29 December 2016

Equipment Test: Earth Resistance

Earth Resistance Meters – A Review


The twin-probe earth resistance meter, being relatively cheap, is often the first piece of geophysics equipment purchased by local archaeological societies. While it may not be the first port of call if you have access to a magnetometer or GPR, there are many situations where it is superior. I've found that earth resistance is the most reliable method for finding Roman roads. Recently, I've had access to multiple pieces of equipment, so I have decided to do a review.

The first of the three machines is the Geoscan RM15. Now replaced by the RM85, which unfortunately I don't have access to, the only major differences that I'm aware of is the inclusion of the multiplexer within the box rather than as an add-on, GPS logging and output via USB instead of the old serial port. If there are further changes that would change this review, I apologise to Geoscan now.

The second machine is the TR Systems meter, which was aimed at local societies and proved very popular before production ceased. Though it is not available any more, its use is so widespread that I include it here for comparison purposes, as many will be familiar with it.

The third machine is the Frobisher TAR-3, a relative newcomer, and like the TR Systems meter, affordable by local societies on a budget.

User Interface

The best way to introduce this section is with images of the interfaces of each machine.

Geoscan RM15 Interface

TR Systems Interface
Frobisher TAR-3 Interface

Both the RM15 and TR machines have a similar interface style, with buttons for each function. The TR machine seems to have taken a design lead from the Geoscan machine, no doubt hoping that familiarity will translate into ease of use. The Frobisher machine has a more minimalist style, with 5 buttons (duplicated, for left handers) controlling a menu system, similar to that used by Bartington in their GRAD601. Ease of use is subjective, and somewhat reliant on familiarity, but some comments can be made.

The Geoscan machine is probably the easiest to use. The TR Systems meter works in much the same way, but has an annoying feature where instead of beeping once when a reading is taken, it will beep when it is starting to take the reading and beep a second time when it is finished. If you take the probes out too early, before the second beep, it will complain furiously, saying something about checking the probes, when you know it is because you took the probes out too early, and you have to wait several seconds before it will allow you to continue. I gather that this 'feature' is due to listening to feedback from users who really should not have been listened to. The Frobisher, lacking the dedicated buttons for each function, is probably the least intuitive, and you will probably need the manual at hand the first few times you use it, until you get used to it. Training is available though. There are inconsistencies with the beeps to record a reading, so at the end of line beep, there is a pause and a further beep which may incorrectly suggest that another reading hasn't been taken, and when you are retaking a reading, there is no beep to say it has been taken. The other strange design decision relates to the end of the grid. It will take 20 seconds to write out the readings to its storage, and then turn itself off, cancelling out the speed increase afforded by the ergonomic design. Hopefully some of these issues will be resolved with firmware updates.

Verdict: 1st – Geoscan, 2nd – TR Systems, 3rd - Frobisher


A big part of the 'experience' of doing an earth resistance survey is lugging the machine around the survey area, over and over again, so how your equipment handles is of great importance. A common criticism of equipment like this is the effect it has on someone with a bad back, both because of the weight of the equipment, and because the height of the bar which you hold on to can make you stoop somewhat. With that in mind, here is a table with some statistics on the three machines.

Weight (sans cables)
Bar Height
Geoscan RM15
TR Systems
Frobisher TAR-3

As you can see, the Frobisher is much lighter and has a higher bar than the other two. My volunteer, Stuart, who has a history of back problems, reported that the Frobisher was his favourite. Another beneficial side effect of a ligher machine is the ability to move it quicker, meaning the survey area is covered quicker. Frobisher can supply whatever bar height required on ordering, including a childrens size frame (40cm-130cm).

Verdict: 1st - Frobisher, 2nd – TR Systems, 3rd – Geoscan

Hardware Options

The biggest selling point of the Geoscan RM85 has a built-in multiplexer, which used to be a separate add-on to the RM15, so parallel and deeper readings can be taken at the same time using the adjustable probe frame (an additional option). The RM85 also has an option of GPS recording if you are into using point clouds.

The TR systems meter had an optional tomography kit for doing manual ERT surveys and producing pseudosections using the free version of RES3DINV.

The Frobisher machine, being new, has yet to accumulate the same level of hardware options as the other machines, but one very useful feature is that the fixed probe cable is easily extendable, meaning more grids can be surveyed without moving the fixed probes. The manufacturer has mentioned that the cable could potentially be done away with entirely, with an entirely separate transmitter, which means very large areas could be done without moving the fixed probes, so faster surveys and no edge matching in software required. A wenner bar is available, and a tomography kit is in production.

Verdict: It really depends what you find useful!


While I can't compare battery life for each machine, I can comment on how easy it is to change batteries.

The Geoscan RM15 and RM85 have an internal battery pack of standard batteries (normal or rechargable). The unit needs to be unscrewed to replace the batteries, but it is possible to do this in the field.

The TR Systems meter has two plastic trays that slot into the side of the machine, so batteries (9V, standard or rechargable) can be easily changed in the field.

The Frobisher TAR-3 has an internal rechargable battery pack that is not user accessible. If something goes wrong with the battery, the unit must be returned to the manufacturer. It is charged via a USB connector, so can be charged in the field using a car charger, or anything that could charge a phone.

Verdict: 1st – TR Systems, 2nd – Geoscan, 3rd - Frobisher

Downloading Data

The RM15 and TR systems meter download via an old 9 pin serial connector, so you would need a serial to USB converter or card to download the data. Fortunately, the replacement for the RM15, the RM85, has now been changed to a USB connector that mimics a serial port, no additional hardware needed. The Frobisher TAR-3 stores data on an SD card that can be read with any card reader, so getting the data onto your computer is much faster.

Verdict: 1st – Frobisher, 2nd – Geoscan, 3rd – TR Systems

Data Quality

The test site was a park through which ran a Roman road. The park is surrounded by buildings, which was an opportunity to see how the three machines were affected by AC interference. The same fixed probe location (0.5m apart) was used for each of the three surveys. The area had been previously surveyed using GPR, and the road is visible in the timeslices starting at about 30cm down, along with some land drains or utilities. The surface is known to be made of flint, and the local geology is on the boundary between Folkestone Formation sandstone and Lower Greensand.

The GPR grid shown above is 30x30m, and the earth resistance test grid occupies the top-left 20x20m of that area. The results, shown below were processed in Snuffler with no filters applied. The display bounds were set to 95% of the readings around the median. There isn't much evidence of noise on any of the three images, and they seem broadly consistent with eachother.

Geoscan RM15
TR Systems

Frobisher TAR-3

Verdict: Not much to choose between them, make up your own mind!


When I bought my TR systems meter, many years ago, the price was £1200. Inflation would make that about £1800. At the time of writing, the Frobisher TAR-3 is £1844 (including a days training), not very different from the TR Systems meter, and aimed at the same budget conscious market. I'm not absolutely sure of the price of the currently Geoscan RM85, but I have been told the basic machine £5000, with the multi-probe array another £1500.

Verdict: Joint 1st – TR systems, Frobisher, 3rd – Geoscan


Given that the TR Systems meter is not currently available, that leaves us with the Geoscan and Frobisher machines. If you want the multiplexer option, then get the RM85, otherwise the lower cost and lighter Frobisher machine will save your back and bank balance.

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