Seismic refraction study investigating the subsurface geology at Houghall Grange, Durham, Northern England

CHARLIE KENZIE Department of Earth Sciences, University of Durham 2013

1. INTRODUCTION

1.1 Purpose of study Seismic refraction is used to investigate the subsurface geology in the area of Houghall Grange. Analysis of the seismic data is used to ascertain the depth to refracted layers and is compared against hypothesized layers in the near sub-surface taken from borehole and leveling data. This paper also provides an introduction to the use of seismic refraction methods and the interpretation of seismic data.

1.2 Geological setting The geology of the surrounding area is indicated by the stratigraphy found at Houghall Pit, a disused coalmine, situated approximately 200m away from the survey profile (Fig.1). The coal shafts suggest that a basement of Carboniferous sands and limestones, at an approximate depth of 15m,

are overlain by drift deposits of silts, clays and conglomerates, which in turn are overlain by a thin layer of topsoil (Shirlaw, 1964). Additionally, the water table is assumed to be the same height as the pond, which is located just to the south of the survey (Fig.1).

2. SEISMIC SURVEY

2.1 Profile geometry The refraction profile was set up with a conventional field geometry, which involved a line of receivers (geophones) connected to a centrally positioned seismograph. A total of five shots were fired, two at the ends of the profile, one in the centre, and two at quarterly points (Fig.2). The end shots were fired at the very end of the profile with the end geophones being moved inward by half an interval (3m). This method allows the most accurate analysis of reciprocal times since the shots are located exactly on the ends of the profile line (Underwood, 2009).

The target depth of the survey is only around 20m, and since a length of profile approximately five times this value is sufficent (Kearey & Brooks 1984), the

Fig.2 Sketch of the seismic profile geometry showing the shot positions and the 24 receivers with a spacing of 6m. Geophone spacings are shown for firing a shot at S4 note that the end geophone (24) has been moved inwards by half an interval (3m).

Fig.1 Survey profile and the location of the pond to the south. The disused coal shafts are approximately 200m to the south of the profile. Durham city is 2km to the NE. © Crown Copyright Ordnance Survey, An EDINA Digimap/JISC supplied service.

ABSTRACT Interpretation of seismic data surveyed at Houghell Grange was carried out using the plus minus method to calculate the depth to the refracted layers. The results show large discontinuities with geological data. The calculated depth to the basement feature is much larger than depths suggested by borehole data. Additional analysis of the data reveals that the data is attributed with large uncertainties and consequently the accuracy and reliability of the results are diminished. It is suggested that the “hidden layer” or continuous velocity problems may have caused errors in the data. In reality, the inaccuracy of the results could have been caused by a variety of possibilities including poor data analysis and computation. This highlights the need for further and more detailed study and more rigorous data analysis.

s3 s4 s5

geophones were spaced at 6m intervals to give a total profile length of 138m (Fig.2).

2.2 Results First breaks of headwaves were generally eyeballed from the seismic data. However, some traces were particulary affected by noise and data from geophones 19 onwards were especially noisy, and were consequently disreguarded. The data from the first 18 geophones is shown below in Table 1.0. The data from each shot, including the forward and backward times, are plotted on plate 1, Fig.3, and the respective lines of best fit are added to each data set. In contrast to geological data, the time graph suggests a two layer case.

2.3 Plus minus method For shallow depth surveys, subsurface layers can be assumed to be near planar, however, the surface of the earth is not planar and the natural relief of the ground causes irregular travel time segments (fig.2). Thus, normal analysis of reciprocal times is unsuitable and the use of the more elegant plus minus method is utilised. If we consider our profile, with four shots s1, s2, s3 and s4 or A, B, C and D respectively (Fig., the minus time between

shots A and B (A/B) is simply given by subtracting the travel times or

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T−( )AB = TA −TB  Similarly for shots B and C (B/C)

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T−( )BC = TB −TC  The minus time graphs for A/B, B/C, and C/D are plotted on plate 1 Fig.4, shots A/C and B/D are shown in Fig.5 and between shots A/D is shown in Fig.6. For a second layer refractor, with dip α, the gradient m of the minus time graph is given by

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m =2cosαV2

 

since we are assuming sub-horizontal layers, cosα is approximately equal to one, leading to the gradient m of the minus graph

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m =2V2

 

In some cases the ends of the minus time graphs show a different gradient, and in the second layer case this is the gradient of the combined V1 and V2 velocities

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m =1V1

+1V2

 

To calculate the plus times we consider a profile with a single geophone G with two shots fired, A and B, at each end of the profile (Fig.6). The plus time GT+ is given by

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GT+ = TAG +TBG −TAB additionally, the plus time for a three layer case is also given by

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GT+ =2z1 V3

2 −V12

V3V1

+2z2 V3

2 −V22

V2V3  

2.4 Velocity of the first layer Although at first glance the travel time graph suggests a two-layer case, it is more likely that, in reality the subsurface is made up of three layers. Geological data indicates that the top layer is comprised of poorly consolidated alluvium deposits and topsoil, and it is probable that such a layer would only accommodate extremely low seismic velocities. A low velocity layer such as this would thus not be shown on the time graph, since first arrivals of the second refractor V2 would reach the receivers before the head waves from the first layer. If we assume that the first layer has a significantly lower velocity than the layers above it then further

Geophone number

Distance (m)

S1 (ms)

S2 (ms)

S3 (ms)

S4 (ms)

1 6 16 31 50 62 2 12 16 25 47 59 3 18 18 23 45 58 4 24 23 20 43 57 5 30 26 18 40 52 6 36 28 12 36 47 7 42 31 13 34 46 8 48 32 15 31 42 9 54 35 12 25 40

10 60 38 21 24 35 11 66 41 26 22 36 12 72 49 32 21 38 13 78 52 40 23 35 14 84 55 - - - 15 90 53 40 28 27 16 96 51 42 31 25 17 102 - 43 40 22 18 108 61 44 35 18

Table 1.0 Seismic data shown for the first 18 geophone stationzs

(1)

(2)

(3)

(4)

(5)

(6)

(7)

PLATE 1

S1 S4

S2 S3

Fig.3 Time travel graphs plotted with data from receivers 1-18 with fired shots S1, S2, S3 and S4.

Fig.4 Minus time graphs for A/B, B/C and C/D. Gradients are shown for each line as part of the displayed equations.

Fig.5 Minus time graphs for A/c, C/D. Gradients are shown for each line as part of the displayed equations.

Fig.5 Minus time graph for data between shots A/D gradients of each line shown included in displayed equations

Fig. 6 General set up geometry for seismic profile with shots A and B and geophone G. Minus times given by Equations 1 & 2, plus times given by Equations 6 & 7.

analysis of the travel time graph shows that the maximum velocity of the first layer V1 is 150 ms-1. We continue the data analysis and interpretation assuming a very low first layer velocity.

2.5 Second and third layer velocities The second and third layer velocities are calculated from the minus time graphs shown in Figs. 4-6 above. Minus times plotted in Fig.4 give the velocity for the second layer, taking the average gradient, the calculated velocity of the second layer V2 is 2000 ± 100 ms-1. Similarly, minus times plotted in Figs. 5 and 6 give the velocity of the third layer V3 and is calculated to be 2250 ± 150 ms-1.

2.6 Depth profile The depth to the refractors at each geophone is given by rearranging Equations 6 and 7. However, since the velocity of the first layer V1 is significantly smaller than that of V2 and V3, the plus time for the second layer refractor can now be given approximately by

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GT+( )V2 ≈2z1V1

 

and therefore for the third refractor

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GT+( )V3 ≈2z1V1

+2z2 V3

2 −V12

V2V3

 

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≈ GT+( )V2 +2z2 V3

2 −V22

V2V3

Additionally, if we assume that the first refractor marks the boundary of the water table and if we assume that the water table is the same height as the water level in the pond, we can estimate the likely depth of the first layer z1 as being equal to that of the water level in the pond.

Leveling data is shown in Table 2.0 opposite and displays the elevation of each geophone station and the relative elevation to the hypothesized water table. Some geophone stations have a lower elevation than the water table, suggesting that generally, the water table is at the surface and indicating that z1=0. The depth to the second refractor is calculated from plus times, taken from the travel time graph (Fig.2), and then by utilizing Equation 10 above. The results are shown alongside the leveling data.

3. DISCUSSION

3.1 Lithology of layers Table 3.0 below shows typical shear wave velocities through some common lithologies. The velocity of the first layer is assumed to be very low, less than 150 ms-1, which is typical of highly unconsolidated topsoil (Reynolds, 1997). The velocity of the second layer is in the order of 2000 ms-1, a value that could be accounted for by a number of stratigraphic units (table 3.0). From the seismic velocity

(8)

(9)

(10)

Table 2.0 Leveling data for each geophone station and also the depth to the second refractor z2 as calculated by the plus minus method.

Fig.6 Depth profile of the layers. Surface topography is shown the firs red line, with the datum taken as the water table (blue line). The water table is assumed to be the first layer. The depth to the second refractor is shown as the second red line.

data alone it is difficult to conclusively say what the lithology of the second layer is.

However, geological data from the coal shafts (Shirlaw 1964) suggests that the drift deposits exist somewhere between the surface and the basement, and additionally, the third layer shows a velocity too high to be accounted for by drift deposits. Furthermore, since we are assuming that the second layer is below the water table, it seems most likely that the drift deposits are water saturated and it is therefore reasonable to interpolate that the second layer is made up of water saturated sands or glacial till.

The third layer shows a slightly higher velocity, in the order of 2300 ms-1, which lies within the range of sand and limestone velocities. Again, geological data allows us to interpolate that the third layer is the basement rock head.

3.2 Depth to the layers Although the velocity profile through the layers suggests a similar structure to that shown by geological data, i.e. a high velocity basement overlain by a slightly lower velocity layer of drift deposits, the depth profile does not. Estimates to the depth of the second refractor, using the plus minus method, show

values far greater than those suggested by geological data. The average depth calculated by means of the plus minus method is 44m, whilst geological data from the coal shafts suggests a depth to the basement of just 15m. Furthermore, in contrast to the previous geological constraint that assumes all the layers are horizontal, the calculated depths show an undulating non-planar layer. Although it is not unreasonable to suggest that in reality the layer is a homogenous concordant layer, the depth profile suggested by our data is geological unreasonable as it predicts a massively undulating surface. Since the calculated values of both the depth and the structure of the third layer seem geological unreasonable, this indicates that errors in the data have carried through the computation and caused inaccuracy in the final results.

This is highlighted, when computing the depth using the maximum and minimum velocity values, instead of the average value. If the maximum velocity of the second layer and the minimum velocity of the third layer is used, so that V2 is almost equal to V3 (V3 ≈ V2≈ 2100 ms-1) then the average depth to the refractor is increased to the order of around 180 m. Conversely, if the velocity of the second layer is set to its minimum value, and the velocity of the third layer is set to its maximum possible value, so that V3 is much greater than V2 (V2 = 1900ms-1, V3 = 2400ms-1) then the average depth of the refractor layer is lowered to the order of 25m.

This highlights that the calculated depth of the layers is associated with large uncertainties, and that relatively small changes in the velocity of the layers causes large differences in the calculated depths. This is of particular concern considering the large range of possible velocities for any given lithology in table 3.0. If you consider a two-layered system with “identical” lithologies, but with differing velocities, this would give rise to vastly different depth profiles.

Much like other geophysical techniques, this highlights the ambiguity of the interpretation of seismic data and underlines the importance of external geological constraints. It is apparent that even if the minimum depth is used as an estimate for bedrock depth, this

Table 3.0 P-wave velocities of some common lithologies (Reynolds 1997)

value is still much larger than the depth suggested by geological data, and additionally, the geometry of the layer is still shown as non-planar. This further suggests that unaccounted uncertainties in the data are causing errors in the computed results. The large extent of the errors in the results are also shown by the large disparity between the calculate velocity of the basement and the expected velocity for a Carboniferous bedrock. Calculated velocities for the basement are in the order of 2000 ms-1, however data in Table 3.0 suggests that Carboniferous sand and limestones accommodate velocities that are typically in the order of 5000 ms-1.

3.3 Possible causes of errors As discussed in the previous section, relatively small changes in the velocity of the layers can cause a large difference in calculated depths. Further analysis of the first break head waves on the initial seismic data set reveals that a change in first arrival times by only a couple of milliseconds causes calculated depths to the refractor to change by hundreds of metres. It is possible that eyeballing the first breaks was not accurate or reliable enough, and caused errors to compile through the computation and thus leading to large inaccuracies in the final results. This underlines the need for further more detailed analysis of the data and perhaps a more elegant method for picking first-breaks.

We assume that at each layer below the first, the velocity is greater than that of the overlying layer. However, it is possible that the third layer, the basement refractor, is hidden because its velocity is lower than that of the sediments above it. Considering the seismic velocities in Table 3.0, it is possible that a layer of highly saturated clay or glacial till could accommodate higher velocities than that of low velocity sand or limestone. In this case rays would not be critically refracted at the top of the basement and not give rise to head waves.

In this case we hypothesise that the refracted head waves picked up in our data may be the consequence of a seismically distinct layer within the basement, perhaps the start of older

basement rocks at around 40 or 50m depth. However, since our survey was only designed to investigate the top 20m of the crust, we cannot be sure that layers calculated for greater depths are reliable. A longer length profile would need to be implemented in order to investigate depths in the order of 50m. Another scenario that could have cause significant error in the data is by considering a layer whose velocity continuously changes. Such phenomena are common in thick clastic sequences, especially clays. The area of High Wood, to the north of the profile, has a geology characterised by a thick layer of boulder clay. If such a layer was present, it is possible that increased compaction and dewatering of the sequence with depth causes the velocity of the layer to also change with depth. This would cause the associated rays to not travel along the top surface of the layer but instead along a curved path within the layer with a turning point at some depth below the interface (Kearey & Brooks 1984). Such a phenomenon would account for the inaccuracies of the data results however, the interpretation of ‘diving waves’ is complex, and is therefore out of the limits of this paper.

CONCLUSION The results of the seismic survey contradict with geological data. The extent of the differences of the results is highlighted in both the calculated velocity and calculated depths of the layers. Although it is suggested that errors in the data are caused by phenomena present in the layers, such as a changing velocity and hidden layers, it seems just as likely that errors are caused by poor data analysis, perhaps due to the eyeballing technique used to pick first breaks. This indicates that more detailed, more sensitive and more rigorous data analysis needs to be implemented. Additional data should also be collected from a similar profile geometry in order to increase the reliability of the data. If time allowed, a 2-D seismic survey may be suitable, to investigate the lateral inhomogenities that may be indicated by the data (Fig.6).

 REFREENCES

EDINA DIGIMAP © Crown Copyright

Ordnance Survey, An EDINA Digimap/JISC supplied service

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