Upland Geophysics
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Meall Greigh
Figure 1 Geophysical Survey Area, Ben Lawers Shielings
The survey location was established with John Atkinson on site through discussion of features and particular areas of the site that were of interest. A base line was established and 40 m grids were surveyed in. Actual survey grids were 20 m, measured in during the first few days of survey.
The RM15 resistivity meter was used to cover the main survey area with resistivity survey.
Figure 3 Resistivity survey with the RM15 resistivity meter
Due to some initial problems with the instrument and the time frame, data collection resolution was reduced for half of the survey area (from sample rates of 0.5m x 1m to 1m x 1m). Despite this sampling reduction, the area has been mapped effectively.
Figure 4 Results of the resistivity survey
The eastern half of the survey area (the first section covered) strongly reflects the underlying geology and does not readily reveal shielings or related features. The western half of the survey area however (with reduced sampling) reflects not only the site topography, but also some of the shielings themselves.Magnetic survey was conducted with the FM256 Fluxgate Gradiometer.
Figure 5 Magenteometry data collection with the FM256 Fluxgate Gradiometer
As with the other geophysical survey techniques, data were collected over every metre of the site surface, including in, around, and over the shieling features themselves. Data collection was at a rate of 0.25 samples per metre at 0.5 m spaced transects.Magnetic gradient readings were recorded across the main survey area and over a 40 x 80 m strip that extended north and up the incline of the slope. Because the rate of data collection with the magnetometer is much faster than with the resistivity meter, this extra area was surveyed with the magnetometer in order to investigate the surrounding landscape and to see if any traces of human activity could be mapped on the slope.
The results of the magnetic survey are quite interesting with many magnetic features appearing in the data. Ground truthing of some of these features during the Thistle Camp, August-September 2003, hope to help identify what they may be.
Figure 6 Results of the magnetometry survey
Ground penetrating radar survey was also conducted over the main survey area. Initial thoughts were that the GPR would not work effectively in this area due to the properties of underlying geology and the high saturation level of the site. Contrary to our assumptions, the GPR worked very well. A SIR3000 GPR unit was employed using a 400MHz antenna with associated survey wheel.
Figure 7 Ground Penetrating Radar data collection with the SIR3000 and a 400MHz antenna
Data collection with the GPR unit was very fast and most of the main survey area was covered with 1m spaced transect intervals. The beauty of GPR is that it records vertically into the earth and enables us to see the underlying structure of the geology and anomalies, as we collect the data in the field. The image below was taken in the field as data were collected. The feature that is most predominant in this transect is the interface between the assumed bedrock and surface soils (marked by the yellow arrows).
Figure 8 Screen shot of GPR data in the field, interface between bedrock and surface soils
GPR has been under-utilized by the archaeological community in general. The undertaking of this full area survey is to demonstrate the effectiveness of survey in the field (speed) as well as the increased information it provides through real-time analysis of the vertical profiles as well as the construction of 3D cubes for more insightful data interpretation.One of the strongest features of this geophysical survey work is that it was conducted entirely by students.
Figure 9 They will follow... and sometimes, take the lead!
The director of the survey, Meg Watters, is a PhD student studying Data Fusion and Solid Modelling at the University of Birmingham. As project director, Meg is working with students interested in landscape archaeology and training in geophysical survey. The hard-working crew took two weeks unpaid vacation time to hunker down on this site and do some impressive data collection.Kiltyrie Burn
| Kiltyrie was surveyed during the week of 7-14 April 2005. The first
three survey grids targeted visible turf structures. Preliminary review
of the results from the resistivity and magnetometry surveys over these
structures did not reveal significant anomalies that could be used to
characterize the structure or related features such as pits, fire hearths
or compacted floor surfaces. The fourth survey grid targeted and area of approximately 80m by 40m that contained a number of visible surface features. The geophysical survey mapped a number of possible archaeological anomalies both related to the visible features and others that were not visible on the ground surface.
Figure 4: (Above) Lower surveya rea of Kiltyrie (area 4) Figure 5 (Right) Resistivity and magnetic gradient survey maps provide information on possible archaeological and geological anomalies. Resistivity results in a. and b. (run through a high pass filter) reveal a number of anomalies. The magnetic results reveal a few anomalies that correspond with results from the resistivity survey. One 10m by 10m grid was selected over a U-shaped turf structure for intensive re-survey with greatly increased sampling rates. This area (outlined in Figures 5b and 5c) was selected based on the results from the geophysical surveys and the visible ground surface features. Results from these surveys are currently being analysed and will contribute to the assessment of the utility of geophysical surveys for the Ben Lawers Historic Landscape Project. |
Figure 3: (Above) GPR results highlighting the relationship of mapped anomalies to depth of bedrock. The top image is a plan map at approximately 0.45m depth that maps a number of anomalies. The yellow line through this map is the location of the vertical profile shown on the bottom of the image. The red arrows connect the position of two mapped anomalies to the vertical display. The bright black and white band that goes across the vertical profile shows the bedrock interface.
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| Meg Watters University of Birmingham [External] |
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