Seismic Techniques
Crosshole Seismic Surveys
Detail
The relationship between the velocity of seismic waves and the density and elastic properties of the materials through which they are travelling means that seismic techniques can be utilised to provide information on various geotechnical properties of the subsurface, such as Poisson's ratio and the shear modulus. The most common method of measuring these properties in engineering studies is through the use of crosshole seismic surveys.
Crosshole seismic surveys involve measurement of the travel time of seismic energy transmitted between two or more boreholes. One hole is used to deploy the source whilst the other hole(s) are used to detect the arrival of the seismic energy. In order to obtain properties such as Poisson's ratio, both P-wave (compressional) and S-wave (shear) data has to be acquired. This normally requires the use of two separate sources. P-wave energy is detected using a string of between 10-24 hydrophones suspended in water in the borehole. As shear waves are unable to travel through water or air they are detected using a single triaxial geophone clamped to the inside of the borehole using a hydraulic system. Data is collected at fixed intervals down the hole (normally 0.5-2m) by moving the shot and detector(s) in parallel. In the case of an array of hydrophones the string is kept fixed until the shot has passed below the depth of the last hydrophone.
The travel times of the P and S waves are derived from the first-arrivals identified on the seismic trace for each shot-receiver position and used with the known distance(s) between the shot/receiver boreholes to calculate the apparent velocities (P and S) for each depth interval. This data is then used to derive a vertical profile of material stiffness properties. Where the borehole separation is small the calculated apparent velocities will equate to the true velocities for each depth, as the energy travels direct from source to receiver. However, where a refracted or reflected wave arrives at the receiver first (first-arrival), this will not be the case, leading to spurious calculations of the material stiffness properties. Where refraction/reflection is considered to be a problem computer modelling of the raypaths should be utilised to help derive true interval velocities.
