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.