Earth
Resistance Meters – A Review
Introduction
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
Ergonomics
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.
|
Machine
|
Weight (sans cables)
|
Bar Height
|
|
Geoscan RM15
|
5.1Kg
|
93cm
|
|
TR Systems
|
4.4Kg
|
93cm
|
|
Frobisher TAR-3
|
2.8Kg
|
105cm
|
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!
Battery
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!
Price
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
Conclusion
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.
