geophysical investigation of rathcroghan mound
TRANSCRIPT
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
10
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)
13
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
14
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)
15
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
17
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
20
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.
21
Fig. 3.2 – ERT profile for Line 1
22
Fig. 3.3 – ERT profile for Line 4
23
Fig. 3.4 – ERT profile for Line 5
24
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.
25
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
27
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
35
References
BARTON, K. & FENWICK, J. 2005. Geophysical Investigations at the Ancient Royal Site of
Rathcroghan, County Roscommon, Ireland. Archaeological Prospection, 12, 3-18.
DAILY, F., RAMIREZ, A., BINLY, A. & LABRECQUE, D. 2004. Electrical resistance tomography.
Leading Edge. 23 (5). 438-442
FENWICK, J., BRENNAN, Y. & DELANEY, F. 1996. The Anatomy of a Mound: Geophysical
Images of Rathcroghan. Archaeology Ireland. 10. 20-23
GAFFNEY, C. F. & GATER, J. 2006. Revealing the buried past: geophysics for archaeologists.
Tempus.
GIBSON, P. J., LYLE, P & GEORGE, D. M. 2004. Application of magnetometry geophysical
techniques for near-surface investigations in karstic terranes in Ireland. Journal of Cave and
Karst Studies. 66. 35-38
LEUCCI, G. 2006. Contribution of Ground Penetrating Radar and Electrical Resistivity
Tomography to identify the cavity and features under the main church in Bortugno (Lecce,
Italy). Journal of Archaeological Science. 33. 1194-1204
MUSSET, A. E. & KHAN, M. A. 2000. Looking into the Earth. Cambridge University Press.
NAESSENS, E. 2013. A geophysical investigation into the geology and archaeology of
Rathcroghan mound, Tulsk, Co. Roscommon, Ireland. Final Year Thesis. National Institute
of Ireland, Galway.
NEAL, A. 2004. Ground-penetrating radar and its use in sedimentology: principles, problems
and progress. Earth-Science Reviews. 66. 261-330
36
NEGRI, S., LEUCCI, G. & MAZZONE, F. 2008. High resolution 3D ERT to help GPR data
interpretation for researching archaeological items in a geologically complex subsurface.
Journal of Applied Geophysics. 65. 111-120.
OGLIVY, R., MELDRUM, P., CHAMBERS, J. & WILLIAMS, G. 2002. The use of 3D Electrical
Resistivity Tomography to characterise waste and leachate distribution within a closed
landfill, Thriplow, UK. Journal of Environmental and Engineering Geophysics. 7. 11-18.
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. Journal of Applied Geophysics. 75. 638-647.
Tucker, M. E. 2001. Sedimentary Petrology. Third edition. Blackwell.
WADDELL, J., FENWICK, J. & BARTON, K. 2009. Rathcroghan: archaeological and
geophysical survey in a ritual landscape. Wordwell Press.
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