused by trouble with the cable.

The take-outs, where the cable connects to the electrodes, need to be waterproof and molded to the cable. Wire-wound take-outs used in the seismic industry are not enough for resistivity applications, because they are virtually impossible to waterproof. In resistivity, work currents of several hundred volts are being transmitted through the cable, while simultaneously, voltages of only a few millivolts are being measured accurately. The slightest amount of water in the cable will ruin the data.

4. The Electrode Stakes

The stainless steel stakes that make contact with the ground are not flashy, but they are an important part of a good ERI system. It is important to have a stake that can easily and quickly make a safe connection to the cable take-outs.

A stake with a stainless steel spring attached and folded over the cable take-out is a quick, secure way to attach the stakes to the cable. Avoid using jumper cables to connect the take-outs to the stake; they are notorious for breaking and causing errors. They also have an unbelievable ability to tangle.

5. The Processing & Presentation Software

The processing software of a good ERI system should be easy to use and seamlessly import field data. Some of the important features include:

Display of contact resistance data.
Statistical noise removal.
Meter/feet/electrode number display.
32-bit graphics and report ready print-outs.
Data display in 2D or 3D.
Volumetric calculation in the 3D processing package.
Rotate, zoom and look-into the resistivity model.
Draggable XYZ slices to display any cross section of the result mode.

Proof Of A Good ERI System

One of the best ways to prove that an ERI system is of high quality is to test it in marine surveys using a normal dipole-dipole array configuration.The conductive seawater will short circuit the signal from the transmitter reducing it to the microvolt range. A high-quality ERI system should be able to measure these signals.

If you can’t do marine surveys to n=8 (eight dipoles away), you don’t have a good system. At AGI, we routinely see our clients use their tools to measure 0.1-0.01 millivolts—that’s 10 times more sensitivity—and better than any competition.