"

1.11 Geophysical methods for soil exploration

Non-destructive methods, where the term non-destructive suggests that no sampling, boring, or excavating of soil is involved, are also used to provide information on soil and rock properties, hydrogeological conditions, the subsoil profile etc. Information obtained through geophysical methods is generally two- (or three-) dimensional (spatial), compared to one-dimensional or point information provided by boreholes/CPT soundings or soil samples, respectively (Figure 1.40).

Graph showing the geotechnical information obtained with different testing methods: at a point (with SPT or samples tested in the lab), along a line (with CPT) and across a 2D cross-section (with geophysical methods).
Figure 1.40. Different methods of soil exploration, compared in terms of area covered by each method.

The main advantage of geophysical methods is exactly their ability to provide spatial information on subsoil stratigraphy and groundwater levels, covering effectively a wide area. Geophysical methods are relatively quick and inexpensive, but their results are mainly qualitative, so they have limited applicability for deriving design geotechnical parameters. However, they can be implemented during early stages of a project, to better plan a detailed site investigation. Note that covering a wide area with geophysical methods requires real estate, to conduct tests properly.

The most common methods mentioned in AS1726 are described compendiously in the following paragraphs, as it is not within the scope of this Part to discuss the interpretation of geophysical tests.

1.11.1. Seismic surveys

Seismic surveying methods, such as Multichannel Analysis of Surface Waves (MASW) is based on the principle that waves travel within the earth crust at different velocities, which depend only on the geomaterial properties. For example, common shear waves propagate with a wave velocity VS that is equal to:

(1.37) {V_S} = \sqrt {\dfrac{{{G_0}}}{\rho }}

where G0 is the elastic small-strain shear modulus of the soil or rock layer, and ρ is its mass density. This implies that if we can measure the shear wave velocity, we can find the small-strain compressibility parameters of the specific material.

Measuring of the seismic velocity is possible by using geophones, which are motion-sensitive instruments that convert ground motion to electrical signals. This can be accomplished by generating a ground wave, simply by pounding a sledgehammer on the ground surface or by using a small amount of explosives. With geophones, we can measure the difference in the arrival time of the wave, t at two geophones spaced L meters apart. The propagation velocity of the wave is equal to V = L/t (Figure 1.41).

Diagram showing the principle of seismic surveys: propagation of a ground wave is recorded by two geophones. Each geophone is recording a velocity time history. From the time shift in the velocity time histories and the distance between the geophones, the wave propagation velocity is measured.
Figure 1.41. Principle of seismic surveys.

In real soils however, this procedure is more complicated, since when a wave encounters an interface between two soil/rock layers with different properties, and thus different wave propagation velocities, secondary waves are generated due to reflection, refraction and diffraction (Figure 1.42). However, we can take advantage of these secondary waves to determine not only the properties of the subsoil layers, but also the stratigraphy, with methods such as the seismic refraction method or the Multi-Channel Analysis of Surface Waves (MASW) method.

Diagram illustrating wave propagation from a source to a geophone array, showing direct, reflected, and critically refracted waves, as result of the existence of a stiff soil layer that underlies the top soft soil layer, on the surface of which the geophones are placed.
Figure 1.42. Direct, reflected and refracted wave paths generated by a wave source and recorded via geophones.

Employing the same principles, interpretation of multiple geophone recordings can provide information about subsoil stratigraphy (Figure 1.43). The seismic refraction method is mainly applied for determining the depth of a major refractor e.g., the bedrock, which has a wave propagation velocity significantly higher than the above softer formation. Other applications of the method include locating weathered fault zones, thickness of aquifer layers etc.

Graphical representation of typical MASW wave propagation velocity contours, plotted along a cross-section. Wave velocities range between 40 and 150 m/sec, and attained their minimum value underneath the soil crust, which is about 2m thick. A cone resistance profile is overlaid on the wave contours, and it is shown that high cone resistance values are associated with higher wave propagation velocities.
Figure 1.43. Typical MASW wave propagation velocity and cone resistance profiles of soft estuarine clay deposits, from the National Field Testing Facility for Soft Soils at Ballina, NSW (after Kelly et al. 2017).

1.11.2. Electrical resistivity method

The electrical resistivity method is used for determining the groundwater table level, the location of any possible subsoil cavities, the identification of the extent of fault zones etc. The concept on which the method is based consists of measuring the electrical potential difference (AC voltage) on the ground surface, created by an AC current. The potential difference reflects the “resistance” to the flow of the electric current through the soil, which is correlated to the soil’s electrical resistivity. Soil resistivity varies with the water content: the pore fluid provides the only electrical path in sands, while both the pore fluid and the surface charged particles provide electrical paths in clays. So, differences in resistivity can be interpreted as differences in soil stratigraphy (Figure 1.44).

Graph with top axis presenting resistivity values and bottom axis presenting electric conductivity values. The range of expected resistivity/conductivity values for different soils, rocks as well as salt water and fresh water is presented .
Figure 1.44. Typical soil and rock electric conductivity/resistivity values (after Samuelian et al. 2005).

Application of this method involves placing four electrodes on the ground surface, applying an AC current to the outer two electrodes, and measuring the AC voltage between the two inner electrodes (Figure 1.45). The spacing of the electrodes is related to the depth of the investigation: the greater the spacing, the larger the depth where we can get information from. A subsurface resistivity profile is created by performing successive measurements at different spacing (Figure 1.46).

Diagram of a measuring instrument above soil, showing AC current application on the soil surface and measurement of voltage values, to infer soil resistivity.
Figure 1.45. Layout of electrical resistivity surveys.
Cross-section showing contours of soil resistivity up to a depth of 18m. In the middle of the cross section there is a deep zone of high electrical resistivity values, which indicates the existence of a fault.
Figure 1.46. Typical electrical resistivity profile.

1.11.3. Ground penetration radar (GPR)

Ground penetration radar (or georadar) is based on the same principles as the radar used in aviation. An antenna is used to transmit and recover high frequency (10MHz to 1000 Mhz) electromagnetic pulses originating from a pulse generator. When the wave hits a buried object or a boundary with different dielectric constants, the reflected return signal is altered (Figure 1.47, Davis and Annan 1989). The method is practically based on the same concept as seismic surveys, only in this case electromagnetic waves are used.

GPR has many engineering applications in general. Its key applications in Geotechnical Engineering include identifying the location of buried objects (utility lines) or cavities, and soil profiling.

Illustration of the use of ground penetration radar method to detect the interface between soil and bedrock, as well as cavities in soil.
Figure 1.47. GPR used for soil profiling and identification of cavities.

License

Icon for the Creative Commons Attribution 4.0 International License

Fundamentals of foundation engineering and their applications Copyright © 2025 by University of Newcastle & G. Kouretzis is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

Share This Book