Geophysics is the study of the Earth, using gravity, magnetic, electrical, and seismic methods. In context of archaeology, this means applying these methods to learn what is underneath the ground without disturbing it as an excavation does.
The Society has several pieces of geophysics equipment, described below.
Our magnetometer is a Geoscan FM256 fluxgate gradiometer. This has two vertically aligned fluxgate magnetic detectors spaced 0.5 metre apart. It operates by measuring minute changes in the Earth’s magnetic field. Fluxgates are directional, so it measures only the vertical component of the field, that is the vector entering the ground and not the component parallel to the ground. This is entirely suitable for work in northern latitudes. The magnetometer can measure changes in the Earth’s magnetic field to 1 part in 500,000, although it is unwise to accept such small changes as evidence of features. It can locate features down to a depth of typically 1.2 metres. Data collection is triggered manually by a thumb – press device attached to the head.
Use of the two fluxgates in differential mode makes it insensitive to movement, but it is very sensitive to the presence of any ferrous metal, whether it be eyelets in the boots of the operator, a car ten metres away or discarded iron objects on or below the ground surface. It also detects interference of the field caused by water flowing in pipes. Signal strengths from buried features were generally in the range ± 10 nT (Earths field is 50,000 nT), but the proximity of iron could cause signals in excess of 200 nT.
The FM256 magnetometer in action.
This shows the principle of operation of the magnetometer. It detects minute changes in the Earth’s magnetic field which occurs when topsoil intrudes into subsoil in areas where the soils have magnetic characteristics.
A magnetometer in use at Harrier.
This is an example of the magnetometer output from Harrier.
BACAS possesses a TR/CIA twin probe resistance meter. This device operates by injecting electrical current into the ground via probes and measuring the voltage across the probes to determine the electrical resistance, R = V/i. The resistance of any soil is determined mainly by its moisture content, and if there is a large stone close to the probes, its reduced water content gives a larger resistance. A line of points of higher resistance may represent a stone wall.
There are a number of configurations of probes for resistance measurement. The TR/CIA device uses the ‘twin probe’ arrangement, whereby current is injected into the ground through a probe on the portable frame on which the meter is mounted, collected on a remote probe and returned to the meter via a long cable. A second probe on the frame and a second remote probe are used to measure voltage. Because the current spreads out as it travels through the ground, it is only perturbed by stones close to the probes.
The device is simple to use and has good controls, so is suitable for all operator skill levels, but it is physically hard work to put in the ground and pull it out after each reading. A reading is automatically triggered as the probes make good ground contact. Because it takes a number of readings and averages them, it is relatively slow instrument and even in skilled hands can only operate at half the pace of the magnetometer. It can detect objects typically 0.5 meters below the ground surface. This is compatible with the magnetometer, as the latter is usually searching for features sunk into an old landscape, whereas the resistance meter is looking for features upstanding in the buried landscape. Note that the resistance meter does not measure absolute resistance, but works from a set value and compares all other values with this.
This shows the principle of operation of twin probe resistance measurement. Current is inserted into the ground from the frame and collected at a remote probe. The voltage between the two points is also measured to calculate resistance. The current spreads out once it is in the ground, so it is only affected by stones close to the probe on the frame. Plotting the points of high resistance may show a line of stones forming a wall.
A resistance meter in use at Harrier.
This is an example of twin probe resistance output.
Electronic Distancing Meter (EDM)
BACAS has a Wild 1600 Distomat EDM.
In using the EDM, the operator lines up the laser head on the target by means of an adjustable telescope. The laser measures time-of-flight to calculate distance to target. The head also includes measurement of angle relative to grid north and angle of elevation. The distance and two angles can be used to calculate eastings, northings and relative height by means of trigonometry, and this is done automatically by the built – in electronics. The absolute height of any nearby benchmark can be used to determine heights above sea level.
Electronic Distance Measurement (EDM) measures distance r to reflector by laser pulse, and measures F. It calculates easting r sin F and northing r cos F. It will also measure angle of declination Y and calculate height difference r sin Y. It must be set up absolutely level, or calculations will be inaccurate.