Archaeological Geophysics Page

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[ Introduction \| Hardware Types Available \| Software Available \| How To Do A Geophysics Survey \| Resources ]

There are three main types of hardware used in Archaeological Geophysics. Whilst there are others, they tend to be little used in Archaeological Geophysics compared to Geophysics in general, so you will probably not come across them.

Resistivity Meters

The first type of equipment available is the Resistivity Meter. For most amateur groups, it is the most commonly used piece of equipment as it is both the cheapest and easiest to use. The hardware usually consists of a box of electronics mounted on a carrying frame, with wires leading down to a pair of metal spikes that are inserted into the ground. An electric current is passed through the ground and the box measures the resistance to the electrical current passing through the ground, hence the name resistivity meter. The amount of resistance is affected by how much moisture there is in the soil. If there is a lot of moisture in the soil, you will get low resistance as the electric current passes through the wet ground easier. Conversely, if there is little moisture, then there is high resistance to the electric current. The problem with this setup is that the resistivity is also dependant on the amount of contact the metal probes have with the ground, so most meters have a cable running to additional set of fixed probes that help it work out how much contact resistance there is.

The amount of water present in the soil is affected by some archaeological features. For example, if you have a wall under the surface, then there is less soil to store moisture, which will evaporate quicker, making that patch of ground high resistance. Conversely, if you have a ditch or pit, it can store more moisture for longer, giving a lower resistance. Resistance readings taken in a grid can then be turned into an image and interpreted to look for relevant archaeological features. The problem with resistivity is that results can be rather poor if the ground is too wet or too dry, i.e. if there is little relative difference between the resistance for different depths of subsoil.

There are currently two commercial options if you want to use Resistivity, the RM15 and the much cheaper M.M Instruments Meter and Frobisher TAR-3. The TR/CIA meter is unfortunately no longer in production. Some people have built meters themselves using the guidelines given in the EPE magazine in two parts (April & May 2003). If you have a really small budget and a knack with electronics, then this may be worth looking at, but it is not for the faint hearted.

Magnetometers

The second most commonly used piece of equipment is the magnetometer. It is rather more expensive and tricky to use, but is favourite with a lot of people because you can do a survey with a magnetometer a lot quicker than with a resistivity meter.

Magnetometers measure the local magnetic field strength. As well as the earths magnetic field, some archaeological features have a measurable magnetic field. Burning will cause substances to become magnetised, metals such as iron have a strong magnetic field, and even the fill of a ditch will show up because there are magnetic particles in soil, so if you have a deeper depth of soil because of a ditch, you will get more of a magnetic field to measure.

There are three main types of magnetometer, all of which do the same thing, but in a different way. The first, the Proton magnetometer is actually rather slow compared even to resistivity, so it is little used. It works by inducing a magnetic field which will cause the protons in a hydrocarbon fluid to align, then by measuring how fast the protons snap back once the induced magnetic field is turned off, a measurement of the local field strength can be taken. Alkaline-vapour magnetometers work in much the same principal, but are much faster, allowing almost continuous readings to be taken. Unfortunately, they are also very expensive putting them out of the reach of most peoples budget. The third type, which is most commonly used, is the fluxgate magnetometer. These consist of a metal core around which is wound a coil of copper wire. The fluxgate magnetometer can also take continuous readings, but is a lot cheaper to produce that the vapour magnetometers. The downside is, unlike the other two types of magnetometer, the fluxgate does not not measure the total field strength, only the portion of it relating to the alignment of the sensor itself. That means that turning the sensor will cause the reading to change. They are also sensitive to changes in temperature, so readings will change of the course of a day.

The way around this is to use a Gradiometer setup. If you have two sensors placed one above the other, then you can take one reading from the other to get what is known as the 'magnetic gradient'. So if you turn the instrument, then both sensors will register the change, effectively cancelling the change out. As one sensor is closer to the ground than the other, it will be affected to a greater extent than the other sensor. Therefore the difference in reading between the two sensors can be taken as a reading for the magnetic influence of the ground beneath the meter, hopefully excluding everything else. The two sensors need to be properly aligned before a survey takes place, and the sensors can be unevenly affected by changes in temperature, so 'drift' measurements need to be taken at a fixed point in the landscape between grids.

Because there is no need for contact between the ground and the instrument, a survey can be done by just walking along the ground. This is quicker than resistivity and also allows more readings to be taken with no increase in survey time. Rather than pressing a button to log a reading, modern meters take readings at pre-defined time intervals, so it up to the user to walk at a fixed rate to a set of beeps made by the meter. A bit of practice is needed as this is not so easy for beginners to do as a resistivity survey. A magnetometer is not affected by groundwater like a resistivity meter, so surveys can be taken throughout the year, but certain magnetic bedrocks will render the machine useless. They are better at picking up ditches than a resistivity meter, but not as good at picking up walls, unless there is a substantial foundation trench or the wall is comprised of a burnt material such as bricks.

The main commercial options in archaeology used are both Fluxgate Gradiometers. The FM256 by Geoscan Research and its predecessors, the FM36 and the FM18, were until recently, used by most people. There is now a cheaper, but still expensive option on the market in the form of the Grad601 by Bartington. The alignment of the sensors is easier than the Geoscan meters, and there is less drift. The sensors also have a greater vertical spacing, giving a greater contrast in readings. Both meters come in one or two gradiometer options, so you can take two sets of readings at once, thereby increasing the speed of your surveys.

Ground Penetrating Radar

The third, and by far the most expensive method used for archaeological geophysics, is Ground Penetrating Radar. Working on the same principle as normal radar for picking up planes, the instrument consists of an antenna which sends out electromagnetic pulses into the ground which reflect off objects and are picked up again by a receiving antenna. The resulting wave will show the strength of reflections over time, with the length of time taken to receive the signal back being an indicator of depth. A greater part of the signal is reflected back when there is an increase in density within the ground, say for instance when the wave travels from topsoil into bedrock, or a wall. This makes the system good for showing changes in stratigraphy.

The instrument is dragged along the ground at a set speed, with the radar taking continuous measurements. This enables a vertical two dimensional representation of what is underneath the ground to be built up from a set of readings. Several of these lines can then be put together to form a three dimensional representation. The process is not as fast as magnetometry, but gives a 3-D image, unlike the 2-D of resistivity or magnetometry.

When performing a survey using GPR, there is a decision to be made about the antenna used. The frequency used by the antenna gives you a tradeoff. A high frequency antenna will resolve smaller objects and layers, but the signal will not be able to travel very far into the ground. Conversely, a lower frequency will travel further down but will not be able to show much detail. In archaeology, most of what is of interest is usually within a couple of meters of the surface, so a high frequency antenna (400-600 MHz) is the best choice. Like the resistivity meter, Ground Penetrating Radar is affected by groundwater, with an increase in moisture attenuating the signal. GPR performs best in very dry conditions, waterlogged ground will not give good results. There are many cheaper GPR units designed for finding utilities. These are not usually suitable for archaeology, both due to the limitations in the setup, since it is designed to look for one thing, and because they are not designed for the rugged terrain you find with archaeological surveys.