Question

The geology of a proposed major industrial site is thought to consist of a thin layer (1-5m) of unconsolidated sandy material overlying dolerite bedrock. The dolerite is known to have a very irregular upper surface and maybe highly weathered in places. Describe geophysical investigation techniques that may be suitable for mapping the thickness of the unconsolidated sand and also the distribution of unweathered basalt at shallow depths. For each method you recommend, outline: a) the basic physical principles of the technique, b) the expected outcomes of the survey.

Answer 1

The geophysical investigation methods that I recommend for determining the thickness of unconsolidated sand and distribution of basalt are a) seismic refraction method b) 2D electrical resistivity Imaging

SEISMIC REFRACTION: BASIC PRINCIPLES AND SCOPE

The seismic refraction method is one of the geophysical investigation technique to determine the characteristics of soil and rock.

Basic principle : A seismic signal is introduced into the subsurface via a shot point using explosives, hammer blow, dropped weight or an elastic wave generator. The energy generated travels either directly through the upper layer or travel down through various layers before returning to the surface. The energy is then detected on surface at a series of receivers called geophones spaced at regular intervals. After a certain distance from the shot point, known as the cross over distance, the refracted signal is observed as a first arrival signal at the geophones. Both compressional waves which provide depth information of interfaces and shear waves which provide additional data about engineering properties of the subsurface media can be used in the seismic refraction method.

SCOPE: Seismic refraction finds application in the determination of rock competence for engineering application, depth of bedrock, groundwater exploration, crustal structure and tectonics. The method relies on the tendency of acoustic velocities to increase with depth, which sometimes make it insensitive to low velocity layers in the subsurface. Based on the analysis of the field data, the seismic surveyor draws a profile showing the thickness of the subsurface and a good idea of what materials they consist of. This method is used in determining the thickness of stratified layers of soil and rock. The results obtained are generalised expressions that relate travel time, offset distance, velocity and thickness of subsurface layers.

2D ELECTRICAL RESISTIVITY IMAGING

Basic principle : The basic principle of electrical resistivity method is the injection of current into the ground using a pair of electrodes. This current causes a potential difference in the ground which is measured by a separate pair of electrodes. The voltage measured can be converted into an apparent resistivity value. This value can provide a range of information regarding the material being tested. Different types of soil composition have different resistivities.

SCOPE : From the results of the electrical resistivity imaging, it is possible to map the stratification of the subsurface layers, areas of weakness in the subsurface and the vertical as well as lateral extents of the different lithologies. The depth to the substratum can also be determined. This method has been conducted for site characterisation for large building foundations. This method can be used as a basis for further development to optimize a procedure to reduce the number of boreholes for conventional site investigation in areas that are environmentally sensitive or where access is restricted.