contrasting impedances. A geophone array at the ground surface detects the refracted wave
arrival at each geophone and allows for measurement of the travel time from source to
detector. Geophone arrays for engineering studies can range from 12 to 48 channels
depending on the survey goals. The measured travel time data at each geophone is
interpreted from multiple shot points to determine the velocity, thickness and depth of the
subsurface layers that exhibit sufficient contrast to be distinguished as seismic layers.
Seismic refraction profiling has several limitations, the most severe of which include
the presence of blind zones (hidden layers with insufficient velocity contrast) and velocity
reversals. The interpretation of refraction data requires the assumption that velocity increases
progressively as a function of depth. A velocity reversal consisting of a lower velocity layer
underlying a higher velocity layer is undetectable at the surface. Field procedures are
relatively slow in the absence of sufficient crews for running seismic lines and setting
geophones. Extraneous noise resulting from cultural features, wind, traffic, trains, or other
sources of seismic waves can be controlled to a certain degree through filtering, signal
stacking, geophone selection, geophone burial, logistics, and noise source control. The use of
explosives as a source requires that extra safety precautions be exercised by field personnel
and that a licensed explosives expert be responsible for explosives control, handling, and
Seismic reflection surveying methods are capable of obtaining continuous vertical and
lateral profiles of the subsurface geology using generally the same equipment requirements as
the refraction method. Shallow seismic reflection applications have, until recently, been
hindered by the lack of high frequency, short pulse seismic sources and by the inability to
overcome severe noise constraints generated by near surface ground "roll" phenomena.
The "optimum window" and "optimum offset" shallow seismic reflection profiling
techniques described by Hunter et al. (1984) have been used to map overburden and bedrock
reflections occurring at depths greater than approximately 60 to 100 feet in areas where large
velocity contrasts are observed. The optimum window shallow reflection technique is based
on the location of a shot geophone spacing that allows non normal incident reflections to be
observed with minimum interference from ground roll or from direct and refracted waves.
The resolution of the reflection method depends on the frequency of the seismic
energy that can be returned from the target reflector to the surface (Pullan et al., 1987), and
on the control of ground roll phenomena through filtering and the use of higher frequency
geophones. Optimum conditions for the technique occur when near surface sediments are
fine grained and water saturated (Pullan et al., 1987).
The shallow seismic reflection technique requires a geophysicist or geologist
experienced in the application and interpretation of the obtained seismic records. In contrast
to seismic refraction data, seismic reflection records can require sophisticated computer
processing and corrections to enhance the coherent features observable in the traces.