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

detonation.

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.

November 1992

4 26