Explanation: Cable & Electrode Options

Electrical Resistivity: Cable Options

Land Cables

Multi-electrode cables for land use are divided into several sections in order to make each section lighter to carry and handle in the field—at AGI, we try to limit each cable section to less than 100 meters. Each section is terminated by a male and a female connector, so that the sections can be connected into one continuous electrode cable and also be used for roll-along surveys.

Our multi-electrode cables are sold in lots of 28, 56, 84, or 112 electrodes.

The most common cable set is 56 electrodes at 6.25 meter electrode spacing. The extra 0.25 meter gives some slack in the cable when the customer uses 6 meter electrode interval. This 56-electrode cable will then be 350 meters long (56 x 6.25); therefore we divide this cable in four sections of 87.5 meters and 14 electrodes each.
Other cables are divided in a similar way in order to make the sections more manageable.

Another thing to consider is the cost of connectors. Mating connectors are expensive, and every additional cable divide increases the price. More divides also increases the chance of failure, because there are more opportunities for moisture and dirt to compromise the connectors.

The most typical AGI system is a land cable with passive takeouts made of stainless steel. The electrodes are molded onto the cable using a polyurethane, which forms a waterproof chemical bond to the jacket of the cable. We also build custom cables to your specification, with any electrode spacing and number of electrodes.

The other type of land cable is our active cable, also called a dual-mode cable. The advantage of the active cable is that resistivity and induced polarization (IP) imaging surveys can be performed with the use of non-polarizable electrodes. Here’s how it works: Each electrode location is occupied by a stainless steel electrode and a non-polarizable electrode placed about one foot apart, with one on each side of the survey line. When this electrode location is used as a current injection point, the stainless steel electrode is automatically used; when this location is used for potential measurement, the non-polarizable electrode is used. The selection is completely automatic and works seamlessly. The active cable can also be used with only stainless steel electrodes.

Marine Cables

A marine environment is very different from a land environment because you’re either laying the cable down on the seafloor, or you’re towing a floating cable behind a boat, which we call marine trawling. Either method requires marine cables, which need to have the following:

Kevlar strength member: If you’re pulling a marine cable behind a boat, it may need to have a kevlar strength member so it is strong enough to withstand the pulling.
Thick cable jackets: A thick cable jacket resists any kind of abrasion. For example, if the jacket does break, it’s important to limit the water coming into the cable because it could short-circuit the cable. There are a couple ways to create a water block to prevent this from happening: one is with a goo that goes inside the space, and another is with a tape that swells when it comes in contact with water. We believe the tape is more effective.
Cylindrical graphite or stainless steel cylinders: These electrodes are molded onto the cable at even intervals. Each cylinder is connected by one wire to the connector at the end of the cable. Note that most marine cables have only one connector, which typically leads to a boat, a dock, or the beach where the instrument is placed.
Electrode streamers: The marine cable used for trawling behind a boat is called an electrode streamer. The streamer typically has two or more dedicated current injection electrodes. These are made of graphite in order to prevent corrosion by electrolysis. The other electrodes, which are used exclusively for potential measurement, can be made of stainless steel or graphite. A streamer has at least two anchor points: one to attach the cable to the hull of the boat, and the other to attach a “sea anchor,” which is typically a bucket at the end of the streamer to keep the streamer straight during towing.

Underwater marine cable used for electrical resistivity imaging of the sea bottom should have all graphite electrodes, since the electrodes are going to function both as current injection electrodes and potential measurement electrodes. The underwater cables need at least three, (and probably more) anchor points: one in each end of the electrode spread for anchors to keep the cable straight and in place, and one more to attach to a boat, buoy, or dock.

Laying a system on the seafloor is like laying a land system on the seafloor, just without any connectors or stakes (the seawater makes the contact between the electrode and seafloor). It’s simply laid down on the sea bottom. If you’re on a boat you’ll need a lead-in, the length of cable that allows the cable to go from the boat, down the water column, and all the way to the first electrode on the seafloor. Those systems can be up to 56 electrodes. At AGI, some of our largest underwater systems have had electrodes spaced at 20 meters, which puts the cable around 1.1 kilometers long (with 56 electrodes), with an estimated depth penetration into the sea bottom of 220 meters.

Borehole Cables

A borehole is a vertical, man-made hole created by a drilling machine. Borehole cables introduce the following issues:

Borehole cables placed in soft sediment need a sleeve of “casing,” which keeps it from closing in on itself once you remove the drill. (Here’s how to create an inexpensive and disposable borehole cable.) The casing must be a PVC sleeve, and not made of steel or anything conductive that will short out the signal. Depending on whether the hole is wet or dry, the type of cable will change. If it’s water filled, use a slotted PVC casing so the electricity can go through the slots in the sleeve. If it’s dry, you have to approach it differently. There must be water present to make the electrical connection, but if you cannot keep water in the borehole because it will pour away into fissures, you need to use mud or concrete in the borehole.
Hard (igneous) rock won’t close in on itself, allowing for safe placement of an electrode string. Determine whether the hole is filled with water or dry. If it’s filled with water and is hard rock, you can simply insert an array of graphite electrodes.

Electrical Resistivity: Electrode Options

Stainless steel electrodes are tough and do not rust readily on land.

Graphite electrodes (AGI U.S. patent 6,674,286) are the best option for water. Unlike stainless steel, they don’t oxidize and corrode in an aquatic environment. Graphite is the same material used in pencil lead, so it’s not as strong as stainless steel, which is why it isn’t used as frequently on land.

When you pass electricity through two stainless steel electrodes in water you have a process called electrolysis—that means one electrode is oxidizing i.e. rusting and the other is plating. They end up with a fine, molecules-thick layer of oxidation that makes them more resistive—but keep in mind, we want to keep them conductive.