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.