Report on Seismic Refraction Survey in Ty Cerrig, North Wales
BY
AYENI, Gboyega O.
March, 2006
CHAPTER 1
1.0
INTRODUCTION
A seismic refraction survey was carried out in the Ty Cerrig Farm to investigate
the geometry of the Cambrian rocks which underlie the quaternary and tertiary
sediments in the area. The seismic refraction line which runs approximately N-S from
the foot of the hill towards the valley was designed in order to allow for interpretation
with the Generalized Reciprocal Method (GRM). Results from previous geophysical
surveys in the area - which formed the basis for the survey design - were compared
with results from this survey. A summary of the acquisition parameters, processing,
interpretation and results of the seismic refraction survey are given in this report.
Discussion of the results obtained from other geophysical techniques (including ERT
and GPR) are also briefly discussed. A comparison of results from the refraction and
ERT surveys is also given since they were run on traverses that were fairly parallel.
1.1
LOCATION
The survey area is located in the flat-lying field at Ty Cerrig Farm (grid
reference SH 587 344), Morfa Harlech approximately 3Km North of the Harlech
Castle. Figure 1.1 shows the location of the survey area. A detailed map of the Ty
Cerrig area is given in Appendix A.1.
Figure 1.1: Maps showing location of the survey area: Ty Cerrig Farm in N. Wales.
(From www.ordnancesurvey.co.uk)
1.2
GEOLOGICAL SETTING
Ty Cerrig Farm is located east of the Mesozoic Tremadoc Bay Basin border
fault (Mochras fault) which runs approx. N-S dividing the Morfa Harlec region into
two parts. The Cambrian basement rock (known as Ynys Llanfihangel-y-traethau) is
bounded by several faults (see Appendices A.2 and A.3 for detailed geologic map
showing the geologic sequences and location of faults in the area). Tertiary and
Quaternary sediments onlap onto the basement outcrop in the study area.
A section through the Mochras Farm borehole (GR 255330 325940) gives a
complete overview of the geologic successions in the Harlec region. A detailed
description of the borehole log (after Woodland 1971) is given in Appendix A.4.
Sediments encountered by the borehole range from Triassic to Recent.
A succinct description (after Allen and Jackson, 1985) of the geologic sequence
encountered in the Mochras borehole is given below:
Upper Triassic: Sequence of sediments deposited in playa environment subject to
flash floods
Lower Jurassic: ~1305m thick marine sediments (alternations of mudstones and
siltstones) suggesting the existence of a major Jurassic basin in the area
Tertiary: Mid-Oligocene to Early Miocene fining upward cycles related to meandering
river channels and flood associated with a southward flowing river, east of the fault and
parallel to subdue fault-scarp.
Quaternary: Two tills of boulder clays (glacial deposits) separated by interglacial sand
and gravels (river deposits) and varved clays (lake deposits). The Quaternary
succession in the Mochras Farm borehole is given in Figure 1.2.
During the late Quaternary, channels were cut in the area (e.g. Figure 1.4). The
sea level was estimated to have been about 90m below the present OD to cut some of
these channels which are filled with argillaceous sediments (Blundell et al, 1964). As
an example, oxbows from a recent channel of the Dwyryd River have been infilled with
marine and estuarine sediments (mainly grey clay and sandy clay) and peat. The peat
occurs towards the present shoreline (e.g. around Ty Cerrig) with fauna suggesting an
estuarine and lagoonal environment. Aeolian sands which occur as a thick layer in the
west of Morfa Harlech cover parts of these deposits (Allen and Jackson 1985). The
isostatic sea-level rise that followed the de-glaciation reached a peak at 5000ma BP,
and is expressed in the region by drowned river valleys (e.g. Traceth Bach to the north
of Ty-Cerrig Farm).
Allen and Jackson (1985) also described a succession of Paleozoic rocks which
have been subjected to gentle metamorphism during the Caledonian orogeny in the
eastern of the Mochras fault. A detailed geology of the Harlech region is given by
Allen and Jackson (1985).
A Bouguer gravity anomaly map of the Harlech district and surrounding areas
is given in Figure 1.3. This clearly shows the contact between the Mesozoic sediments
and the high density Cambrian rocks. A buried river channel (see Figure 1.4) west of
Ty-Cerrig (approx. N-S negative anomaly) had been identified along the traverse Y-Y
by Blundell (1969). An interpretation of an aeromagnetic map of the Harlech district is
given in Appendix A1.5. The results of a Geological Survey seismic reflection survey
in the Tremadoc Bay described by Bullerwell and McQuillin (1969) is given in
Appendix A1.6.
Figure 1.3: Bouguer anomaly map of Harlech region showing the contact between the
Mesozoic-Tertiary sediments and the high density Cambrian rocks; and a buried river
channel (negative anomaly) west of Ty-Cerrig. (after Allen and Jackson 1985)
Figure 1.4: Interpretation of the seismic refraction survey (marked as Y-Y’ on Figure
1.3 and Appendix A1.2) by Blundell et al. (1969) showing Tertiary infill of a buried
river channel overlain by Quaternary sediments.
1.3
GLACIATION
The regional glacial history of the Harlech district forms a major part of the geologic
evolution of the area. Woodcock and Strachan (2000) described three major glaciations
within the last 0.5Ma. These are Anglian, Wolstonian and the Devensian which were
separated by temperate interglacial periods (GPR Girl, 2005). Table 1.1 is a summary
of these glacial events.
Epoch
Beginning
Time
(Ma)
Holocene
0.01
Late
0.11
Pleistocene 0.15
0.35
0.43
Mid
0.48
Pleistocene
Marine
Oxygen
isotope stage
1
2-5d
5e
6-10
11
12
Sequence
Climate
Flandrian
Devensian
Ipswichian
Wolstonian
Hoxnian
Anglian
Interglacial
Glacial
Interglacial
Glacial
Interglacial
Glacial
Table 1.1: Summary of the Mid to Late Quaternary Stratigraphy for the British Isles. (Bowen,
Rose et al. 1986; Lowe and Walker 1997; Woodcock and Strachan 2000). [After GPR girl,
2005)
The most recent glaciation (Anglian) was the most extensive in Britain and Ireland,
though the Late Devensian Glaciation has generally removed the effects of previous
glaciations in Wales (Whittow 1992). The limits of the Anglian and Late Devensian ice
sheets as described by Bowen et al (1986) are shown in Appendix A1.7. Various
studies showed that several channels acted as drainage conduits for the local ice caps of
Scotland, Northern England, Wales and Ireland during theses major events (GPR Girl,
2005).
Early research established that there are two Welsh glacial drifts visible on the coast of
Tremadoc Bay. GPR girl (2005) chronicled these researches and the various concepts
on the glacial advances as interpreted by early and recent researchers. The movements
of local Welsh piedmont glaciers into Tremadoc and Cardigan Bays and of the Irish
Sea ice across the Lleyn peninsula as interpreted by Garrard 1977 are shown in
Appendix A1.8.
Some recent contributions to the understanding of glacial studies in the region as
described by GPR Girl (2005) include the following:

uncoupling of the Irish Sea ice and local Welsh ice (Thomas et al, 1998) around
16ka leaving individual piedmont glaciers to retreat back into the Irish and
Snowdonia valleys respectively; and

the fact that most of North Wales was unglaciated by 13.6 – 12ka (Evans, Clark
et al. 2005) with evidence of the Loch Lomond stadial period in the cirques of
Snowdonia (Bowen, Rose et al. 1986) approximately 10-11ka (Shotton 1986).
An overview of the isostatic and eustatic effects on the distribution and re-distribution
of sediments in the Harlech region is also given by GPR Girl (2005).
1.4 Previous Geophysical Investigations
The results of regional gravity, aeromagnetic and seismic studies carried out by
Blundell, et al (1969), Allen and Jackson (1985) and various studies by the British
Geological Survey suggest that a buried river channel runs west of the survey area, with
the Mochras fault as the eastern plane of the valley (GPR Girl, 2005). (See figures 1.3
and 1.4).
Several shallow geophysical studies have been carried out on fields in the Ty Cerrig
farm by researchers and students from the University of Leeds. As shown in Appendix
A1.9, the majority of the surveys have been located on the western and central part of
the upper field (GPR Girl 2005). A summary of some of these studies is given below:
Survey type
Name
Shallow seismic and gravity
Year
completed
1993
Ground penetrating radar
1993
Mark
Johnson
Oliver
Lambert
Colour
key
Blue
Red
Seismic reflection
1995
unknown
Green
Seismic refraction
2004
Master’s
class
purple
Resistivity
2004
Master’s
class
yellow
Table 1.2: Summary of the previous surveys mentioned in the text (After GPR Girl 2005)
Oliver Lambert 1993
Oliver Lambert conducted a series of profiles using shallow reflection, seismic
refraction and gravity methods. The seismic reflection surveys (shown in blue on
Figure 1.11) were completed using a hammer and plate source and a buffalo gun source
(for line ty93-01 only), with geophone and shot spacing of 2m and a near offset of 4m.
One of the gravity profiles was conducted along the line ty93-01 using LaCoste &
Romberg gravimeters with a station spacing of 10m and 20m.
Lamberts interpretation from the depth converted reflection line ty93-01 indicates that
the Cambrian basement dips underneath Quaternary sediments in a southerly direction
reaching a depth of around 80m in the lower field where it appears to level out. Figure
1.12 displays the results from modelling the gravity anomaly using the depths found in
the reflection survey. Line ty93-03 (the closest to the eastern side of the field) also
shows this southerly dip within the upper field, but a depth section was not provided
(see Figure 1.13).
Seismic survey 1995
In 1995 a seismic reflection survey was conducted along the south end of the upper
field (shown as the green line, ty95-21 on Figure 1.11). Unfortunately the only
information available on the acquisition of this data set is the location of the profile
which as CMP points 100 on the west end of the profile and 400 on the east end. Figure
1.14 displays the migrated seismic section that had a 60% velocity field applied to it.
The orange line indicates a possible interpretation for the location of the basement
reflector that shows the reflector dipping upwards at the edges. At the west end this due
to being closer to the Cambrian outcrop.
Figure 1.12: Oliver Lambert’s gravity modelling using the depths from the seismic line ty93-01b
(depths are relative to sea level).
Figure 1.13: Non migrated seismic section of line ty93-03 which is the closest seismic line to
survey area. The orange line represents Lambert’s interpretation of the basement reflector and
the red line indicating a possible fault location.
Figure 1.14: Migrated seismic line ty95-21 collected by the Masters Class in 1995. The orange
line shows a possible interpretation of the basement interface. (Vertical axis is time in msec).
Depth Profile
0
10
20
30
Offset (m)
40
50
60
70
0
5
Depth (m)
10
15
20
25
30
Figure 1.15: Results from the seismic refraction survey conducted in 2004 using General
Reciprocal Method to calculate depths to the basement. The black line represents calculated
loci depths with the associated errors marked as crosses (Williamson 2004).
Master’s class 2004
1) A seismic refraction survey was conducted on the north-west part of the upper field
(see purple lines in Figure 1.11) using a combination of a hammer and plate source and
a buffalo gun source. This consisted of an 89m north-south line which was the
combination of four 1m geophone spacing profiles (starting from grid reference SH
58565 34432) with a perpendicular line (crossing at 45m) which was 115m long and a
5m geophone spacing. The data was processed using a planer interface model for the
top layers and an undulating model (via the General Reciprocal Method) for the
basement interface.
The results of the north-south line indicate that the basement dips underneath
Quaternary sediments (Figure 1.15 shows the resultant geological section completed by
Williamson 2004) with no indications of Tertiary sediments (Figure 1.15 shows the
resultant geological section completed by Williamson 2004).
2) On the south-east area of the upper field a resistivity tomography survey was
performed, which consisted of two parallel lines, 180m long with a bearing of 264 o
from true North (line 1 started at grid reference 58845 34284 and line 2 was 20m north,
parallel to line1; see yellow lines on Figure 1.11). The data was acquired using an
initial electrode spacing of 3m with a Wenner Array set up (See appendix C for basic
theory behind resistivity acquisition).
Figures 1.16 and 1.17 show the inversion results from using finite element modelling
and logarithmic apparent resistivity. Both sections show high resistivity values at the
surface representing dry sandy soil, then below the water table there is large mass of
relatively constant resistivity that represents the sand and gravels of the Quaternary and
Recent sediments. Below the Quaternary sediments there is another resistivity
boundary that could represent the top of the basement rocks (see appendix C for a table
of resistivity values for common geological material).
Figure 1.16: Least squares inversion of resistivity line 1 with vertical scale as Depth (m) verses
distance (m).
Figure 1.17: Least squares inversion of resistivity line 2, 20m north of line 1.
Mark Johnson 1993
Mark Johnson conducted a ground penetrating radar survey in conjunction with Oliver
Lambert’s seismic survey, using 70 MHz antennae at a constant offset of 1m spacing,
in a continuous survey mode (shown as the red lines on Figure 1.11). Data was
collected in a course grid of 20m spacing with dip and strike lines (see Figure 1.18).
Figure 1.18: Layout of the GPR grid conducted by Mark Johnson.
Once simple processing of the data had been conducted, depth conversion was
attempted using a constant overburden velocity of 0.06 m/ns which is typical of
saturated sands (see Section 2.1). Figure 1.19 presents Johnson’s interpretations from
his GPR data which indicate that the basement dips southwards to depths of around
22m, about 50m along the line DIP1. Figure 1.20 shows a depth to basement contour
map.
Figure 1.19: Interpretations of Johnson’s GPR
lines. The top picture is of the line DIP 1 that runs along Lambert’s seismic line ty93-01. The
picture on the left shows the interpretations for the strike lines and the picture on the right
shows the interpretations of the dip lines 2-4.
Figure 1.20: Depth to basement contour map.
Appendix A1.9 : Diagram showing the locations of the surveys conducted by previous
students. The yellow and purple lines represent the resistivity and seismic refraction surveys
conducted by Masters Class 2004/2005, the green represent the seismic reflection survey
conducted in 1995, the blue represent the seismic reflection survey conducted by Oliver
Lambert 1993 and the red represent the 75 MHz GPR survey conducted by Mark Johnson
1993. (After GPR girl 2005)