Geophysical Investigation of the Leamonaghan Monastic Site, Co

Geophysical investigation around the site of the former monastic settlement, Rahan, Co. Offaly

P. J. Gibson

Report No: EGU 01/06

Environmental Geophysics Unit

Department of Geography

National University of Ireland, Maynooth

Maynooth

Co. Kildare

2006

Site summary sheet

Site Name:

Former monastic settlement and associated archeological features, Rahan, Co. Offaly. Irish Grid Reference approximately E 225900 N 225400.

Survey type:

Magnetometry using a Bartington 601 fluxgate gradiometer. Data collected in a zig-zag pattern at 1m traverse interval and 0.25m sample interval. Approximately 120,000 data points obtained mainly in 30 x 30m grids.

Resistance using a TRCIA twin electrode resistance meter with 0.5m electrode spacing. Data collected in a zig-zag pattern at 1m traverse interval and 1m sample interval. Approximately 33,000 data points obtained mainly in 30 x 30m grids.

Resistivity using a multi-core Campus Geopulse. Data collected at 1m spacing for 4 par lines and 1.5m spacing for 1 line.

Processing software:

Data were processed using ArcheoSurveyor Surfer, Res2dinv, GPS Pathfinder and ArcMap.

Summary of results

The resistance data have shown different parts of the site have different resistances. In addition, a number of features could be observed.

A broad curvilinear low resistance feature can be observed in the eastern part of the image. It may represent an in-filled boundary ditch. Southwards it is more difficult to discern but if continue it would just enclose the small church at Rahan. (A, Figure 4). It is about 110m in length.

Very closely spaced parallel linear patterns can also be observed in the field and are also obvious on aerial photographs of the site. They relate to ploughing patterns and may be medieval in age.

Broadly spaced linear patterns with an approximate east-west trend are detected near the northern edge of the study area. These are parallel to and beside the outer enclosing northern embankment of the site and represent small embankments and ditches in this vicinity. Just to the east of these is a region with very high resistance values suggesting that stone may be present.

There are a number of other linear features most of which probably represent old field boundaries. One to the west of the small church is at least 70m in length and coincides with a linear depression on the ground.

The resistivity data generally showed non-archeological features. However, Line 1 was taken beside the large church and across where the 12th century church can be seen on the north side of the enclosure. There is a very distinctive high resistivity anomaly isolated in the section between 9-11m, which extends to a depth of at least 2m. Either side of the anomaly there are distinctive dips in the readings. This feature is possibly archaeological and may be related to the 12th century church.

Line 2 resistivity section was taken across the front of the small 15th century church with the Romanesque doorway. Readings show a low shallow resistivity layer underlain by a deeper higher resistivity layer. The resistivity values on this section are the lowest of the 5 sections and confirm the resistance data which showed this church in a low resistance area. The doorway of the church is at 11-12m and this is where a low resistivity zone is observed..

The magnetic data has provided considerably more information than the resistance data.

There is a distinct magnetic zonation in the study site.

Zone A encompasses parts of the enclosing ditch on the northern edge of the study area. It covers are area of around 50m x 30m and shows clear evidence on at least 2 sides of straight linears at right angles to each other.

Zone B is NW of the small church in Rahan and is a magnetically quiet area suggesting little human activity.

Zone C is east of the large church in Rahan. It is magnetically noisy but not to the same extent as zone A. It is a very regular area and suggests a zone of linear patterns.

Zone D is magnetically quite but it is sharply defined by a curvilinear break from the magnetically noisy zones C, E and F.

Zone E occurs over the banks and ditches on the northern part of the site though there does appear to be a crude approximate E-W trend which probably reflects the different magnetic signatures for the banks and ditches.

Zone F is a regular small area of low variability , most probably bounded by ditches.

Zone G: This is quite similar to zone F and is in fact parallel to it. Again there is a magnetic zone surrounding a core of low magnetic variability.

A long curvilinear feature appears to connect the large anomalous magnetic zone which has been interpreted as a building and continues past the front of the small church. It is over 70m long and it may be a boundary or possibly a path. Part of it is quite magnetically noisy.

A band of ridges and furrows caused by ploughing are very distinctive. They terminate in the north at the possible building though continue westwards past it.

Clear evidence of part of a feature which appears to curve around at least part of the larger church. Fences and metal made it difficult to obtain data in this area. Some other features parallel to it.

Geophysical investigation around the site of the former monastic settlement, Rahan, Co. Offaly

Introduction

The primary aim of this research is to geophysically investigate the area around Rahan churches which has been seen a main ecclesiastical centre since the 6th century when it was supposedly founded by St. Carthach (Mochuda). It adopted the rules of St. Augustine around 1200. The site contains the remains of 3 churches. The Church of Ireland nave and chancel was added in1732 to a 12th century church. The former has incorporated into it a large Romanesque 12th century rose window. In the graveyard is another small church and some distance away is a small Romanesque church rebuilt in the 15th century.

This work was undertaken in early 2006 under permit number 06R023. Data were collected by Paul Gibson, Dot George and Lorraine O’Reilly. The authors of this report would like to thank the following for supporting this research:

Offaly County Council especially Amanda Pedlow
Caimin O’Brien, Archaeological Survey of |Ireland.

In addition, this work could not have been undertaken without the full co-operation of the landowner, Sean Grennan who allowed access to the relevant areas. An especial thanks goes to him.

Geophysical techniques

Geophysical data were collected at the Rahan monastic site in early 2006 using resistance, magnetics and resistivity. Magnetic data were collected for 30 x 30m grids with a traverse spacing of 1m and a station spacing of 0.25m. Resistance data were collected for 30 x 30m grids with a traverse spacing of 1m and a station spacing of 1m.

A short outline of the theoretical principles behind each of these techniques is followed by sections considering:

Resistance characteristics at Rahan.
Magnetic characteristics at Rahan.
Resistivity lines at Rahan.

Resistance and Resistivity theory

Resistance surveying has been employed for a long time in archaeological investigations because anthropogenic features are often very regular in shape and provide good resistance contrasts with the background. The variation in subsurface moisture content dictates to a large extent the changes in resistance. Infilled defensive ditches are often wetter than surrounding regions and are associated with low resistivity values whereas buried walls or foundations may appear as high resistance features. It should be stressed though that features detected by resistivity means cannot be unambiguously identified until excavated as each site is different. For example, a ditch may have been infilled with stones and it may then have a relatively high resistance and water can collect under buried walls resulting in low resistance. A TRCIA twin electrode resistance meter was employed in this study (see Figure 2).

Resistivity data were also collected using a 2D electrical resistivity imaging technique. A Campus Geopulse resistivity meter was employed in this instance. Two dimensional electrical imaging (also termed tomography) allows the acquisition of apparent resistivity variations in both the vertical and horizontal directions. Electrical imaging was undertaken using an insulated multi-core cable with a number of fixed interval take off points to which electrodes are connected. The cable is connected to the resistivity meter which in turn is connected to a laptop computer which contains the relevant software to run the process. A parameter file was written to instruct the computer which sets of 4 electrodes to use and the currents to apply. For this study the data lines were obtained with an electrode spacing of 0.5 cm which gave greater detail in the upper layers of the subsurface.

In order to determine how the true resistivity varies with depth the data must be modelled using a computer program. One such program that is widely used in resistivity work is the inversion program RED2DINV.

Measuring the Earth’s magnetic field

A magnetometer is a device used to measure the intensity of the Earth’s magnetic field at a specific location. There are various types of magnetometer which operate using different physical principles, though the one employed in this study was a fluxgate magnetometer.

The fluxgate magnetometer consists of two short ferromagnetic bars around which identical primary coils are wound though in opposite directions. This array is surrounded by a secondary coil. An alternating current is applied to the primary coils which in the absence of an external magnetic field produces zero output in the secondary cool because the primary coils are oppositely wound. However, when positioned such that the cores are aligned with the component of the Earth’s magnetic field which is of interest, a voltage is induced in the secondary coil because the Earth’s field is increased by one core and decreased by the one whose windings are in the opposite direction. This induced voltage is proportional to the strength of the component of the Earth’s magnetic field to which the cores are parallel. It is possible, by changing the orientation of the fluxgate magnetometer, to measure the total field (F), the horizontal field (H) or the vertical field (Z). However, very accurate alignments are required in order to obtain accurate readings of H and Z. These high accuracies can be obtained at base stations where three fluxgate magnetometers can be permanently fixed such that one obtains values of Z and the other two measure the northerly and easterly components of the horizontal field.

When a magnetic reading is obtained using a magnetometer, this represents the resultant of the addition of the Earth’s magnetic field vector and the anomaly due to all the subsurface sources. However, it is shallow sources that are of most importance in archaeological investigations but the relative contribution of shallow and deep sources cannot be determined from a single magnetic reading. However, this problem can be overcome by using the magnetometer in gradiometer mode and acquiring two simultaneous readings from two sensors located at different heights. This is accomplished by placing sensors a fixed vertical distance apart. In this study a Bartington Grad-601 gradiometer was employed with a vertical separation of 1 m between the two sensors.

(a) (b)

Figure 1: (a) Twin electrode resistance equipment; (b) Bartington magnetic gradiometer.

Resistance data at Rahan

In all over 40 grids of data were collected at Rahan, where each grid is 30 x 30m, thus approximately 33,000 resistance data points were collected with a spacing of 1m in both the x and y directions. Figure 2 shows the location of the data collection and the northern part of the Rahan field that has been surveyed. The colour coded image shows that there is a clear patterns to the distribution of high medium and low values of resistance, Figure 3. In general the high resistance values are observed in the northwest part of the study area, the low values in the southeast with the medium values mainly around the central part. To a large extent, these variations probably represent variations in soil type and/or drainage. However, there are some sharply defined changes in resistance. Note how the small church at Rahan is enclosed within a very low resistance area, delineated in blue. This blue trends turns through a right angle and suggests that the church was once in a separate area and the soil evolved differently due to possible different agricultural practices. There are also a number of sharp changes in resistance for the high values, where the change occurs over a clearly defined linear break.

A grey tone image and interpretation are shown in Figure 4. There are clear linear features observable on this image which in all likelihood are archaeological in nature.

Feature A: This is a broad curvilinear low resistance feature in the eastern part of the image. It may represent an in-filled boundary ditch. Southwards it is more difficult to discern but if continue it would just enclose the small church at Rahan. (A, Figure 4). It is about 110m in length.

Feature B: A series of low resistance linear features can be seen to the east of feature A, i.e. they are enclosed by it. They may represent old ploughing patterns, B, Figure 4.

Feature C: Very closely spaced parallel linear patterns can also be observed in the field and are also obvious on aerial photographs of the site. They relate to ploughing patterns and may be medieval in age.

Feature D: Broadly spaced linear patterns with an approximate east-west trend are detected near the northern edge of the study area. These are parallel to and beside the outer enclosing northern embankment of the site and represent small embankments and ditches in this vicinity. Just to the east of these is a region with very high resistance values suggesting that stone may be present.

Feature E: A broad linear feature characterized by low resistance values extends across the resistance image for a distance of 80 metres.

Feature F: To the west of feature E is a number of short linears. Interestingly these tend to be concentrated within an area where resistance values are slightly lower than the surrounding areas.

Feature G: This is a curvilinear narrow high resistance boundary. It is quite close to the fence and an adjoining house so it was not possible to discern its full extent

Figure 2: Location map of resistance data for Rahan.

Figure 3: Plot of resistance data at Rahan, colour coded.

Figure 4: Plot of grey tone resistance data and interpretation.

Magnetic data at Rahan

In all over 40 grids of magnetic data were collected at Rahan, where each grid is 30 x 30m, thus approximately 120,000 magnetic data points were collected with a spacing of 0.25m in the y direction and 1m in the x direction. Figure 5 shows the location of the data collection and the northern part of the Rahan field that has been surveyed.

Figure 6 is a colour coded magnetic image of the study area and Figures 7 and 8 represent blow-ups of parts of the study area. These figures show Surfer grey-scale versions of the data in which deviations from the norm are show in blue or red. In theory a region which has had no anthropogenic activity should give a background value of around 0 nT. However, archaeological sites are often associated with magnetic anomalies due to the activities that occur around them and zones of activity are best observed when colour coded.

Figure 6 clearly shows there are a number of distinct zones in which there is enhanced activity and other areas which are magnetically quiet. 8 zones have been identified though it is possible that some could be further subdivided.

Zone A encompasses parts of the enclosing ditch on the northern edge of the study area. This area is shown in more detail on Figure 7. It covers are area of around 50m x 30m and shows clear evidence on at least 2 sides of straight linears at right angles to each other. Interestingly this area is also characterized by high resistance values which would also support the view that there was a building here.

Zone B is NW of the small church in Rahan and is a magnetically quiet area suggesting little human activity.

Zone C is east of the large church in Rahan. It is magnetically noisy but not to the same extent as zone A. It is a very regular area and suggests a zone of linear patterns.

Zone D is magnetically quite but it is sharply defined by a curvilinear break from the magnetically noisy zones C, E and F.

Zone F is a regular small area of low variability , most probably bounded by ditches.

Zone G: This is quite similar to zone F and is in fact parallel to it. Again there is a magnetic zone surrounding a core of low magnetic variability.

Zone H: This is near the entrance to Rahan field and again this zone has little magnetic variation. It’s southern edge is delineated by the modern roadway to the church, though the strongest linear feature is parallel and a short distinct from the modern roadway.

Figure 5: Location map of magnetic data.

Figure 6: Colour coded magnetic data for Rahan.