Seismic Reflection Profiling

OUTLINE

Seismic reflection profiling involves the measurement of the two-way travel time of seismic waves transmitted from surface and reflected back to the surface at the interfaces between contrasting geological layers. Reflection of the transmitted energy will only occur when there is a contrast in the acoustic impedance (product of the seismic velocity and density) between these layers. The strength of the contrast in the acoustic impedance of the two layers determines the amplitude of the reflected signal. The reflected signal is detected on surface using an array of high frequency geophones (typically 48-96). As with seismic refraction, the seismic energy is provided by a ‘shot’ on surface. For shallow applications this will normally comprise a hammer and plate, weight drop or explosive charge.

DETAIL

In most reflection surveys shots are deployed at a number of different positions in relation to the geophone array in order to obtain reflections from the same point on the interface at different geophones in the array. Each common point of reflection is termed a common mid-point (CMP) and the number of times each one is sampled determines the ‘fold coverage’ for the survey. Traces relating to the same CMP are stacked together to increase the signal-to-noise ratio of the survey before being combined with other CMP’s stacked traces to produce a reflection profile. In order to stack related CMP traces a stacking velocity is applied to each trace. This accounts for the difference in two-way travel time between the normal incidence reflection (vertical travel path below the shot) and those at increasing offsets from the shot (known as the normal moveout or NMO). The stacking velocity will vary down the trace to take account of the increase in velocity with depth for each reflection event. The simplest form of seismic reflection profiling is the constant-offset method. This technique uses a single geophone offset from the source by a fixed distance. The two are moved along the survey line in equal steps with a single trace being recorded at each position. The main advantage of this technique is the limited amount of processing that needs to be applied to the data due to the almost vertical orientation of each ray path. However, in order to avoid problems with interference from ground roll and the shot airwave, the offset distance has to be selected with care.