Seismic surveys for engineering projects on land are mostly undertaken by the refraction technique. Twelve or more geophones are used in a traverse while the seismic pulse is provided by explosives, falling weights or hammer blows.
Results are presented in a time/distance graph (Fig. 1) which, in favourable conditions, will show a curve composed of straight line segments with increasing traverse distance and with sharp segment intersections.
Results are presented in a time/distance graph (Fig. 1) which, in favourable conditions, will show a curve composed of straight line segments with increasing traverse distance and with sharp segment intersections. The slope of each segment represents a velocity and it is conventional to designate the velocity of the near surface layer as V1, the next velocity as V2 and so on. The depth (d1) to the first horizontal refractor is given by:
An ideal time distance graph from multiple horizontal layers should show several sloping straight-line sections of the graph, each corresponding to increasing layer velocities; depths to layer interfaces and layer thicknesses may be computed by formulae of increasing complexity.
If the refractor is dipping it is possible to gather data by reverse shooting (keeping the geophones as before but putting the shot point at the other end of the traverse) to calculate the dip of the refractor. If the refractor is undulating there are interpretative techniques which allow the calculation of the depth to the refractor under each geophone, so that, for example, an irregular rockhead surface under a soil cover may be mapped. Mostly engineering sites are examined by a grid network of traverses, plotting the levels of refractors and drawing refractor contours. However, it must be remembered that the real geological situation is three dimensional and not two dimensional.
Thus, on a hillside or above an irregular rockhead the first arrival recorded by a geophone may come from a refractor not immediately beneath the geophone although it is common practice to display refraction profiles as if they were vertical crosssections.
It must also be remembered that the accuracy with which the refractor shape can be plotted depends (just as it does in surface topographical surveying) upon the density of observation points.
The depth to which the ground may be examined by the refraction method depends upon length of traverse, velocities and attenuation factors of the strata and the energy of the source. In much seismic refraction work a great part of the pulse is absorbed by near surface soils, particularly if the seismic shock is generated by falling weights or hammer blows. For engineering purposes investigations seldom require information to depths of more than 100 m and traverse lengths of the order of 400 to 500 m are suitable.
Data is recorded digitally to allow for processing and computer calculation of results. The technique is particularly suitable for the investigation of ‘long’ engineering works involving excavation of materials, such as the construction of roads, canals, railways, etc. Thus, for example, Tan et al. (1983) have described the use of refraction surveys for the investigation of a proposed road construction in Singapore (Fig. 2).
Twelve, twenty-four or more geophones refraction seismic surveys are the task of a geophysicist but simple surveys using the single geophone (single channel) seismograph are now commonly undertaken by engineering geologists. As will be seen later in this chapter the reason for these surveys may be other than simply to find the depth of a refractor but many engineering geologists now use the single channel hammer seismograph as geologists use a hammer. The basic idea of this device is instead of using one shot point and twelve geophones, to use one geophone and twelve shot points the seismic pulse being given by a sledgehammer blow on a steel plate resting on the ground. The earliest model single channel seismographs simply recorded the time of the first arrival and numerous hammer blows were needed to be assured that the true first arrival had been recorded.
Many an engineering geologist saw strong healthy labourers reduced to shivering blistered wrecks in the course of a day’s survey in ar eas of seismic ‘noise’. However, the invention of the enhancement seismograph, which allows successive hammer pulse records to be ‘stacked’, changed this and surveys now are undertaken with less effort and greater reliability. The energy that can be put into the ground by a hammer blow is, however, rather limited and single geophone surveys are employed generally when the refractor (usually rockhead under soil) lies at depths of 10 to 15 m or less below surface.
David George Price -Engineering Geology Principles and Practice