OR/13/006 Geophysics

Entwisle, D C, Hobbs, P R N, Northmore, K J, Skipper*, J, Raines, M R, Self, S J, Ellison, R A, and Jones, L D. 2013. Engineering geology of British rocks and soils - Lambeth Group). British Geological Survey. (OR/13/006). * Geotechnical Consulting Group (GCG)

The use of surface geophysics to investigate and characterize the Lambeth Group is problematical for two reasons. Firstly, in urban areas, where most of the engineering development and investigations are carried out, it is difficult to apply traditional geophysical techniques; secondly, the Lambeth Group is lithologically complex, exhibiting both vertical and lateral variation (Hight et al., 2004). Page and Skipper (2000) demonstrated this variability when they identified at least 20 different recognizable sediment types from their work on exposure sections and high-resolution cored borehole logs throughout south-east England. This lithological variation may result in an overlap of physical properties and hence a reduction in the overall geophysical contrast.

The urban environment, in particular, poses a major challenge for many of the geophysical techniques due to a combination of anthropogenic effects. For instance, magnetic and electromagnetic surveys (including ground-penetrating radar) may be seriously affected by anthropogenic noise such as buried pipes, concrete reinforcing bars and electrical cables both above and below ground. Standard seismic methods and ground contacting resistivity profiling or 2D imaging/tomography techniques are restricted by the presence of buildings and large paved or bituminous surfaced areas. In addition, the seismic reflection technique suffers from significant signal degradation due to the high levels of vibration noise associated with urban areas. In contrast the microtremor survey method (Okada, 2003) is the one technique that is ideally suited to the urban environment as it uses background microseismic and anthropogenic noise in its measurements.

Geophysical methods applicable to the Lambeth Group

The generally applied geophysical methods and their suitability with respect to the Lambeth Group are shown in Table 4.1.

The detailed heterogeneity of the Lambeth Group is best observed in borehole logs as exemplified by Jubilee Line Extension borehole 404T (see Appendix 1). The gamma ray logs reflect the presence of gamma ray emitters due to the radioactive decay, primarily, of potassium, thorium and uranium. In the Lambeth Group it generally reflects the varying sand/silt/clay and in someplace calcium carbonate content. Peaks often correspond to increased clay content either because the clay mineral contains potassium, as in the case of illite or they absorb uranium and thorium. Very low values may be associated with calcium carbonate-rich deposits such as chalk, limestone or calcrete as in parts of the Lower Mottled Clay and the pedogenically altered Upnor Formation. Examples from London (Ellison et al., 2004) show that the higher values are found in Lower Shelly Clay and Upnor Formation, whilst the intervening Lower Mottled Clay has a generally low gamma count. These variations have proved useful for correlation over relatively short distances; but may not be successful on a regional scale due to the rapid lateral and vertical variation in lithology.

Table 4.1    Generally applied geophysical methods and their suitability with respect to the Lambeth Group.

Method	Advantages	Disadvantages	Comments
High-Resolution Seismic Reflection	Generates high-resolution seismic images; maximum resolution of a few metres.	Expensive, good results require water-saturated consolidated deposits. Needs low-noise.	Good delineation of hard bands, shelly limestone and gravel beds.
Seismic Refraction	Relatively cheap method for determining thickness of weak sediments overlying bedrock.	Low resolution; assumes increasing velocity with depth.	Possibly useful in determining the depth to base of Lambeth Group.
Surface wave methods	Best seismic technique for measuring the moduli of sediments. Can discriminate useful signal against all other types of noise, especially useful in urban environments, whilst also being able to map velocity reversals. Field data is easily collected using standard seismic equipment as surface waves comprise the strongest energy. Derives shear wave velocities and hence shear moduli from ~1 to 100 m below surface. Large area can be covered in relatively short time period, hence it is highly cost effective and time efficient.	Can be limited resolution and may miss thin layers of anomalous velocity. Only average shear wave velocities derived. Passive methods work best when noise is coherent and directed parallel to array set-up.	Can map velocity reversals and may be able to map out limestone, hard bands and gravel beds in relation to lower velocity sand/clayey beds.
Ground Penetrating Radar	High-resolution image (sub-metre resolution) of near-surface; much cheaper than seismic reflection.	Strong signal attenuation in conductive ground (clays); Penetration of 12–20 m possible in resistive ground.	Possibly useful if the Lambeth Group is at or near surface.
Resistivity Tomography	High resolution 2D image of the sub-surface enhanced by inversion processing. 3D imaging and volumetric analysis possible.	Technique requires a relatively large amount of space; very difficult to operate in urban areas. Also quite slow data collection, which is restricted to the top 35 m.	Good for showing lateral lithological variations and for delineating sand/gravel bodies within clay.
Resistivity Sounding	Quick method for mapping horizontal layers with appropriate resistivity contrasts.	Relatively slow data collection. Interpretation is 1D and more than one model may match data.	Could be useful for assessing overall thickness of Lambeth Group.
Ground conductivity (EM)	Maps variation in conductivity, usually related to clay content; useful for conductive horizons; 50 m exploration depth.	Difficult to operate in culturally noisy environments. Limited vertical discrimination.	Good for detecting near-surface (i.e. <20 m depth) sand channels within clay bodies.
Transient Electromagnetism (TEM)	150m depth from small loop set-up to map depth conductivity variations; smaller ground volume involved than resistivity sounding.	Difficult to operate in noisy environments. Low resolution in top 10 m; most interpretation is 1D and assumes horizontal layers.	Useful on constrained sites, but susceptible to urban noise.
Downhole ‘wire line’ logging	High resolution, good for stratigraphic correlation and bulk physical properties. Logs can be run in cased boreholes.	Point source data that can be difficult to interpret and correlate in laterally variable environments.	Electrical logs show clear delineation of divisions in monotonous strata like the Thanet Formation, but natural gamma logs appear best suited for Lambeth Group.
Microgravity	Apart from borehole logging this is the only technique that can be used over relatively small grids in noisy urban environments.	Relatively expensive and slow data capture and processing. Requires accurate height of each data point.	Can be used to detect near-surface collapse zones due to dissolution, or map, relatively large bodies of lower density sands that cut into clay (e.g. channels and buried valleys).

Figure 4.1    Correlation of gamma-ray signatures in the Lambeth Group and Thanet Formation (Ellison et al., 2004).

Ground Probing Radar (GPR)

Ground Penetrating Radar uses radio waves in the range of 1–1000 MHz to map the internal structure of the ground. It is an efficient and cost effective technique that works best in dry resistive lithologies (Davis and Annan, 1990), but has a limited depth of investigation in the UK (generally less than about 15 m) due to signal attenuation from a predominance of conductive clay in superficial deposits and bedrock. However, it may be useful in delineating and characterizing channel sands and/or laminated beds (where they are mainly sand). An example of this type of radar section is shown in Figure 4.2a where a 50 MHz antenna was used to examine the relatively dry, clean sands and gravels of the Blakeney Esker in East Anglia (Busby and Raines, 1999). Data is observed down to about 14 m, whilst the sub-horizontal reflectors show an indication of the Esker’s depositional history. Similarly, detailed reflections are observed over resistive ground near Sellafield, Cumbria, UK (Busby and Merritt, 1999), which was interpreted as a kettle hole infilled with later horizontally bedded silty sand and gravels (Figure 4.2b). A comparison between signal resolution and depth of penetration can be observed in slumped (mine induced) Carboniferous strata near Ebbw Vale. In Figure 4.3a the 100 MHz antenna section shows good stratigraphic resolution and limited depth profile, whilst a greater depth of signal penetration is noted in the lower frequency 50 MHz antenna section (Figure 4.3b), but offset by a lower bed resolution. This method may be suitable where site conditions are favourable and where the Lambeth Group is near or at surface.

Figure 4.2    GPR reflection sections a) orientated along the Blakeney, Esker, Norfolk (after Busby and Raines, 1999) and b) GPR reflections interpreted as a kettle hole infilled with horizontally bedded, on lapping sediments (after Busby and Merritt, 1999).

Figure 4.3    a) GPR measured with 100 MHz antenna, b) GPR measured with a 50 MHz antenna.

Electrical Resistivity Tomography (ERT)

Electrical resistivity tomography techniques generate 2D slices and 3D models of even complex geological environments and should be used in conjunction with seismic or GPR surveys as they provide complementary information about the subsurface. This is a powerful geological mapping tool, for use in engineering and environmental applications, including hydrogeological mapping. Reliable models of the subsurface can be created where ERT is used in combination with a ‘ground truthing’ boreholes at locations informed by the resistivity results (Loke, 1997, 1999).

One of the limitations of the technique is that increasing the depth of investigation requires longer electrode arrays and, therefore, larger available areas of ground. The site must permit an electrode array length of about 10 times the depth of investigation (e.g. an array length of 300 m would provide a depth of investigation of approximately of 30 m). This was observed in the Three Valleys Tunnel project (Baker and James, 1990), where a resistivity survey was deemed inconclusive due to the limited array length.

Nevertheless, given the space and relatively low levels of electrical noise, it is the one technique that should identify the vertical and lateral variation present in the Lambeth Group across a site. For example, the Harwich Formation (a good marker horizon dominated by glauconitic sands); occasional beds of Paludina Limestone; sand-filled channels generally; and the gravel beds at the top of the Upnor Formation, would all be expected to have relatively high resistivity. In contrast, intermediate resistivity might be expected from the laminated beds and shelly clays, whilst the clays of the Reading Formation (not in the east of the London Basin) would have relatively low resistivity. This assumes little overlap in the physical properties of the lithologies, which may not always be the case.

Space notwithstanding, the technique has some other limitations that might need to be considered when characterising the Lambeth Group. Realistically, resistivity data can be acquired to between 35 and 50 m below ground level as may be required in some areas of London, but the resolution tends to decrease with depth. An obvious resistivity target within the Lambeth Group is where sand-filled channels cut into the clay of the Reading Formation. A similar example of this scenario can be observed in Figure 4.4 where ERT was used over a landslide (Chambers et al., 2011; Wilkinson et al., 2011). Here, an approximately 35 m thick, relatively resistive sandstone (Staithes Sandstone Formation, SSF) is sandwiched between two relatively conductive mudstone formations (Whitby Mudstone Formation, WHM and Redcar Mudstone Formation, RMU). However, identifying resistive bodies, particularly if relatively small, beneath 20 m of superficial deposits and London Clay Formation would be difficult, as the current would tend to flow mainly in the conductive clay, hence decreasing the depth of investigation. The technique is ideal for mapping rapid lateral changes in resistivity as indicated by Figure 4.5, where a 2D resistivity section of a cambered slope in the Cotswolds (Raines et al., 1999) show small sediment infilled graben/half graben structures, lying between limestone blocks.

Shallow seismic reflection

Shallow seismic reflection (when and where site conditions permit) may also indicate some of the affects of variable lithology. For example, in The Three Valleys Tunnel survey (Baker and James, 1990) marked reflections were recorded at the top and base of the Lambeth Group and horizons of major contrast in the acoustic impedance (i.e. density x seismic velocity), possibly denoting limestone or gravel beds.

Figure 4.4    Inverted dipole-dipole ERT images. Inferred boundaries between the WMF, SSF and RMF are shown by dashed white lines.

Figure 4.5    2D pole-dipole resistivity section of a cambered slope at Aston Farm, North Cotswolds.

Surface wave methods

The growth in the use of seismic surface waves in earthquake and foundation engineering over the past decade has been remarkable. Their main attraction is the ability to derive values of shear wave velocities and hence shear moduli, at depths ranging from less than a metre to 100 metres below the surface, as a practical alternative to drilling expensive boreholes (Milsom and Eriksen, 2011).

Surface waves (Rayleigh and Love waves) are seismic waves propagating parallel to the earth’s surface without spreading energy through the earth’s interior: their amplitude decreases exponentially with the depth, and most of the energy propagates in a shallow zone, roughly equal to one wavelength. Surface waves are dispersive, resulting in a different wavelength of propagation for each frequency that propagates over different depth intervals within the ground (Reynolds, 2011). Thus, field survey methods that can propagate and record multi-frequency surface waves can be applied to characterise the shear wave velocity and stiffness properties of the near surface. Passive surveys utilise so-called background ‘noise’, whereas active surveys use a vertically impacting point source to produce Rayleigh waves. Different field set-ups range from use of dual geophones to 2D multi-geophone arrays. This enables characterisation of 1D profiles, 2D sections or pseudo 3D volumes, which can be applied to map engineering interfaces and disturbed ground via disruption to the subsurface stiffness (Park et al., 1999, 2007).

In engineering geophysics Rayleigh waves are considered the most important as their velocities are related to those of shear waves in the same elastic media. The exact relationship depends on the Poisson’s ratio, but generally they are within 10% across a range of materials (Milsom and Eriksen, 2011). The recent popularity of surface wave surveys are due to the fact that they are non-invasive and can be quickly mobilised to provide shear wave velocity, and thus, small strain stiffness (shear modulus) information, from which heterogeneity can be assessed, (Foti, 2000; Menzies, 2001).

In the Multi-channel Analysis of Surface Wave (MASW) method, data is gathered using the same receiver array configuration adopted in shallow seismic refraction and reflection surveying (Gunn et al., 2012, 2013). The method utilises the dispersion property of surface waves for the purpose of shear wave profiling in 1D (depth) or 2D (depth and surface location) format (Park et al., 1999, 2007) and illustrated in Figure 4.6. The active method generally permits the determination of apparent phase velocity (or dispersion curve) within the frequency range 5–70 Hz. Hence, the active method can provide information concerning the top 30–35 m, depending on the stiffness of the site. The passive method, in contrast, has a much lower frequency range (5–15 Hz) and consequently provides information on deeper layers, below 50 m, again depending on site stiffness (Roma, 2010). ‘Microtremor’ is the name given to the background low-amplitude seismic waves that are present everywhere at the earth’s surface. Microtremors with frequencies above one Hertz are generally associated with man-made sources (such as road traffic, trains, machinery, etc.), while those below one Hertz are generally associated with natural phenomena such as wind action and variations in atmospheric pressure.

The microtremor survey method has been adapted and applied as the Refraction Microtremor (ReMi) method by Louie (2001) and is equivalent to the passive MASW method. It uses standard seismic refraction equipment and a linear geophone array to measure the ambient noise or ‘microtremors’ to derive a shear-wave velocity profile down to about 100 m with 15% accuracy. In the inversion procedure, the Rayleigh wave dispersion curve is picked from a wavefield transformation, and iteratively modelled to derive the S-wave velocity structure.

A ReMi survey conducted over a buried mineshaft at Brighouse in Yorkshire, (Raines, et al. 2011), showed the technique’s potential. In Figure 4.7a plots of shear wave velocity versus depth are shown for the various geophone groups. An advantage of this method over seismic refraction is observed in Figure 4.7a, where small velocity reversals are noted between 5–6 m below ground on Geophone groupings G9-G-16 through to G17-G24 respectively. The profiles at this site could be associated with weathered sandstone (600 m/s) overlying weathered mudstone or siltstone (400 m/s) as proved in various nearby boreholes.

The contoured shear wave velocity data shown in the 2D velocity section of Figure 4.7b suggests that the method has successfully mapped relatively low velocity structures beneath the made ground or colliery waste (5–6 m thick) that are associated with the backfilled mineshaft and edge of the former sandstone quarry. As this method can be used to measure velocity reversals it may be possible to delineate the relatively low velocity Upper and Lower Mottled clays, where they interdigitate with and/or underlie some of the limestone bands and gravel beds.

Figure 4.6    A 3-step processing scheme for Multichannel Analysis of Surface Waves data; a) Typical 8-channel field record; b) Dispersion curve constructed from phase velocity — frequency transform; c) Dispersion curve inverted to produce shear wave phase velocity with depth.

Figure 4.7    a) Plots of shear wave velocity (m/s) versus depth for geophones groups, G1-G8, G3-G10, G5-G12 and G7-G14 (top); G9-G16, G11-G18, G13-G20, G15-G22 and G17-G24 (bottom), b) 2D Pseudo-section of contoured shear wave velocity data from geophone groupings shown in above.