The Work

Geophysics Survey - 2004

Posted by steven on 06/04/2005 at 10:26 AM

Introduction

Geophysical surveys allow archaeologists to detect archaeological features buried below ground without digging. There are a number of different techniques, each requiring its own piece of equipment, and each technique detects different physical properties of the ground. Thousands of readings are collected in data-loggers and transferred to a computer for analysis. Maps showing the discoveries below ground can then be printed. Three main techniques have been employed during the recent investigations of the High Pastures site and the area around it including:

Magnetic Survey (magnetometry) - This technique records subtle changes in the local strength of the earth’s natural magnetic field. By collecting readings on a regular grid pattern, a map of hidden features underneath the ground can be made. The magnetometry survey area extended to 1800 sq. metres and included the core of the site as represented by the major stone-built structures. 

Earth Resistance Survey (resistivity) - This technique measures variations in soil moisture, which often reflect the presence of buried structures, building foundations (less moisture) or ditches and pits (more moisture). By pressing the frame down at precise intervals of 1m on a marked grid, the operator forces the probes into the ground. A small electrical current then passes through the ground.

Ground Penetrating Radar (GPR) - The principles here are much the same as those used by radar at airports to detect flying aircraft, only in this case the equipment sends a short pulse of radio energy into the ground and measures the time taken for any reflections from buried features to return to the surface. This allows the depth of buried structures to be estimated from the length of time it takes for the reflections to reach the surface: the longer the delay, the deeper the structure. Once a large area has been surveyed, it is possible to attempt a 3D reconstruction of an entire site. The resistivity survey and the GPR traverses covered an area of 225 sq. metres, centred on the ‘U’-shaped enclosure.  Protruding stones and banked features, representing the upstanding archaeological remains, obstructed some of the GPR traverses and these are indicated in the resulting data.


Figure 1.  GPR traverses in progress within the U-shaped structure at High Pastures - December 2004.

The geophysical survey of the High Pastures site was carried out over 2 days from the 7th – 8th December 2004.  The weather was dry and fine on the first day but deteriorated to rain and gale force winds throughout the second day. The main objectives of the geophysics survey were to trace the possible location of a former entrance into High Pasture Cave, leading directly into Bone Passage where the main archaeological deposits are concentrated, and attempt to correlate its position with the archaeological features recorded on the surface.  From the earliest fieldwork at the site it became apparent that the archaeological material in Bone Passage had been introduced via another entrance into the cave system.  This was partially confirmed during the cave morphology survey when a potential rubble-filled shaft was identified at the terminus of Bone Passage, which could be seen extending up towards the surface.  The magnetometry survey included a larger survey area in order to investigate the wider potential of the site for buried archaeological features.  The results and conclusions set out below are drawn from the Geophysical Survey Report published by Stratascan (Carpenter, 2004).

Magnetic Survey Results

The most dramatic feature shown up by the magnetometer data is a large, strongly negative linear anomaly cutting across the south-eastern corner of the westerly grid and continuing through into the easterly grid.  As the area has been affected by substantial volcanic activity there is a strong possibility that this anomaly is caused by an igneous dyke.  To the south of this anomaly on the far side of the eastern grid is a very weak positive anomaly - this seems to display a cornered shape in a right angle.


Figure 2.  Using the Fluxgate Magnetometer at the High Pastures site, in worsening weather conditions - December 2004.

In the western grid there is an area composed of discrete positive and negative anomalies, which is curvilinear in shape.  This seems to correlate with the curvature of the stone arrangement seen at the surface, indicating that they are of similar origin.  To the north of this circular area is a large discrete anomaly and to the south there is a smaller discrete anomaly, which are possibly caused by ferrous objects.

The south-east corner of the western grid square is dominated by a cluster of positive rectilinear anomalies of varying magnitude.  To the east of these in the eastern grid is a strong discrete anomaly, which could possibly have been caused by a ferrous object.

The magnetometry survey highlighted three discrete anomalies.  It is possible that all three of these anomalies are produced by a ferrous material and so could highlight areas of metalworking.  The curvilinear anomaly to the east of the two grids seems to correlate with the curvature of the surface structures and so it is possible that it is of a similar origin.  A number of rectilinear positive responses were also detected within this area, which differ in shape from the circular features seen at the surface.

Resistivity Results

The resistivity survey revealed several areas of high and low resistance.  In the western edge of the grid is a large area with lower than average resistance.  This anomaly is roughly circular in shape with a linear limb extending from its base in a southeast direction.  The circular shape of this anomaly correlates to the curvature of the surface features.

Moving east across the grid a linear low resistance anomaly can be seen.  This is orientated in a similar direction to the linear anomalies picked up by the magnetometer data.  It is possible that anomaly such as this could be caused by a cut feature.

The eastern half of the grid is dominated by a large patch of low resistance ground, which seems to be curving eastwards to the north of its extent.  The position of this feature corresponds to the large U-shaped surface structure and so it is possible that these are connected.  In the centre of this arched region is a patch of high resistance ground, this could be connected to the centre of the U-shaped structure, maybe indicating the presence of a floor or maybe a shaft in-filled with rubble.

The most pronounced anomaly picked up by this method was the curvilinear patch of low resistance, which curved around a patch of ground with high resistance.  These areas correlated to the U-shaped surface feature and so are probably connected to this.

Ground Penetrating Radar Results

The objective of the survey was to locate the opening of a rubble-filled shaft coming up to the surface from a passageway that has been located below the surface.  A rubble-filled shaft, such as the one we are attempting to locate, would show up on the GPR radargrams as a region of complex ground, possibly increasing in strength with depth as the rubble becomes less compact and the number of air voids increases.  The manual abstraction of the radargrams highlights two strong complex areas labelled A and B within the U-shaped surface feature.  These areas were also highlighted in the timeslices and can be seen most vividly at depths of 1.8m and 2.2m.

The smaller northern area A is more centrally positioned within the U-shaped structure than the larger southern area of interest marked B.  This area also is directly aligned with the directional trend of the high level passage (Bone Passage) and so there is a higher possibility that this anomaly marks the location of the shaft/entrance.  One of the radargrams transecting this region is pictured below:


Figure 3.  Example of a radargram showing anomaly B within the U-shaped structure. The darker complex area within the diagram indicates disturbed ground and possible air-filled voids that may indicate the location of the original entrance into Bone Passage.

The larger strong complex area to the north is also within the U-shaped structure and the magnitude of the anomaly increases with depth, as can be seen from the example radargram shown below.  The definition between the strong complex region and the bounding less-complex areas can be seen particularly well in this example.

Other areas have been highlighted in the radargram abstraction but labelled with a low possibility of being the shaft/entrance location.  These areas are strong and complex but are slightly weaker than the areas mentioned above and also are not positioned within the U-shaped surface feature.

The GPR survey showed up a number of strong complex responses throughout the survey area, however the largest magnitude responses came from two distinct regions within the U-shaped structure.  These areas are also clearly highlighted in the time slices.  The northern anomaly (A) is in line with the directional trend of the high level passage where archaeological material has been discovered (Bone Passage), making it the most likely position for the opening of the rubble-filled shaft.  However, the southern anomaly (B), which is also bounded by the U-shaped structure, is slightly stronger with more pronounced sides and so is also a possible location for the original cave entrance.


Figure 4.  Abstraction of the GPR results. The structures on the plan are represented by the dark hatchered areas, while the location of the cave passages below are shown in red (Bone Passage hatchered red). The green box outline represents the area surveyed using the GPR and the cross-hatchered areas represent the following:

Pink - High energy areas at 1.3m depth

Blue - High energy areas at 1.8m depth

Red - High energy areas at 2.2m depth.

These areas indicate the probable location for the original entrance into Bone Passage, especially Area B

References

Carpenter, A.H. (2004) Geophysical Survey Report: Strath, Isle of Skye.  Stratascan Ltd. 


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