06 January 2013

What to look for in a magnetometer

I have been looking at buying a GPR recently, and trying to get decent comparative information between makes and models is difficult to say the least. It occurred to me that other people must have the same problem when buying a magnetometer, so I thought I would write a quick guide detailing what knowledge I have gained over the years. I wont be discussing anything too expensive, like alkali-vapour magnetometers, as they are outside most peoples budgets. I will stick to fluxgate magnetometers. There are three makes I will discuss.

Geoscan

Geoscan make the FM256, a fluxgate gradiometer with 0.5 metre sensor spacing. I have personally used its predecessor, the FM36. I am not entirely sure of the differences between the two models, though I gather than the main points I will touch on have not changed.

Bartington

Bartington make the GRAD601, a fluxgate gradiometer with 1 metre sensor spacing. I personally have a GRAD601-2, the version with two sensor columns.

Foerster

Foerster make the Ferex. I personally have no experience of these devices, and what I have to say on the matter is merely what I have learned from other people and the internet.

With all that in mind, I will discuss the differences between the devices in various categories.


Sensitivity

One of the most important things to consider is the sensitivity of the instrument, and how good it is at picking up the slight changes we are looking for in archaeology. In my personal experience, the Bartington is more sensitive than the Geoscan, perhaps helped by the longer sensor columns. Apparently, the Foerster is not very sensitive. The Bartington wins here, not sure who comes second, but I would guess the Geoscan.

Setup & Stability

Fluxgate instruments, being directional in nature, need to be balanced before use. The Geoscan instrument has a manual process, where physical knobs are turned to align the sensors. The process is somewhat time consuming, and is not helped by the device suffering a lot from thermal drift, so you may find yourself realigning the sensors after each grid. The Bartington is much better here. It has an electronic balancing process, which calculates the differences in sensor alignments and compensates electronically. It is also helped by being very temperature stable. I only tend to balance it a couple of times in a day. The Foerster is apparently set up in such a way that it doesn't need balancing. I'm not quite sure how this works, but it seems to do so, thus the Foerster wins here, with Bartington second.

Array Options

The Geoscan instrument has an option to carry two separate devices on a carrying frame, with a single button to start recording. Each device has to be balanced and downloaded separately. The Bartington comes in the single column GRAD601-1 variety or the dual column GRAD601-2 variety. You do not need  separate balancing and downloading for each column with the GRAD601-2. Because of the lack of setup needed with the Foerster instrument, it is easy to have a large array of devices, perhaps towed behind a vehicle even. Foerster wins this one with the Bartington second.

GNSS Integration

The geoscan instrument has no GNSS integration. The Bartington has an option of a separate data recorder that uses GNSS, but that will not do normal gridded recording. The Foerster has full GNSS integration, which helps with its cart and vehicle towed setups. The Foerster wins this one with the Bartington second.

Reliability

The Geoscan has a very good reputation for reliability, these things never seem to go wrong. The Bartington has a poor reputation for reliability. Personally I've had to have my machine repaired twice. Once to replace the motherboard in the data recorder, and once to have a sensor column rebuilt after water got in, causing thermal drift. They can be damaged by rain, especially after seals have perished. Other people I have talked to have had a similar experience. I don't have any information on the reliability of the Foerster instruments, but I would guess somewhere between the other two, so Geoscan wins this one with Foerster second.

Cost

I am somewhat lacking in information here. All I can say that is concrete is that my GRAD601-2 cost me £10,500 a few years back. I gather that the GRAD601-1 and FM256 are roughly the same price, but for two sensor columns, the GRAD601-2 is much better value than buying two FM256's. I know nothing about the Foerster prices. I can't call who wins this one, get some quotes.

Conclusion

What I would recommend going for depends on how you will be using it. If you are surveying using a gridless GNSS technique, then the Foerster is probably your best bet. If you are doing a gridded survey, I would recommend the Bartington. If anyone out there has further information to contribute to this guide, especially regarding the Foerster instruments, please leave a comment below.

4 comments:

  1. the post was really good and as well as informative for a beginner like me, am just starting my journey as a geologist and your post helps me to set a magnetometer and GPR for my system.

    ReplyDelete
  2. When I was involved in archaeological geophysical survey, I used both the FM256 in dual mode and the Bartington 601-2. The main difference between the FM256 and the FM36 is in how much data it can hold and the configuration options, as far as I can recall. I think it is more stable too, so doesn't need zeroing as often - altough good practice to zero regularly anyway.

    In terms of data collection on 20m or 30m grids, the bartington system can be quicker, and is easier to set up and use. It is easier to carry on its harness system, which means that you can survey a grid faster. This leads to the problem of sensor wobble though, and although the 1m sensor may be more sensitive than the 0.5m Geoscan sensor, I would bet that on a lot of surveys this is counteracted by a reduction in data quality caused by sensor wobble due to surveyors walking too quickly, on uneven ground, and the data striping effect discussed below.

    Where the FM256 dual system wins is, I think, in data quality. Because you balance the two sensors separately, you can do so over the exact same point, so both sensors are zeroed in the same way and to the same value. the frame they come on also holds them in a more stable fashion. The Bartington system zeroes both sensors at the same time and does so within a circular zone, with the diameter equal to the width of the system. This can often result in data striping, removable in post processing, but obviously not ideal. It can take a long time to find a spot large enough and with sufficiently similar nT values to get a good zero location, and you can forget finding one at 0.1nT sensitivity on very noisy sites.

    It is, as ever with geophysical instrumentation, a trade off between speed, ease of use and data quality. The issues with the Bartington can be avoided with careful choice of zero location and ensuring survey speed and traverse location is kept to accurate limits. Unfortunately the competition to provide surveys at the lowest prices probably means that these precautions are often not taken and it becomes just a race to see how much ground can be covered in a day.

    ReplyDelete
    Replies
    1. Thanks for the in depth comment Sam. The 40m grids available in the newer model of Bartington instruments makes it even faster. I see what you mean about the balancing issue with the dual barty, but there hasn't yet been striping that I haven't been able to defeat with Snuffler. The thermal drift of the geoscan instruments will stop the destripe by sensors/direction working, so you are limited to destriping each line. I walk at a fairly sedate 1.3m/s, so I don't get much sensor wobble or step errors. Cheers

      Delete
  3. The magnetic gradiometer which the magnetic field can be recorded at the same time, it will do better job for mapping near surface magnetic bodies than total field, the magnetic gradiometers are provides more geometric information than the total magnetic field, it is more sensitive to smaller bodies and objects to one side of the flight line, making it possible to define the body better when only two flight lines cross the object. and finally it can used to improve gridding.

    ReplyDelete