GEOPHYSICS and MELLOR ARCHAEOLOGY

Anyone familiar with "Time Team" will know what "geophys" looks like in action. Actually, in practice, good geophysics moves much more slowly and methodically, but the reality would not make such dramatic television! We use the same techniques as the Time Team, an important characteristic being that these investigations do not disturb the ground and in consequence are generally acceptable to farmers and landowners. In all these methodologies, except metal detection, the readings are logged automatically, and on transfer to a computer the results can be displayed on screen or printed out.

Electrical conductivity measurement.

This is one of the oldest-established geophysical methods used in archaeology. At its simplest, the investigator pushes 2 metal probes, usually attached to a simple "walking frame", into the soil and an electrical circuit in a control box measures the conductivity between them (see Figure 1). The control box has the capacity to record a large number of readings, and the operator ensures that these are taken at pre-determined distance intervals. Soil conductivity is dependent on mineral composition, and is also greatly affected by water content. Thus, wet soil conducts well, moist clay less well, and sand or solid rock less well again. Conductivity is also affected by buried man-made features: for example, a demolished wall or a filled-in ditch, both invisible at the surface, may show up as local variations in electrical conductivity. These will appear as darker or lighter regions or lines when the data are displayed as intensity maps. Surface conductivity measurements are sensitive to variations in soil materials to depths of around 1m or more, and many archaeological features lie within this range. Figure 2 shows a conductivity plot of an area surveyed at Shaw Cairn. Regions of low conductivity show up in red/yellow, and in this case result from buried rocks within the site of the original cairn. Areas of higher conductivity, showing up in green or blue, result from less disturbed and/or damper soil, which at this site is largely peat. (back to Contents)

Magnetometry.

The instrument contains two sensors at the ends of the 1m white "pole" shown in Figure 3, and measures the intensity of the earth's magnetic field gradient over any given point. The operator carries the magnetometer at a constant height above the ground surface, traversing a regular pattern, and slight variations in the magnet field are measured and recorded. These variations are caused by changes in the nature of the materials in the make-up of the soil, such as its natural mineral content, or from artefacts, such as buried ditches or brick structures, up to ~2m from the surface. Small pieces of metallic iron, such as nails or wire, near the surface, produce small intense deviations, which can be filtered out in the data processing. Magnetometry and conductivity surveys often appear to show similar features, but because the two techniques measure different physical effects they may respond differently to many underground features, and are effectively complementary. Thus, although the magnetometry "map" of Shaw Cairn (Figure 4) shows obvious similarities to the conductivity map, there are important differences in detail.

Around the Mellor Hilltop Site, magnetometry has been particularly successful in locating the path of the so-called "outer ditch", as the soil infill of the ditch generates a higher magnetic gradient than the parent bedrock into which the ditch has been cut. Magnetometry located the ditch in the fields to the east of the Church car park (Figure 5), and these observations were confirmed by the excavation of several trenches, trench 50 in Figure 6 being the most recent.

Ground Penetrating Radar (GPR).

In this technique, a radio transmitter sends short radar pulses at regular intervals vertically downwards into the ground. Any sharp changes in the material through which the pulses travel reflect a portion of the signal upwards, and a suitable radio receiver detects these reflections. These occur in a sequence at increasing time intervals, because the distance travelled by the reflected pulses increases with the depth of the reflecting surface or object. The measured time delays can readily be converted to depth measurements. In practice, the radar electronics are contained in a small "box", attached to a distance-measuring wheel, and these are dragged in a straight line (transect) along the ground surface behind a cart carrying a computer (Figure 7). As the data are obtained, they are immediately transferred to the computer and the readings can be displayed a function of depth at the measured points along the transect. If a number of such transects are obtained, so as to cover a rectangular area, the data may be combined into a 3-dimensional "block", which may then be viewed either as a set of vertical profiles, or as a series of horizontal "slices" at gradually increasing depths in the soil – the latter views are particularly revealing, as they mimic the layers which would be revealed during an actual excavation.

Depth profiles were obtained by scanning along the path starting at the gate to Mellor Church (Figure 8). The purpose was to determine whether the so-called iron age "inner ditch", uncovered in the Old Vicarage Garden (just over the wall to the left of the picture), continues under the church path and graveyard. The transect profiles suggest a V-shaped reflective feature lying between ~6 to 10m from the gate, and reaching a maximum depth around 2.7m. In the light of the known size, shape and nature of the rocky infill of the deep inner ditch in the close-by garden, the GPR observations have been interpreted as revealing a continuation of the ditch, cut into the sandstone bedrock, and crossing under the Church Path about 8.5m in from the gate.

GPR was also used to investigate the outer ditch in a field to the northeast of the Old Vicarage. Figure 9 shows a general view of the field, with two excavations in progress (Mellor Hall is just visible behind trees to the right of the picture). At one location, an area 8m wide was scanned, and a series of horizontal sections from this scan, descending at 10cm intervals, is shown in Figure 10 (these are shown sequentially, row by row, left to right, to a total depth of 2.5m). Between the 9th and 12th slices, around 1m in depth, a linear feature appears across the top right-hand corner of the area, which is thought to be the remnants of a field drain, although feature this has not been excavated. At about the 13th slice (~1.3m depth), a linear feature starts to appear, crossing near to the middle of the area, and is most intense at a depth around 1.5m. Below 1.8m, the ground is far less disturbed. The feature was identified by excavation as a continuation of the outer ditch, which has now been tracked to about one third of the way across the field (to the more distant orange-fenced area seen in Figure 9.

Metal Detection.

A metal detector is now a familiar object at many archaeological sites (Figure 11*), although in the past their use was frowned upon by "serious" archaeologists. However, metal detectors are now deservedly recognised as a valuable adjuncts to an excavation. A modern detector will react to small metallic objects up to about 60cm deep in soil, and additionally distinguish between different types of metal, e.g. iron, aluminium and copper. Metal detectors have been useful at all stages in the Mellor excavations, both to screen the spoil and to detect metal objects still buried under the surface. Numerous coins, broaches and other metal objects have been found in this way (Figure 12 - an Elizabeth I half-groat, found in 2007).

* Metal detection close to the Jacobite line at Culloden ______________________________________________________________________________