OR/13/006 Appendix 4 - Geophysical techniques
|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 electrical properties of earth materials are dependent on porosity, saturation, pore water resistivity and clay content and functions relating to the shape of the pores and the electrical conductivity of the clay. In general, saturated rocks and soils have lower resistivity than unsaturated rocks. In most rocks and soils electric current is conducted electrolytically by the interstitial fluid, and resistivity is controlled more by porosity, and pore water conductivity than by the resistivity of the rock matrix. However, many clay minerals are capable of conducting electrons; hence the current flow through a clay layer is both electronically (via the mineral) and electrolytically (via the bound water). A relatively low clay content (<10%) can significantly reduce measured resistivity in both weathered/fractured bedrock and clay or clayey deposits. Because of these phenomena the resistivity of rocks and minerals display a wide range that is unmatched by any other physical property.
Two resistivity survey modes are possible:
i) Conventional resistivity-depth soundings or ‘vertical electrical sounding’ (VES) is carried out using either Schlumberger, Wenner or Offset Wenner Arrays. The resistivity technique involves injecting a switched direct or low frequency current into the ground via two current electrodes. A potential difference is then measured between a pair of closely spaced inner, in line electrodes (see Kuetz 1966, Keller and Frischknecht 1966). When the current electrode separation is relatively small most of the current is constrained to flow close to the surface. By expanding the current electrode separation systematically about a fixed centre, successively deeper sections of the earth are investigated, thereby yielding a measure of the variation of resistivity with depth.
The measured resistance (voltage/current, V/I) is multiplied by a geometric factor, according to the disposition of current and potential electrodes, to give an apparent resistivity of the ground in ohm.metres (ohm.m). This represents a weighted average of the true resistivity of all lithologies through which the current passes.
The apparent resistivity sounding data are analysed in terms of a layered earth (1D) model comprising a series of resistivities and comparable depths, or layer thicknesses. Automatic computer software, such as Interpex RESIXplus use this initial approximate model to generate a theoretical apparent resistivity curve (Ghosh, 1971), which is then compared and matched to the observed field curve. After a series if iterations, the final interpretation will normally result in between three and five identifiable distinct electrical layers.
Apart from the assumption of lateral continuity an important problem confronting the interpreter of VES data is that of equivalent solutions. That is, a range of different arrangements of layer resistivity and thickness will yield very similar sounding curves. There is also a limited ability to resolve thin layers. The best way to resolve these ambiguities is to incorporate local borehole control (i.e. layer thicknesses and, where available, resistivities) at the modelling stage. In the absence of such control, data from complementary techniques (e.g. EM34 ground conductivity, see below in electromagnetic methods) may be used to constrain the model. A series of interpreted soundings can be combined to construct a geoelectrical section.
A further difficulty with VES interpretation could result from the lack of clearly defined resistivity contrast of the lithologies present. For example, there may be considerable overlap in the range of resistivities displayed by gravels (dry and saturated) and weathered bedrock.
Better results are obtained if resistivity measurements are positioned parallel to the geological strike and away from services such as water mains and metal fences. In addition measurements along the sides of roads invariably produce poor results (Anon, 1988, McDowell, et al., 2002).
ii) The resistivity tomography technique is useful for investigating areas of complex geology where the use of resistivity depth soundings and other techniques are unsuitable. The method is used to characterise vertical and lateral changes in subsurface electrical properties, by means of automated resistivity tomography or imaging technique. The main advantage results from an increase in resistivity information from the underlying strata, which can be plotted and interpreted in the form of a 2D section.
A 2D resistivity image is measured by moving the electrodes along a profile, whilst maintaining a constant separation between them. By repeating the profile at increasing electrode spacing or ‘n’ levels increases the depth of investigation and thus resulting values of apparent resistivity can be plotted against distance.
The pseudo-section is processed and inverted using a commercial software package, such as RES2DINV (Loke 1997, 1999, Loke and Barker, 1996b) for 3D inversions. This results in a colour contoured section, which reflects the qualitative spatial variation of true resistivity across the section.
The electrical conductivity of the ground is determined by its response to an induced magnetic field. The technique (a variation of the conventional Slingram method) measures the terrain conductivity by passing an alternating current through a transmitter coil placed on or near the ground. This current produces a primary magnetic field that induces small currents in conductors in the underlying strata. These currents, in turn, produce a secondary magnetic field, which is sensed by the receiver coil together with the primary signal. The resultant field will have the same frequency as the primary field but, in general, not the same phase or direction. The ratio of the secondary to the primary field is approximately linearly proportional to terrain conductivity at low values of terrain conductivity, which, therefore, permits a direct readout at the instrument of apparent conductivity in milliSiemens per metre (mS/m). These readings are referred to the mid-point between the two coils. Resistivity (in ohm.m) is the reciprocal of conductivity (S/m) e.g. 100 ohm.m = 10 mS/m; hence the two parameters are comparable and readily convertible.
As it is an induction method there are no grounded electrodes, and the problems of contact resistance, which can occur in galvanic resistivity prospecting, are avoided.
The Geonics EM31 and EM34 are non-contacting terrain conductivity meters, which operates on the principles of electromagnetic induction as described by McNeill (1980a). The EM34 is a two-man operation using two separate coils linked by a reference cable with measurements taken for coil operations of 10, 20 and/or 40 m length. McNeill (1980b) defines the depth range as 0.75 and 1.5 times the coil separation in a homogenous and conductive layer. Although this condition is rarely met, the effective penetration will be an average value. Hence, when operated in the vertical coil (Horizontal dipole) mode, approximate depths of investigations are 7.5 m, 15 m, and 30 m respectively. Similarly, there is a substantial increase in the depth of investigation when operated in the horizontal coil (vertical dipole) mode of approximately 15 m, 30 m and 60 m respectively.
The EM31 is operated by one man and comprises a transmitter and receiver coil at the end of a 3.66 m rigid boom, whilst a central console houses an analogue meter and data logger. The instrument is normally supported across the shoulder of the observer with measurements taken at waist height (approximately 1 m). When the boom is rotated from the vertical to horizontal coil position, (horizontal to vertical to dipole) the depth of investigation increases from approximately 3 m to 6 m respectively.
The seismic methods (reflection and refraction) are the most effective and by far the most expensive of all the standard geophysical techniques. In the seismic method, an elastic pulse or a more extended elastic vibration is generated at shallow depth and the resulting motion of the ground at nearby points on the surface is detected by small seismometers or ‘geophones’. Measurements of the travel-time of the pulse to geophones at various distances give the velocity of propagation of the pulse in the ground.
The ground is generally not homogeneous in its elastic properties, and this velocity will, therefore, vary both with depth and laterally. Where the structure of the ground is simple the values of elastic wave velocity and the positions of boundaries between regions of differing velocity can be calculated from the measured time intervals. ‘Velocity’ boundaries coincide with physical changes in the ground; usually coincide with geological boundaries and a cross-section on which velocities interfaces are plotted may, therefore, resemble the geological cross-section, although the two are not necessarily the same (Griffiths and King, 1981).
Seismic methods are of major importance in the fields of engineering site investigation and hydrogeology, where depths of interest lie in the range 10–200 m. Seismic refraction surveys are used for estimating the depth of high-velocity ‘bedrock’ or of a well defined water table, in addition to evaluating the mechanical and hydrogeological properties (degree of fracturing, porosity, degree of saturation etc.) of a concealed foundation material or aquifer.
iii) The Surface Wave Survey Method
When seismic waves are generated, there is a special type of wave propagating along the free surface called surface waves whose penetration depth is wavelength-dependent; the longer wavelength influences the deeper portion of the earth. Because of this property, surface waves are usually dispersive, meaning different frequencies have different propagation velocities, whereas body waves (refraction, reflection, head, etc., waves) rarely take such property to a noticeable extent. Two types of surface waves are generally known: Rayleigh and Love waves. The disturbance (vibration) direction of the former is mainly perpendicular to the surface, whereas it is parallel for the latter. Theoretically, the dispersion property of surface waves is determined by several elastic properties including density (rho), and depth-variation of S- and P-wave velocities (Vs and Vp). Among these parameters, the depth-variation of Vs is the most influencing factor. Because of this, surface waves are often used to deduce Vs properties of near-surface earth materials. In comparison to using conventional body-wave methods to achieve similar Vs information (for example, S-wave refraction, reflection, down-hole, cross-hole surveys), the surface-wave method has several advantages:
- Field data acquisition is very simple and tolerant because surface waves always take the strongest energy.
- The data processing procedure is relatively simple and easy even for the non-experienced.
- A large area can be covered within a relatively short time period.
- Because of all above reasons, it is highly cost effective and time efficient.
Utilization of surface waves for geotechnical engineering purposes has a history dating back to the early 1950s. Since the early 2000s a multichannel approach called the MASW (multichannel analysis of surface waves) method has been widely used (Park et al., 1999, 2007).
Ground penetrating radar
The GPR technique is similar in principle to sonar methods. The radar transmitter produces a short pulse of high frequency (25–1000 MHz) electromagnetic (EM) energy, which is transmitted into the ground through an antenna. Reflections are generated in the ground by changes in electrical impedance, which is dominated by changes in the relative permittivity or dielectric constant (K) of the ground. These reflections are subsequently detected at the ground surface by another antenna attached to the receiver unit. In general, the strength of the reflected signals is proportional to the degree of contrast of dielectric properties across the interface, the larger the contrast the stronger the signal reflected back to the ground surface (Davis and Annan 1989). The results are plotted in section form as two way travel time (TWT) (i.e. the time taken for passage of the signal from transmitter to reflecting horizon and back to the receiver) against traverse position. The distance in metres is shown along the top of the profile section, whilst the left hand vertical axis indicates the TWT in nanoseconds (ns). The corresponding right hand vertical axis shows the depth in metres, derived from an average EM transmission velocity value for limestone of 0.1 metres/nanosecond (m/ns). It should be noted that ground surface is represented by the first thick black line of radar reflections.
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- KELLER, G V AND FRISCHNECHT, F C. 1966. Electrical methods in geophysical prospecting. Pergamon Press, London. 519p.
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- LOKE, M H. 1997, 1999. Electrical imaging surveys for environmental and engineering studies. A practical guide to 2D and 3D surveys.
- MCNEILL, J D. 1980a. Electromagnetic terrain conductivity measurement at low induction number, Geonics Ltd. Technical Note TN-6.
- MCNEILL, J D. 1980b. Applications of transient electromagnetic techniques, Geonics Ltd. Technical Note TN-17.
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- PARK, C B, MILLER, R D, XIA, J. 2007. Multichannel analysis of surface waves (MASW) — active and passive methods. The Leading Edge, 26, 60–64.
- DAVIS, J L, and ANNAN, A P. 1989. Ground penetrating radar for high-resolution mapping of soil and rock stratigraphy. Geophysical Prospecting, 37, 531–551.