E O S 4 0 3 : F i n a l Y e a r P r o j e c t

Geophysical Investigation of Rathcroghan Mound, Co. Roscommon, Ireland.

Stephen Kenny

December 2014

Table of Contents: Page

Abstract 1

Chapter 1 – Introduction

• 1.1 Thesis Outline 2

• 1.2 Site Overview 3

• 1.3 Geology/Soils/Land Use 4

• 1.4 Previous Work 6

• 1.5 Aims and Objectives 8

Chapter 2 – Methodology

• 2.1 Electrical Resistivity Tomography (ERT) 9

• 2.2 Ground Penetrating Radar (GPR) 11

• 2.3 Survey Design 16

2.3.1 ERT 18

2.3.2 GPR 19

Chapter 3 –Data Processing and Results

• 3.1 ERT Results 20

• 3.2 GPR Results 25

Chapter 4 – Discussions and Conclusions

• 4.1 Line 1 30

• 4.2 Line 4 30

• 4.3 Line 5 32

Chapter 5 - Conclusion 35

Acknowledgments 35

Appendices 36

References 37

Abstract

From the 7th-11th of July, 2014, non-invasive geophysical techniques were used on the

protected site of Rathcroghan mound and its environs to obtain a better understanding of the

subsurface features and geology in the area. The survey area in Rathcroghan is located

approximately 5km to the northwest of the village in Tulsk, Co. Roscommon (Fig. 1.1).

Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) were used on

the mound itself over three survey lines running north-south. These surveys showed a range

of geophysical and archaeological features that would have gone otherwise unseen from the

surface, as well as depth to bedrock profiles and determination of bedrock, interpreted as

ooidal limestone. Past historical (and mythological) significance plays a large role in the

interpretation of these features.

1

Chapter 1 – Introduction

1.1 - Thesis Outline:

This thesis is broken down into 4 chapters and each will deal with a specific part of the

investigation that went into Rathcroghan mound:

• Chapter 1 will look at the background of the site in Rathcroghan including the

geology and soils of the area and previous work that has been done.

• Chapter 2 will deal with the methods and theory behind the different techniques used

in Rathcroghan and the survey design used on site as well.

• Chapter 3 involves the data processing techniques used and the results from the

various survey methods.

• Chapter 4 will wrap everything up in the form of some discussions as well as

conclusions about the site and what was seen.

2

1.2 - Site Overview

The Rathcroghan Complex is a vast site of great pre-Christian importance stretching some

10km2 and containing over 60 ancient monuments, ranging in age from the Neolithic to the

medieval periods (Barton & Fenwick, 2005). These monuments range from standing stones to

ring barrows and forts, to other, prominently circular, ring works. At the approximate centre

of this complex is the Rathcroghan Mound, which is the particular site that was worked upon.

Many of the other large features can be seen from atop the mound. This complex is located

roughly 5km to the north-west of the village of Tulsk, Co. Roscommon, just off the N5 (Fig.

1.1). The site also holds some mythological and spiritual significance to the history of Ireland

at that time

Fig 1.1 – Ordnance Survey Ireland map of the Rathcroghan Complex with survey area highlighted containing Rathcroghan Mound

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Rathcroghan Mound is a flat-topped circular mound with a diameter of 88m and at its highest

point stands at 6m above the surrounding land. It is relatively featureless at the top, with

some minor elevated platforms and such, but it does contain a ramp at both the east and west

side of the mound, which allows for easier access to the top. It is thought that the eastern

ramp is an original feature whereas the western ramp is a result of later quarrying activities.

Rathcroghan mound is significant in Irish folklore as it is believed to have been the possible

seat of the Kings of ancient Connacht, as well as being a royal burial site. The story of the

Táin Bó Cualinge, The Cattle Raid of Cooley, was said to have been launched from this site

by Queen Medb into Ulster to steal the prized bull and fight against Cú Chulainn (Barton &

Fenwick, 2005).

From previous work done on Rathcroghan, it has been seen that a circular ditch encompasses

the mound and has a diameter of approximately 370m. This ditch cannot be seen today but

has shown up on radar and magnetic data surveys and is almost constant the entire way

around the mound for the areas that have been surveyed. This may have been a defensive

structure like a revetment in prehistoric times, and no entranceway has been found as of yet

although it is thought there is possible evidence for one to the east (Barton & Fenwick, 2005).

1.3 – Geology/Soils/Land Use

The area around the Rathcroghan complex is predominantly Carboniferous limestone which

is said, in early literature, to be part of the Machaire Connacht, or “the plain of Connacht.”

This limestone been shaped by extensive glacial activity, present tofay in the form of

drumlins, eskers and moraines. Glacial till or drift can be seen to overlay some of this

limestone as well. It is documented that during the last ice age in Ireland, a major north-

east/south-west axis of ice movement was observed, especially in the west of the country.

This direction of movement is supported by the presence of till-covered limestone hillocks

ranging from 1-2km in length that reach up to 500m in width around the area of Tulsk. These

can be seen to fade out the closer you move towards Rathcroghan. Towards the end of the ice

age, debris flows from the glaciers formed eskers and moraines. These can often be seen in

pairs and run north-south, at right angles to ice movement. These ridges range in height from

3-5m and 100-500m in length, which made for ideal ground for the construction of ancient

buildings and forts, including Rathcroghan Mound itself. (Waddell, Fenwick, Barton, 2009)

The limestone itself is high in reef limestone and is fossiliferous, often showing an ooidal

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texture, and can usually be seen at a depth of about

1m (Barton & Fenwick, 2005.) Two small quarries

are present on site, to the north and south in relation

to the mound. An examination of the smaller of the

two to the north, which is across the N5 road, was

carried out. In this quarry a highly weathered face of

this limestone was studied (Fig. 1.2), which was

ooidal and had a soil covering of about 1m. Ooids are

tiny sedimentary grains which have a calcium

carbonate core, and form by constant movement

(“rolling”) along a sea floor which allows them to

grow and form a rounded shape. These ooids form in

high energy conditions, which mean this limestone

was more than likely part of a high energy

environment (Tucker, 2001).

As was mentioned earlier, Rathcroghan served as an ancient site for the high Kings of

Connacht. They used the land, in particular its higher areas, to build their forts and buildings

for protection. In the day to day workings, they may have farmed the land, as was seen on a

magnetic gradiometry survey done in the area that revealed evidence of ancient ridge-and-

furrow cultivation. It has also been noted that these practices have not been undertaken within

living memory (Barton & Fenwick, 2005). In modern times, sheep have grazed the land and

modern agricultural practices like mowing silage have taken place around the mound, with

the sheep allowed to graze on the mound itself. These practices, along with the grazing of

sheep, have upset the topsoil and may have affected any shallow structures that were once

present.

Due to Rathcroghan being a protected site, a problem arises when trying to identify the soils

without the use of excavation. Instead, any assumptions that can be made about the soils must

be done so by looking at natural holes or openings in the top grass layer. In some places, the

sheep have worn away small nooks in the mound. One such area was found on the eastern

side near the ramp (Fig. 1.3), and a simple hand soil test was conducted.

Fig. 1.2 – Area of exposed limestone at northern quarry

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A small piece of the soil was taken from just underneath the grass cover and examined. By

using water to see if the particles stuck together, and by testing to see if it formed a ribbon

shape, it was estimated that the soil was most likely a clay-loam, or possibly a silty-clay. This

was done quite roughly, so a more accurate analysis of the soil would be needed to get a more

detailed answer.

1.4 – Previous Work

Barton and Fenwick (2005) conducted several different types of geophysical and

archaeological surveys in Rathcroghan, focusing primarily on Rathcroghan mound itself.

Micro-topographical surveys enabled them to accurately map the topography on the mound

as a basis. Extensive magnetic work was done including magnetic susceptibility and

gradiometry. For the susceptibility a depth of investigation at 0.1m and a coarse sampling

interval of 2m was used, which was later narrowed down to 0.5m in order to better map the

subsurface. It was found that a circular zone of approximately 30m in diameter at the centre

of the mound had quite high susceptibility values in comparison to the rest of the relatively

low-valued mound that resulted from anthropogenic use, mostly likely the spreading of ash.

The gradiometry survey was conducted at 0.5m station interval and 1m traverse separation to

map the surrounding fields. After examination of the mound, it was decided that a more

detailed 35m x 35m survey with a 0.25m interval would be done. From the broader survey on

the fields they found extensive evidence of ridge-and-furrow cultivation which is not modern-

Fig. 1.3 – Area worn away on eastern side of the mound

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day, as well as remains of a revetment of some sort around the base of the mound. In the

more detailed survey on the mound summit, a series of detailed concentric rings dotted

around the mound’s summit separated by positive magnetic anomalies were seen. A square-

square resistivity survey was used on the mound as it is very good at imaging small-scale

features below the surface. A 0.5m electrode spacing with 0.5m intervals was used. The

mound itself portrayed quite a high resistivity compared to the surround fields, except for an

arcuate anomaly about 10m to the southeast of the mound. As was mentioned earlier, the two

ramps showed contrasting values indicating that only the eastern ramp is original. A twin-

probe resistivity array was also conducted using the same sample intervals, which produced

very similar results to the square-square array, except for slightly more detail on the

anomalies.

Electrical resistivity tomography (ERT) and ground penetrating radar (GPR) were used in the

Midlands of Ireland around the vicinity of Tullamore, Co. Offaly and Mullingar, Co.

Westmeath, to better understand the nature of the glacial and post-glacial Quaternary

sediments in the area. Five ERT profiles were conducted using a Wenner-Schlumberger array

with one profile at 2m spacing and 48m long, one at 5m spacing and 120m long and three

profiles at 10m spacing and 240m long. The programme RES2DINV was used to process the

data and calculate pseudo-sections of the area. GPR data was taken with the Pulse EKKO

system with 50MHz (one profile), 100MHz (two profiles) and 250MHz (two profiles)

antenna frequencies, each with varying step sizes, separations and time windows. Two

common midpoint surveys were also conducted to measure velocities in the subsurface.

Using these techniques, Pellicer and Gibson (2011) observed a range of glacial structures and

sediments including diamictons, esker gravels, glacial-lake sediments as well as fans

composed of silt, sand and gravel.

ERT and GPR were used by Leucci (2006) in southern Italy to examine the subsoils under a

church which had been weakened and was due for renovation. Due to the presence of

structures such as scaffolding within the church, only a limited numbers of survey lines could

be taken as a result of interference form the metal stuctures. A 400MHz GPR antenna was

used in the church with an acquisition time range of 100 nanoseconds. Only 9 profiles were

completed due to the aforementioned structures. A 10m x 15m survey grid was laid out for

the ERT and data was collected along 1m-spaced parallel lines. For the electrodes, special

electro-cardiogram electrodes were used so as not to damage the floor of the church, of which

16 were used in each line. A dipole-dipole array was used with 1m electrode spacing. This

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array was used so as to give a good vertical picture of the subsurface to about 5m depth. Both

RE2DINV and RES3DINV were used to plot the ERT data, and a programme called

REFLEX was used to process the GPR data. A large cavity, approximately 6m x 6m and

1.5m deep, was found under the church as well as various fractures and voids, with some

void spaces containing water. These surveys pinpointed the precise locations of the areas

beneath the church that required the most work and restoration.

The geologically complex subsurface of an ancient 4th-2nd century BC settlement in southern

Italy was in danger of having sewerage pipelines running through it. Simple archaeological

work had been done and remains were found at a depth of 0.3m but a detailed survey of the

area was required with the use of GPR and ERT. A Sir-2 GPR system was used with both a

200MHz and a 500MHz antenna (the 500MHz was used more often due to the shallow depth

of the remains) along parallel lines with a spacing of 0.5m. The area around the site had a

thick covering of topsoil, and was found to contain many fractures in the karstified rock as

well as larger areas of infill. These factors made it difficult to interpret much from the GPR

survey, so 3D ERT was used to supplement the picture. A 23m x 18m grid was set out in

order to map a strong elliptical anomaly that was found by the GPR. A dipole-dipole array

was used in both the x and y directions, with the x direction containing 19 lines with 24

electrodes, and the y direction utilizing 24 lines with 19 electrodes. Both sets of lines used a

1m separation and electrode spacing. RES3DINV was used to make-up pseudosections for

the area. These surveys allowed Negri, Luecci and Mazzonne (2008) to find many ancient

wall structures and tombs, and clarified the extent of the overall site. Small pebble walls were

excavated as a result of the surveys shortly after they were completed.

1.5 – Aims and Objectives

• To examine Rathcroghan mound and its environs

• To map the subsurface under the mound and on the ditch

• To identify (if any) features present such as cavities/air pockets

• To further expand on previous work that has been done in the area and to help

complete the picture

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Chapter 2 – Methodology & Survey design

2.1 – Electrical Resistivity Tomography (ERT)

Electrical resistivity tomography is a widely used geophysical technique that involves measuring

the resistivity of a material by sending an electrical current into the ground using electrodes.

Resistivity is a physical property that determines how easily a material will allow an electrical

current to pass through it. It is a relatively new technique (field-capable systems not developed

until 1989) that involves the use of a minimum of four electrodes; two current and two potential

electrodes (Daily, Ramirez & Binley et. al., 2004). Different variations of electrode layouts

(array types) can be used to obtain different information like depth and resolution; the most

common being the Wenner, Schlumberger and Dipole-Dipole array types (Fig. 2.1). Although

resistance is what is being looked at, apparent resistivity (Pa) is measured in the field by these

methods as resistance is not an accurately measurable property in a heterogeneous material such

as subsurface soils. The apparent resistivity values are made into pseudosections that are

inverted at the processing stage using a programme like RES2DINV and the true resistivity of

the material is produced.1 A table of standard resistivity values for different materials is given in

Fig. 2.2.

1http://www.epa.gov/esd/cmb/GeophysicsWebsite/pages/reference/methods/Surface_Geophysical_Methods/Electrical_Methods/Resistivity_Methods.htm

9

The electrodes used in these surveys are usually made of bronze, copper or steel with copper-

plating and are pushed into the ground at equal spacing. The electrodes are joined to the survey

cable which is then connected to the processing unit. These units run a specified array type that

the user inputs before starting the survey, and a quick check of all the electrodes on the lines is

Fig. 2.2 – Typical resistivity values for various materials

Fig. 2.1 – ERT array types, where I = current electrodes and V = potential electrodes

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done by the unit to ensure that the electrodes are all working correctly, after which the array

will run on its own. Often an electrode can encounter near-surface pebbles or rocks which can

hinder its ability to accurately measure the resistivity and must be moved slightly. If a problem

arises due to a bad contact between the electrode and the soil, water (salt-water if possible) is

often poured around the base of the problematic electrode to try and improve the contact. 1

ERT has become a popular method of sub-surface imaging and has been used for a number of

different surveys. A common use is the finding and surveying of landfill areas and leachate,

such as in Thriplow, UK where an ERT survey was conducted over a landfill where boreholes

data was inconclusive and a number of discrete areas of leachate were discovered within the

landfill (Oglivy et.al, 2002). Another common use which has been done in Ireland is the

application of ERT to identify subsurface karst features such as caves. Two areas in Ireland were

surveyed and previously unseen cave and collapse features were observed on the ERT profiles

(Fig. 2.3) buried under the karstic limestone (Gibson, Lyle & George, 2004).

Fig. 2.3 – ERT profiles from Co. Kildare, Ireland showing previously unmapped features.

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2.2 – Ground Penetrating Radar (GPR)

Ground penetrating radar fits into the electro-magnetic (EM) group of geophysical surveys. It

uses radio waves over a range of 80MHz to 1GHz sent from a transmitter into the ground, some

of which are reflected back to the surface and collected by a receiver, and some which continue

further down into the subsurface. Most modern GPR survey equipment uses a transmitter and a

receiver, together called an antenna, mounted on a frame. The antenna size is related to the

frequency required, as a small antenna is used for more accurate, high frequencies and larger,

sometimes vehicle-mounted, antennas are used for lower frequencies that can see to greater

depth but have poor resolution. In essence, the smaller the antenna size, the poorer the

penetration, but the better resolution can be achieved, so a compromise must be met. A number

of factors must be taken into account when deciding the appropriate antenna, such as depth of

the area of interest, accessibility of the site and most importantly, the properties of the soil. In

particular the electrical properties such as conductivity (Fig. 2.4) will play a huge part, as high

conductivity will cause the energy from the waves to be dissipated, as will seawater (Gaffney &

Gater, 2006).

Within the antenna, the transmitter emits wave pulses lasting only nanoseconds. It is important

to keep these pulses as short as possible as EM waves travel extremely fast. Once these pulses

are sent, the receiver picks up the waves that have been reflected back up from below the surface

(Fig. 2.5). Reflectors like large stones, pipes, walls etc. (as well as boundaries of changing

properties) act to send the signal back, and the time it takes for the receiver to pick these signals

Fig. 2.4 – Table of electrical properties for various materials

12

up is known as two-way time (TWT). These TWTs are then used to estimate the depth to the

object based on how quickly the wave was reflected (Mussett & Khan, 2000).

In some cases, rather than having the transmitter and receiver

move as one, a common mid-point (CMP) survey can be used,

whereby the transmitter and receiver act as separate antennae

and are moved away from a central point at equal incrememts

(Fig. 2.6). This type of survey is important when dealing with

how the wave velocities in the ground vary related to depth, as

the velocity depends on the material the wave passes through.

The velocity of a wave is heavily affected by the relative

permittivity of the subsurface. As seen in Fig. 2.4, most

rocks/soils have a value between 3-40, and this in turn is

affected by the water content. Water has the highest value on

the table with a value of 1, which means in certain conditions

water can act as a reflector. This has made GPR a useful tool

in the delineation of water tables (Mussett & Khan, 2000).

Fig. 2.5 – Example of an antenna with transmitter sending signal into the subsurface and receiver catching reflections

Fig. 2.6 – CMP survey (bottom) with the more popular common offset method (top)

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Not all the waves sent down by the transmitter are simply reflected back equally. On almost all

GPR surveys, the “air wave” is the first recorded pulse as it simply travels through the air from

the transmitter to receiver, and often does so at the speed of light. The ground wave is next to

arrive, which like the air wave, travels along the ground in the space between the transmitter and

receiver. Another anomalous wave is the lateral or “refracted” wave (Fig. 2.7). This pulse’s path

is altered by changes in the subsurface that cause the wave to reflect back in between the

transmitter and receiver at the air-ground interface (Neal, 2004).

Data from GPR surveys are often loaded with noise and other background interference,

depending on location. For example the presence of electricity wires, metal buildings, telephone

wires as well as smaller things like mobile phones and walkie-talkies in the near proximity of a

survey can affect the antenna and introduce unwanted noise (Neal, 2004). Post-processing of

the data is required in order to produce a better image of the subsurface. Programmes like

EkkoView are popular processing methods that allow the use of filters, gains, background-noise

subtractions as well as topographical corrections on GPR data. For example, GPR was used on a

church is southern Italy to find any cavities or features beneath the church as it was under

construction (Leucci, 2006). A 400MHz antenna was used within the church and the data was

later processed using various gains and filters. GPR was also used in the Midlands of Ireland

(Pellicer & Gibson, 2011) to map internal structures within Quaternary sediments.

Fig. 2.7 – Plot of various types of waves on a GPR survey

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A 50MHz and a 200MHz antenna were used with post-processing including topographical

corrections and migrating of data (Fig. 2.8).

Fig. 2.8 – GPR data from Midlands of Ireland with topographical editing and data migration (50MHz top profile, 200MHz bottom profile)

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2.3 – Survey Design

For the purpose of this survey, three lines in total were worked upon over the mound and its

close environs. Fig. 2.9 shows the survey lines running north-south, of which lines 1, 4 and 5

were worked on this year, as lines 2 and 3 were completed last year. Lines 1 and 5 were taken

just off the western and eastern side of the mound, respectively, to get a contrasting image of the

subsurface both on and off the mound, with line 4 on the mound just about running over the tip

of the western ramp. Electrical resistivity tomography and ground penetrating radar surveys

were carried out on these lines. GPS coordinates were established along each line and at each

end of the lines before any work was carried out. This was obtained by the setting up of a base

station on the top of the mound and calibrating all the GPS readings back to it.

Fig. 2.9 – Map of survey area showing survey lines (blue) and N5 road (green)

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2.3.1 – Electrical Resistivity Tomography:

A Wenner-Schlumberger array (Fig. 2.10) was used for the resistivity measurements as it gave

good resolution, moderately good depth and very good lateral resistivity variations.

An electrode spacing of 1m was used over 96m long lines. The lines consisted of one 48m

section, followed by four 12m long “roll-alongs” adjoining it. These roll-alongs were conducted

by taking the first 12m section from the 48m line and adding to the end of that line, extending it

12m. This process was repeated three times to obtain a 96m line. The lines were laid out using

multi-core cables connected to an IRIS Syscal Pro meter, which ran the Wenner-Schlumberger

array and collected and stored the data. The lines were connected using connector boxes that had

to be wrapped in plastic to avoid moisture damage (Fig. 2.11).

Fig. 2.10 – Outline of a Wenner-Schlumberger array

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2.3.2 – Ground Penetrating Radar:

A GPR survey was conducted over the three lines using a wheel-mounted 250MHz antenna

(Fig. 2.12), which provides a depth of investigation of approximately 2-4m and gives good

resolution. On lines 1 and 4 the survey was run north-south, while problems in the instrument

led to line 5 being run both north-south and south-north as well. The antenna wheel contained an

odometer which allowed the distance to be measured accurately.

Fig. 2.11 – IRIS Syscal Pro meter for measuring apparent resistivity

Fig. 2.12 – 250MHz GPR antenna 18

The antenna is also connected to a controller unit (Fig. 2.13) which is worn by the operator and

gives real-time analysis of the radar waves and produces a rough profile of the GPR traces as

you move along.

Fig. 2.13 – GPR antenna in use in the field

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Chapter 3 – Data Processing and Results

3.1 – Electrical Resistivity Tomography - Data Processing

As shown above, resistivity data in the field was collected by the IRIS Syscal Pro meter and

stored in “blocks” of data. These blocks were then exported from the meter to a laptop for

processing. The data, for each of the three survey lines, contained the values for the 48m section

of the line as well as the separate 12m roll-alongs (as explained above in 2.4.1). These separate

pieces had to be stitched together using a program called Prosys II, where topography data was

also added. Once the data was edited, the program RES2DINV was used to process the raw

resistivity data and produce profiles. The system ran the data and produced three profiles; one

with the measured apparent resistivity values, a second with a calculated apparent resistivity

section and a third with an inverted model (Fig. 3.1). This model was the predicted best-fit

section for apparent resistivity which was then inverted again, along with topography in order to

get the final model of the subsurface. Values on these sections are given in ohm metres and a

colour scale is provided mapping the variations.

Fig. 3.1 – Primary/basic data modelling of ERT data showing three different profiles

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Once the final inversion is done, a completed profile with topography is produced. The data that

has been inverted is done so in iterations, and the user must define the amount of iterations that

is to be shown on the profiles. Too few iterations and the data becomes generalised, whereas if

too many iterations are used, too fine a profile is gathered and the data becomes inaccurate.

Results:

The ERT results for line 1 (Fig. 3.2), line 4 (Fig. 3.3) and line 5 (Fig. 3.4) are shown below.

These are the topographically-corrected models with resistivity in ohm metres along the bottom

and elevation in metres along the y-axis. The dark blues and greens represent low resistivity,

while the darker purple sections are the highest resistivity values.

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Fig. 3.2 – ERT profile for Line 1

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Fig. 3.3 – ERT profile for Line 4

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Fig. 3.4 – ERT profile for Line 5

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3.2 – Ground Penetrating Radar – Data Processing

The GPR data collected in the field was stored in the controller unit of the radar antenna (Fig.

2.13) in directories which were then uploaded to a computer. The data was edited using

EkkoView Deluxe, which allowed for the reversing and repositioning of lines that were done in

different directions in the field as well as processes such as gaining and filtering. These

processes edit the data and make it clearer and more accurate. A process known as “Dewow”,

along with Background Subtraction, was one of the first used, which cuts out some of the

unwanted background noise in the traces. Both of these processes are called filters, while gains

amplify the signals in certain areas. Automatic Gain Control (AGC Gain) was used to amplify

some of the signals that were lost in the lower part of the trace. Topography was also added.

These processes were applied and the resulting trace was produced in another programme called

EkkoView2. Here, any gains/filters that were applied were fine-tuned to produce the best

possible profile of the data (Fig. 3.5).

Results:

The three GPR profiles below are for line 1 (Fig. 3.6), line 4 (Fig. 3.7) and line 5 (Fig. 3.8) and

each are running north-south. The length of line is along the x-axis with elevation on the

primary y-axis and time in nanoseconds on the secondary y-axis. The collection of black and

white traces indicate the depth of overburden as GPR profiles do not show bedrock very well,

only topsoil.

Fig. 3.5 – GPR profile with Dewow, Background Subtraction and Automatic Gain Control. The single GPR traces are seen in black and white and are combined in EkkoView Deluxe to produce a profile.

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Fig. 3.6 – GPR profile for Line 1

Line 1 North South

26

Line 4

Fig. 3.7 – GPR profile for Line 4

North South

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Fig. 3.8 – GPR profile for Line 5

Line 5 North South

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Chapter 4 –Interpretations and Discussions

The geophysical surveys showed a wealth of buried subsurface features that would have gone otherwise unnoticed from simple above-ground observations. For the purpose of this section the three survey lines that were conducted will be broken up and the different survey results will be compared, as the comparison of multiple survey types allow for the accurate characterisation of subsurface findings.

4.1 – Line 1

Line 1 was taken on the eastern side of the mound running north-south. ERT and GPR surveys were conducted on the line also running north-south.

The electrical resistivity survey done on line 1 (Fig. 3.2) indicated a relatively shallow overburden ranging from 0.9-2m in thickness. Within this overburden there is quite a low-to-medium range of resistivity values from 45.6 ohm metres (Ωm) to just over 160Ωm, shown by the dark blues, and greens on the colour scale. These ranges are indicative of clay/gravel-rich soils characterised by the extent of glacial till in the area (Waddell, Fenwick & Barton, 2009). The dark purple region indicates bedrock, which has been interpreted as limestone (Barton & Fenwick, 2005). Limestone has quite a high variability of resistivity values depending on how weathered it is (Mussett & Khan, 2000), and this limestone is thought to be karstified due to its relatively low values, never exceeding 1000Ωm throughout the survey area. This is further supported by the presence of heavily weathered, ooidal limestone seen in the northern quarry area (Fig. 1.2). No discernible archaeological or geophysical features were seen on the ERT profile.

The GPR profile (Fig. 3.6) simply reflects what was seen in the ERT section, including the depth to bedrock characterised by the lack of signal, except for some areas where the signals reflect to depths greater than 3.5m. A very faint dipping anomaly can be seen the northern end of the profile, which may be as a result of the reflections being skewed by something, possibly the extended section 3.5m in depth, or it could be the very edge of a feature just out of sight on this line.

4.2 – Line 4

Geophysical surveys were done over line 4 in order to take in the mound structure and subsurface as well as briefly touching on the western ramp structure. All lines were run north-south.

The ERT profile (Fig. 3.3) shows a much more variable and erratic subsurface than was seen in line 1. The overburden is much thicker atop the mound, ranging in thickness from 1.6m to over 7m at its thickest. This range of thickness may be attributed to anthropogenic use, as it is

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thought that the past inhabitants of the mound built up the topsoil on the pre-existing lower structure using materials in the vicinity of the mound, to the height that can be seen today (Waddell, Fenwick & Barton, 2009). This human influence can also be attributed to the vast range of resistivity values seen within the mound structure, as material used to build up the mound would have simply been thrown on top of the mound and would have been mixed. The range of values includes lows of 25.6Ωm up to 516.05Ωm plus within the overburden itself, and support the use of glacial till material like clays and gravels taken from the mounds environs. Again, the dark purple section is limestone bedrock with slightly higher values than line 1, reaching 819.17Ωm at its peak. Some curious features can be seen on the ERT profile, such as the tip of the western mound, but the most striking feature is the pocket of very high resistivity surround by much lower resistivity material to the southern end of the line. This area has an extremely high resistivity that could be interpreted as an air-filled (or partially filled) chamber, as air is a very poor conductor so would have very high values. This may represent a possible burial chamber of some kind.

The GPR profile for line 4 (Fig. 3.7) supports the presence of this air pocket in the same region by a cluster of traces known as a parabola, which signals possibly the top or “ceiling” of the chamber. However the GPR trace shows a differing depth to bedrock from the ERT line as it is more uniform in thickness throughout, with the exception of a slightly thicker section at the very southern region of the profile. This may be as a result of the poor depth penetration of the 250MHz antenna used, which doesn’t see far below 3-4m. One interesting feature seen on the GPR trace which is not evident on the ERT profile is another parabola-like feature to the left of the chamber, and a slightly less visible feature further to the north (Fig. 4.1). This feature could be the remains of an ancient ditch or ring-structure that was once visible on the mound and may have acted as a defence structure or wood wall foundation. Evidence for this can be seen on a magnetic gradiometry survey that was done on the mound by Barton & Fenwick, 2005 (Fig. 4.2). On this survey, two concentric ring structures were seen within the mound itself, with the larger of the two being 32m in diameter. The diameter of this ring correlates with the distance seen between the two features on the trace.

Fig. 4.1 – Possible ring stricture seen on GPR trace from line 4

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4.3 – Line 5

Line 5 was carried out on the eastern side of the mound similar to line 1 but much closer to the mound itself. The original line ran north-south but instrumentation problems led to the line being re-done north-south and also south-north, which was reversed in EkkoView Deluxe.

Line 5 shows a similar trend to that seen in line 1 in terms of its ERT profile (Fig. 3.4). An even shallower depth to bedrock is seen here but is quite uniform in thickness throughout, only ranging from 0.9-1.3m in thickness, and no major features can be seen. The resistivity values have a slightly narrower range than was seen on line 1, however the values here are much higher overall, with the minimum value being 70.2Ωm (compared to 45Ωm on line 1) and the maximum value at 944Ωm (compared to 613.52Ωm). These values coincide with a magnetic conductivity test (Fig. 4.3) that was conducted on the area around the mound (Naessens, 2013). Conductivity is the inverse of resistivity, which means that high resistivity is equal to low

Fig. 4.2 – Gradiometry map of Rathcroghan with blown up section on the mound showing concentric rings (outer circle is the mound proper)

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conductivity, and vice versa. This can be seen on the scale on Fig. 4.3, where the area to the west of the mound has a much lower conductivity (i.e. a higher resistivity) than the eastern side. This could be due to a number of reasons such as a more saturated soil being present on this side of the mound. Also to note the mound has a variation in values, further supporting the theory of the building up of the mound.

The GPR profile for line 5 shows approximately the same depth to bedrock as the ERT line, especially the southern end of the line, but most of the profile is obscured by diagonal dipping traces (Fig. 4.4). These anomalies can be seen to continue to depths of over 5m and have reflected very strongly, which means there is something in the subsurface creating these traces. By looking at the previous gradiometry survey done in the area, a number of features found on this survey are seen to interact with survey line 5. By looking at Fig. 4.5, a possible entryway to the mound facing due east can be seen in the area as two parallel lines, as well as a cluster of much smaller, closer parallel lines running roughly northeast-southwest just off the western side of the mound that have been interpreted as ancient cultivation plots (Barton & Fenwick, 2005). These features, and in particular the cultivation plots, would cause the GPR radio waves to be disrupted as they came into contact with these subsurface reflectors and were reflected back obscurely, resulting in the diagonal anomalies seen on line 5.

Fig. 4.3 – Magnetic conductivity survey of Rathcroghan mound and its environs

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Fig. 4.4 – GPR profile for line 5 showing diagonal dipping anomalies marked in yellow

Fig. 4.5 – Magnetic gradiometry map showing line 5 (green), with a blown-up section showing the possible entryway (yellow) and the ancient cultivation lines (white) 33

Chapter 5 – Conclusion

Rathcroghan mound and its environs show a wealth of archaeological features within the subsurface that would not have been identified without the use of geophysical techniques like ground penetrating radar and electrical resistivity tomography. These non-invasive techniques are the ideal survey candidate for this area, as Rathcroghan mound is a protected site and no excavations are permitted.

The use of more than one survey type is a key point to the success of this project. By using multiple surveys, a better picture of the subsurface can be obtained as opposed to using just one method. For example the ring structure that was seen on the GPR profile was unseen on the ERT line, further increasing the value of various survey types.

In terms of future work, I would like to go back to the site and run some surveys perpendicular to the survey lines taken to try and get a better image of the subsurface within Rathcroghan mound, especially over the area with the possible air chamber and ring structures.

Acknowledgements

I would like to thank my supervisor Dr. Eve Daly and also Yvonne O’Connell for all their help in the field and in the processing stage back in NUIG, along with Shane Rooney. I would also like to thank Dr. John Murray and all the Earth and Ocean department for all their help and guidance throughout the year. And finally I would like to thank my project partners Bláthnaid McKevitt, Sarah Bergin, Alida Zauers, Caue Hess and Anselmo Ruy Zuqui.

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Appendices

Figure List

Fig. 1.1 – 1.3: Taken from own photos obtained on site, 2014

Fig. 2.1:

http://www.epa.gov/esd/cmb/GeophysicsWebsite/pages/reference/methods/Surface_Geophys

ical_Methods/Electrical_Methods/Resistivity_Methods.htm

Fig. 2.2: Adapted from Looking Into The Earth, Mussett & Khan. 2000

Fig. 2.3: Gibson, Lyle & George. 2004. Application of magnetometry geophysical techniques

for near-surface investigations in karstic terranes in Ireland.

Fig. 2.4 – 2.5 & 2.7: NEAL, A. 2004. Ground-penetrating radar and its use in sedimentology:

principles, problems and progress.

Fig. 2.6:

http://www.epa.gov/esd/cmb/GeophysicsWebsite/pages/reference/methods/Surface_Geophys

ical_Methods/Electromagnetic_Methods/Ground-Penetrating_Radar.htm

Fig. 2.8: PELLICER, X.M. & GIBSON, P. 2011. Electrical resistivity and Ground Penetrating

Radar for the characterisation of the internal architecture of Quaternary sediments in the

Midlands of Ireland

Fig. 2.9: Aerial photograph edited using QGIS

Fig. 2.10: http://jeeg.geoscienceworld.org/cgi/content-nw/full/16/3/115/EEGO160303F05

Fig. 2.11: Taken from own photos obtained on site, 2014

Fig. 2.12 – 2.13: Picture courtesy of Joe Fenwick, NUI Galway.

Fig. 3.1-4.1 & 4.4: Taken from own data processing

Fig. 4.2 & 4.4: Edited from - BARTON, K. & FENWICK, J. 2005. Geophysical Investigations at

the Ancient Royal Site of Rathcroghan, County Roscommon, Ireland

Fig. 4.3: NAESSENS, E. 2013. A geophysical investigation into the geology and archaeology

of Rathcroghan mound, Tulsk, Co. Roscommon, Ireland. Final Year Thesis

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