PurposeThe purpose of the seismic survey is to determine and
interpret the geophysical measurement i.e. depth and subsurface
topography of the Quaternary and Tertiary sediments over Paleozoic
Cambrian rock at Ty-Cerrig Farm, Morfa Harlech, Gwynedd, North
Wales (GR: 258570.1, 334298.7). The aim is extended to study the
spatial and lateral formation of Mochras fault system which forms
the boundary between Quaternary and Tertiary sedimentary topography
to the west and Paleozoic Cambrian rock to the east. The latter
determines the location of the Paleo-valley which is discovered
from the previous boreholes as well as gravity and seismic survey
(cited Blundell et al, 1969). The current survey also acts to
determine the extension of tertiary and quaternary layer depth on
the survey location which can prove the existence of the
paleo-valley.Geological BackgroundThe survey is conducted at local
farm called Ty-Cerrig which is located at Morfa Harlech, Gwynedd,
North Wales (see figure 1). The area is an English countryside
setting with northern side stands a Paleo-Island hill called Gwrach
Ynys (in Welsh Ynys is literally means island which interestingly
relates the hill to the sea surrounding setting few centuries ago)
and further south and east are surrounded by Cambrian mountains
range (cited Allen and Jackson 1985).
Figure 1: The location of Ty-Cerrig in Morfa Harlech, Gwynedd,
North Wales. The location is located at British Ordnance Survey of
258570.1 easting and 334298.7 northing in small red square. (Map
source: http://digimap.edina.ac.uk/)The geological properties
surrounding the survey area consist of Cambrian grits and volcanic
rock to the east and Quaternary and Tertiary sedimentary to the
west. The distinctive regions are separated by Mochras fault which
runs through north-south direction (cited Allen and Jackson, 1985).
The previous boreholes survey indicates the existence of channel
runs southward from Tremadog Bay (west of the survey area) which
creating bay and lagoon in the vicinity. The boreholes evidence
also shows the existence of peat soil (lignite) which indicates the
establishment of fauna estuary during Quaternary age. However,
following the de- glaciation of Welsh Ice and Irish Ice around
5000MA, the sea-level increased and inundated the delta-channel
system. Over time, the glaciation process deposited glacial clay
and overlaying the Paleo-Cambrian valley which runs aligned to the
pre-existed valley (cited Woodland A.W. 1971). By finding the
glacial composites like silt and clay from the Quaternary age over
a dipping valley-shaped Paleo Cambrian rock, the existence of
delta-channel system few millennia away will be proven in this
survey.
Figure 2: The figure shows the result from the Mochras borehole
during Quaternary period. The boulder clay and varved clay proves
the existence deglaciation and delta-channel system during the
period. (cited Woodland A.W. 1971 and Allen and Jackson,
1985)Recent geological survey in 2013 proves the earlier borehole
hypothesis when they conducted a seismic survey near Cardigan Bay
(cited S. Hesselbo et. al. 2013). The seismic reflection survey
shows the existence of deposition sedimentary during the era of
Mesozoic until Quaternary over the Paleo-Cambrian valley as
depicted in Figure 2.
Figure 2: The figure shows the topography map of seismic survey
conducted in 2013 at the previous Mochras borehole. Noted the
Mochras fault divides the Paleo-Cambria rock to the east and
Quaternary sediments to the west. This proves the existence of
channel ran southward created fauna estuary during Tertiary and
early Quaternary period (cited Allen and Jackson, 1985 and S.
Hesselbo et. al. 2013).TheoryThe refraction method implies the
measurement of the seismic waves travel times generated by an
impulsive energy source such as hammer or weight drop. The wave is
refracted from the layers underneath based on the Snells Law where
wave will be diffracted at certain angle when propagating through
different density layers. Furthermore, the density associated with
the layer governs the speed of the propagating wave. Equation 1
shows the relationship between the refracted angle and speed of the
wave:
(Equation 1)
Where 1 is the angle of incidence and 2 is the angle of
refraction. The wave refracted is transferred through energy then
detected by geophones, amplified, and recorded by special equipment
designed for this purpose called geodes. The instant of the energy
reaching the device is recorded as arriving pulses. The raw data,
therefore, consists of travel times and distances, and this
time-distance information is then manipulated to derive the
velocity variations with depth. The process of the wave propagation
till the recording is illustrated in Figure 3.
Figure 3: The figure shows the travelling wave propagates
through different layers with different speed. The wave is
refracted and eventually reflected back to the surface. The
geophones pick the energy refracted and store them as a travel time
impulse. (Source:
http://www.engr.uconn.edu/~lanbo/G228Lect0604Refract.pdf, 2014)
From the direct wave, the information about the velocity of the
first layer can be derived using Equation 2. (Equation 2)
where m is the slope of the line and also representing the
slowness of the direct wave. For the refraction survey, the
critical angle (minimum angle for refractions to take place) is the
fundamental to derive the formula relating the speed of wave
propagation and depth. As such, the critically refracted wave is
presumed to travels along the boundary between two layers with
different velocity properties. As it travels, the wave releases the
energy to the upper layer in form of seismic wave, travelling
upward at critical angle and detected as the first arrival in each
geophones. These first arrivals are widely known as the head wave.
(Cited Redpath 1973) There will be distinctive slopes appear at any
given refractor in the seismic records associated with the head
wave arrival which more information such as time intercepts and
slowness (inverse of velocity) can be derived. The depth of the
layer with the respective velocity can be calculated based on the
Equation 3.
(Equation 3)
where T is the intercept time for the nth refractor layer. Vn is
the speed of the nth layer, Vj is the velocity for n-1 layer, hj is
the depth for n layer and x is the calculated depth. From equation
3, the depth can be determined when all parameters involved derived
from the head wave slope.
Overall, the preceding cases apply on the assumption that the
boundary layer consists of infinite horizontal planar. However, not
all boundaries in the real world consist of infinite horizontal
planar. In fact, most boundary layers dealt in the real world
consist of undulating boundary which needs specific rule in dealing
with them as portrayed in Equation 4 and Equation 5. The
derivations of these equations come from the simple numerical
method called Palmer Generalized Reciprocal Method (GRM).
(Equation 4)
(Equation 5)where tv is the time velocity function which
corresponds to the time taken for wave to travels from the surface
refractor to the geophones and tg is the corrected time depth for
any given XY spacing and apparent velocity. The travelling wave
time over the refractor layer are better depicted in Figure 4.
Figure 4: The GRM is a method to find the optimum XY spacing for
the corresponding time-depth function on the undulating boundary.
The tv and tg are plotted and the straightest curve for tv
corresponds to the optimum XY. From the curve, the apparent
velocity vn is determined by inversing the slope. tv and tg
parameters e.g. tB1 and tA2 are obtained from the seismic shot time
vs offset. (Source: Clark. R, 2014)
Equation 4 and equation 5 are the manifest of one of the most
important tool when dealing with undulating boundary. The GRM
delineates the undulating refractors by recording forward and
reverse deepest travel time. These travel times will be used to
find the optimum XY spacing (geophones spacing) which the upward
travelling rays geometrically emerge from a single point on the
refractor layer (cited D. Palmer, 1981). The GRM method produces
Equation 4 and 5 which are used to create smooth undulating
boundary on the refractor. Inevitably, the method can be only used
successfully when two conditions are fulfilled (cited D. Palmer,
1981):
a) The head wave refractor calculated using the Hawkins
Time-Depth Method must be the deepest layer detected.b) The reverse
coverage shots are implemented which means both forward and
backward shot at the same site recorded.
Later, the depth for the refractor is then determined from the
Equation 6.(Equation 6)Where z is the calculated depth, vn is the
apparent velocity, and v1 is the velocity for the first layer. The
velocity for the nth layer is determined by Equation 7.(Equation
7)where is the dipping angle for the refractor.
Survey DesignThe survey is conducted to determine the depth and
topography of the Quaternary- Mesozoic sedimentary over Paleozoic
Cambrian rock. With the amount of resources available with the
survey team, the decision is made to apply 188m spread line with 4m
geophones spacing. The survey layout on the field is depicted in
the Figure 5.
Figure 5: The survey design at the location. The survey design
is marked with the red line on the map. The survey line runs from
east to west. The line spread is 188m with geophone spacing 4m. The
total numbers of 48 geophones are used on the survey line. There
are forward-reverse offset shots at -80m and 286m to map deeper
refractor. The British Grid coordinates for the offsets are denoted
in the legend. The full BG coordinates for each geophone are
provided in Appendix A. (Map source:
http://digimap.edina.ac.uk/)Instrumentation
Figure : The setup of the instruments during the survey.The
survey uses two 24-channel geodes (seismic recorder) to records the
seismic signal from the geophones. A software package called
Multiple Geode Operating Software(MGOS) is used to operate the
Geodes. Table 1 lists out all the equipments needed to conduct the
survey.InstrumentsQuantity
Geodes2
Geophones48
Trigger cable and strike plate1
Towed Elastic Weight Drop 40kg1
Seismic Cables (reels)4
Ethernet Cable (reels)2
Toughbook1
Battery6
Hammer1
Table 1: The table shows the equipments needed for the
survey.
Shots Technique
Figure 6: The shots location with the respective SEG-Y file ID
(Source: Clark R. 2014)The reverse-forward shot technique is used
to comply with the condition of GRM and to find dipping at any
refractor. The procedure begins with the laying of 48 geophones out
on the field connected through 2 geodes in a straight line
according to the Figure 6. The geophones spacing is set to be 4m
apart. The totals of 6 shots using elastic weight drop and hammer
have been conducted with different source locations. The last shot
uses a hammer for shorter 6m spread to record the direct wave. The
table 2 shows the different shots denoted with various SEG-Y
filename and the respective source location. Multiple stacks have
been used for each shot to improve S/N. The average of 6 stacks per
shot has been recorded during the survey.ShotShot TypeBG Source
Coordinates(Easting, Westing)
FFID 1101Zero offset258570.12, 334298.69
FFID 1109Far Offset 258490, 334298
FFID 1108Quarter length offset 258618, 334299.1
FFID 1106Mid offset 258666.48, 334299.52
FFID 1107Reverse quarter length offset 258710.71, 334299.9
FFID 1104Reverse zero offset 258758.67, 334300.31
FFID 1105Reverse far offset258839, 334301
Table 2: The table shows the shot ID with the respective
location coordinateGeophysical ResultsBased on Figure 7, all shots
from the first picks show 3 observable layers which have
distinctive slopes. The results are divided into 3 parts to ease
the derivation of the time intercept and slowness.
Figure 7: The graph shows the first arrival picks from the
seismic data. The first arrival picks are denoted with their
respective shot ID. The blue region shows the first pick for the
first layer, the yellow region is the first picks for the second
layer and the red region is the first picks for the third layer
(Raw SEG-Y data are in Appendix A). Geophysical Results: The Direct
Wave (Shot ID: FFID1110)
Figure 8: The graph shows the first arrival picks FFID 1110. The
shot is taken with 0.25m XY spacing to derive just the direct wave
from the survey. From equation 2, the velocity for the first layer
is equivalent to 0.22 0.17 m/ms. (Raw SEG-Y data are in Appendix
A)Geophysical Results: The Head Wave
Figure 9: The graph shows the first arrival picks FFID 1101,
FFID1104, FFID 1106, FFID 1107, and FFID 1108. The slope for the
head wave is derived from the first picks to measure the velocity
of the second layer of the survey area. The absolute average slope
for all 4 slopes data denoted as Q1, Q2, Q3, and Q4 are 0.22 ms/m.
From equation 2, the velocity for the second layer is 1.59 0.05
m/ms and the average depth for the layer 1 using Equation 3 is 1.9
0.1 m. (after Clark. R, 2014)Geophysical Results: The Undulating
Boundary
FIGURE 10: The first arrival picks for survey FFID 1105 and FFID
1109. Those surveys are the far offset forward-reverse shot at -80m
and 286m. The shots are taken to determine the depth of the
undulating boundary using GRM. The forward-reverse first break
refraction lines show they are not intercepted symmetrically at the
middle of CMP. This proves the refractor is dipping from east to
the west. (Raw SEG-Y data are in Appendix A)The third layer first
break picks show the unconformity layout of the surface refractor
which have been portrayed by uneven slope in forward-reverse far
offset shots. These undulating refractor conditions are treated
with the Generalized Reciprocal Method (GRM) to find each common
midpoint (CMP) velocity and undulating depth. Using equation 4 and
equation 5, Tv and Tg are plotted for XY=0m until XY= 40m with
interval of 4m. The values of Tv and Tg for every XY spacing
against CMP as depicted in Figure 11.
Figure 11: The graph shows Tv and Tg plotted against CMP. Based
on the statistical calculation of the graph, the least RSS error
per geophones for slope Tv goes to XY spacing equal to 16m. The RSS
per geophones for XY=16m is the least which make the line the
straightest among all XY spacing curves. Further calculation using
Equation 6 and Equation 7 derive the depth for each CMP and the
velocity anticipated for layer 3 which is 4.69 0.03 m/ms. The full
depth calculations with the CMP data at XY=16m are shown in
Appendix A. In addition, there is a dipping anticipation at the
refractor which runs from east to the west due to unsymmetrical
forward-reverse shots at midpoint. The calculation made deduced
that the dipping angle is equal to 2 degrees. This undulating
dipping sub-surface play critical role in answering the main
objective of this survey which will be further discussed in the
next section.
Discussions: Sub-surface Cross SectionComment by ee14nbhm: Check
the velocity layer data and compared the stratigraphy and deduce
the layer.Analyzed geological background and compare with the
result. Analyzed the hidden layer probabilityAnalyze the error and
reciprocity checkSuggest the improvement for the surveyReplace the
tg tv plot with the std plotBased on the data analysis, the cross
sections of the sub-surface for the 3 layers boundary are shown in
Figure 12.
Figure 12: The 3 layer sub-surface separated by the refractor
boundary. The velocity for each layer and the corresponding depth
are shown in the MATLAB graphic generator.The analysis of the
stratigraphy is conducted and the velocity for each layer is
compared with the anticipated velocity from the geological
background of the surveyed area. Figure 13 shows the comparison
between the anticipated velocity and the velocity from the
survey.
Figure 13: The survey area is compared with the data set from
the earlier report (cited Blundell gravity data profile). The
velocity for each layer in the refraction survey matches the
velocity predicted by Blundell et.al. The red square shows the
approximate location for the current refraction survey to the east
of the fault. The diagram proves the first hypothesis that
sedimentary layer overlay the Palaeozoic Cambrian and passed
through the Mochras fault. The Quaternary sediments created
post-rift basin in this fault system. The absence of the tertiary
layer in the survey area suggests that the tertiary sediments was
part of the syn-rift which is halted after the post-rift Quartenary
evolved (it might due to new smaller fault triggers the post-rift
basin system)The existence of the dipping sub-surface between the
sedimentary second layer and third layer top Cambrian rock also
proves the earlier hypothesis from the previous borehole survey
where the area was part of the delta-channel system. The existence
of the Quaternary layer also suggests that the surveyed area was
the extension of the delta-channel system basin over Palaeozoic
fault valley.Discussion: Beyond Second LayerAn analysis of the
second refractor is conducted to detect any hidden layers within
the second layer Quaternary to further refine the result to match
the multiple Quartenary layers of Mochras borehole (See Figure 2).
The critical distance for the third refractor is calculated based
on the second layer depth calculated from the GRM. This critical
distance is considered the optimum XY spacing predicted for any
given depth. The result shows the average predicted XY spacing is
30.5 meter, more than the calculated XY value of 16m from GRM. This
discrepancy between predicted XY and calculated XY spacing proves
the presence of few hidden layers within layer 2. The Mochras
borehole result in Figure 2 shows few layers consist of sand and
gravels, boulder clay, and varved clay intertwined to create the
Quartenary succession. The result from the borehole can be used to
deduce the hidden layer is due to thin layering (in the case of
sand and gravels) or velocity inversion (in the case of higher
velocity boulder clay and slower velocity varved clay).Discussion:
The Undulating BoundaryThe deglaciation process few millennia back
deposits boulder and till clay at the Quartenary period. The
deposition and glaciation process during Quartenary period also
eroded the surface of top Cambrian rock which created undulated and
unconsolidated Cambrian layer. The data from Allen and Jackson
(1985) shows the p-wave velocity for Cambrian rock formation has
the range between 3.90- 6.31 m/ms. As the top surface Cambrian
formation eroded, the layer became unconsolidated and the p-wave
velocity is expected to be in the lower range (which is proven with
p-wave velocity from seismic refraction is around 4.69
m/ms).Discussion: Error and Reciprocity CheckThe reciprocity check
has been conducted to test for consistency of the forward-reverse
shot undertaken. This step is essential to verify the forward shot
seismic data obtained are matching with the reverse shot. Moreover,
the step will further improve the accuracy and precision of the
seismic data at any shot direction taken. The reciprocity check is
vital to be conducted during the survey after each set of
forward-reverse shot taken to make sure the data acquired are
viable to be processed. The reciprocity check step is taken by
finding the intercept time for each reverse and forward shot. The
time-intercepts are then deducted and the differences are analyzed
statistically as depicted in Table 3 for this survey.
Table 3: The difference in time intercept for each reverse and
forward shots based on the shot ID. The staticstical data shows the
standart deviation for the diference are witin the reasonable
values which proves the consistency of the seismic
data.ImprovementThe hidden layer expected needs to be treated to
match the Mochras borehole result. The velocity-depth model
prediction needs to be done to lay the possible velocity for any
given depth for the second Quartenary layer. This step is essential
to refine the lithography of the second layer and give better
understanding of the rift-basin system in this area. Conclusions In
the nutshell, Reference1. Allen, P.M. and Jackson, A.A. 1985.
Geology of the country around Harlech. British Geological Survey
Memoir, sheet 149, HMSO, London, 111pp.2. Blundell, et al. 1969.
Geophysical Investigations of Buried River Valleys around Cardigan
Bay. Geol. J. Vol. 6, 161-181. 3. North Wales Geophysical Field
Class Handbook (2014 Edition). 2014. School of Earth and
Environment, University of Leeds.4. Redpath B. Seismic Refraction
Exploration For Engineering Site Investigations, NTIS US Department
of Commerce, May 19735. S. Hesselbo et. al. Mochras Workshop
Report. 2013. British Geological Survey. 6. J.M. Reynold, An
Introduction to Applied and Environmental Geophysics, John Wiley
and Sons Ltd (1997)7. Rosli S. Nawawi. M.N.M. Tonnizam E.M.
Groundwater Detection in Alluvium Using 2D Electrical Resistivity
Tomography (ERT), EJGE vol. 17 (2012).
Notes:Explain in details the approximate/expected depth based on
the previous geological survey. Why 40m is adequate?
FIGURE 11B: The absolute average slope for all 4 slopes data
denoted as Q1, Q2, Q3, and Q4 are 0.22 ms/m. From equation 2, the
velocity for the second layer is 1.59 0.05 m/ms and the average
depth for the layer 1 using Equation 3 is 1.9 0.1 m. The absolute
average slope for all 4 slopes data denoted as Q1, Q2, Q3, and Q4
are 0.22 ms/m. From equation 2, the velocity for the second layer
is 1.59 0.05 m/ms and the average depth for the layer 1 using
Equation 3 is 1.9 0.1 m.
V1=0.22 0.17 m/msh1,f=1.8 0.1mh1,f=2.3 0.1mh1,f=1.9 0.1mh1,r=2.2
0.1mh1,r=2.0 0.1mh1,r=1.9 0.1mh1,r=1.7 0.1mV2=1.62 0.05
m/ms0.06m/msV2=1.75 0.18 m/msV2=1.57 0.06 m/msh1,f=1.9 0.1m
A refraction seismic survey has been conducted at Morfa Harlech,
Gwynedd, North Wales (BNGR: 258570.1 easting, 334298.7 northing) to
study the Mochras fault system which consists of Quaternary and
Tertiary sedimentary basin over the Paleo-Cambrian Rock valley. A
seismic refraction line with 188m spread has been set up with 48
geophones and 4m spacing. A shorter 6m spread line with 24
geophones and 0.25m spacing has been set up later to cover the
direct wave. The totals of 8 forward-reverse shots at different
offset positions have been conducted to determine the topography
layers of the sub-surface. A weight-drop has been used for 7
offsets shots while a hammer is used for the shorter 6m spread. The
result is then analyzed by collecting the first break pick time
from the raw SEG-Y data and plotted against geophone spacing. The
slopes for the plot are retrieved and the velocity and intercept
time are used to derive the depth of the layer from the critical
angle equation. For undulating boundary, the General Reciprocal
Method (GRM) is used to analyze the depth and velocity of the
layer. The seismic refraction survey result shows the depth for the
first layer is 1.9 0.1m and the corresponding velocity is 0.22 0.17
m/ms. The average depth for the second layer is 32 0.1m and the
corresponding velocity is 1.59 0.05 m/ms while the velocity for the
third layer is 4.69 0.03 m/ms. The velocity analysis shows the
first layer consists of the unconsolidated weathering soils, the
second layer is Quaternary sediments, and the third layer is
weathered unconsolidated Cambrian rock. The results also prove the
existence of Paleo-valley and the extension of the Quaternary
sedimentary rift basin over the fault Paleo-valley system
underneath the survey area.