Geotechnical Studies

In-Situ soil and rock properties can be calculated using various geophysical technologies. Properties that can be directly measured include P-Wave velocity (Vp), Shear-Wave velocity (Vs), Density and Electrical Resistivity/Conductivity. Geophysical methods include Seismic Refraction, MASW, ReMi, ERI, Soil Resistivity, Gravity and Natural Gamma.

• Seismic Refraction is a seismic technology that can be used to measure the P-wave velocity (Vp) of the subsurface geology. Vp is directly influenced by the elastic modulus of geological material, i.e. the stiffer or more consolidated the geologic material, the larger the value of Vp. Vp can also be used to estimate P-wave modulus.
• Multichannel Analysis of Surface Waves (MASW) is a surface wave seismic technology that is used to calculate the Shear Velocity (Vs) profile of the subsurface. MASW is suited to a geological setting of interchanging stiff and less stiff geologic layering, as this technology can deal with seismic velocity inversions. Vs can be used to estimate Shear-modulus if material density is known.
• Vertical Seismic Shear-Wave Profiling (VSSP) measures the Vp and Vs of the soil profile through a downhole seismic tool and a surface seismic source. By measuring the Vp and Vs of the soil profile it is possible to estimate Poisson’s Ratio.
• Refraction MicroTremor (ReMi) is a passive surface wave seismic technology, providing a Vs profile of the subsurface. The difference between the ReMi method and the MASW method, is that ReMi records the ambient seismic noise of the geology (e.g. vibrating tree roots) and does not need an active seismic source like MASW (e.g. drop-weight). Similarly, ReMi can also use an MASW set-up and provides a Vs profile over a larger depth range. Vs can be used to estimate Shear-modulus if material density is known.
• Soil Resistivity Testing calculates the apparent resistivity of the subsurface through the 4-electrode Wenner method and is presented as a 1D sounding of apparent electrical resistivity versus electrode a-spacing. The technique is useful for earth-grid designs in the construction of electrical towers, substations and power stations.
• The Gravity method is effective on both a local and regional scale at estimating the depth and shape of subsurface structures by measuring the gravitational acceleration of the earth. The gravity method can be most useful in modelling basement structure where densities of the buried causative bodies are known and when regions of outcropping rock are present to assist with tying-in basement model boundaries.

Voids and cavities can be detected using various geophysical techniques. It is possible to calculate depth, extent and location of these features in the subsurface. Karst terrain, tunnels, caves, old mine workings or utilities can be modelled and interpreted depending on their nature, depth and geological setting.

• Ground penetrating Radar (GPR) measures changes in the dielectric properties of the subsurface. Voids and cavities often yield sharp changes in electrical conductivity due to the void/cavity being filled with air, fluid or other material which has a large electrical contrast to the surrounding host geological material. GPR is a rapid technique that is effective for interpreting the top and in some cases the bottom of voids, by identifying hyperbolas in the GPR reflection section.
• Electrical Resistivity Imaging (ERI) measures the apparent resistivity of the subsurface. It tends to be used in tandem with GPR. As ERI also exploits electrical changes in the subsurface it can be used to quantify the size and depth of cavities as well as their nature. For example, voids in an ERI cross section will be mapped as a conductivity low if they are above the water table but a void full of brackish water will be represented as conductive high.
• Gravity measures changes in the density of the subsurface. If a cavity presents a difference in density (e.g. an air-filled void or tunnel) gravity can be an effective technique for detecting these features. Gravity tends to be used in tandem with ERI or GPR as it targets changes in material density where as GPR and ERI target the contrasts in electrical properties of voids.

Geophysical technologies can be used to measure depth to groundwater, as well as fresh/saline interfaces and changes in the salinity. Depending on the investigation requirements different techniques are needed to gain a greater understanding of the groundwater.

• Groundwater can produce a strong radar reflection in Ground Penetrating Radar (GPR) data, this is due to a strong contrast in the dielectric properties of the saturated geology and the relatively dry overburden. Saline regions are generally electrically conductive, so can have an attenuating effect on the radar wave. Therefore, a reduction in amplitude response in radar data may also be indicative of a highly conductive clay/saline groundwater region.
• Electrical Resistivity Imaging (ERI) is highly dependent on the porosity and water content of the earth material and can target changes in apparent resistivity due to the presence of groundwater. As well as detecting the depth to groundwater ERI can also be used to correlate levels of salinity and map fresh/ salt water interfaces. ERI can also be used to map calcrete layers and near-surface clays layers.
• Electromagnetic (EM) methods such Frequency Domain (FDEM) systems (e.g. EM31, EM34) are a fast-effective technique at interpreting presence of near-surface groundwater or clay regions. Similar to electrical methods, the presence of fluids or the level of porosity within the near-surface geology can influence electrical conductivity. Conductivity maps can be used to map brackish water, estimate fresh/brackish interfaces and highlight clay regions. Time Domain (TDEM) methods target groundwater at artesian depths and can also be used to interpret aquifers and aquitards.
• Gravity can be used to provide information on the general basement structure within the investigation area which may influence regional groundwater paths. Gravity used in conjunction with TDEM methods are especially effective on investigation at the regional-scale.