Spectrum geophysics

Seismic refraction is a surface method that uses the properties of acoustic waves to measure the velocity of the material through which the waves travel. The seismic refraction method has a long history with the oil industry, and as such the equipment is easy to obtain and deploy. Because this method uses travel time to measure the seismic properties of materials it is highly precise in the measurement of seismic velocity and quite accurate in the measurement of material thicknesses in many instances. Because seismic velocity is diagnostic for different types of material, and generally increases with degree of induration or hardness, the seismic refraction method not only can measure depth to a hard layer but can be used to non-invasively classify the type of material (e.g. soft sedimentary vs. igneous) and rippability of the layer encountered. In addition, seismic layers often correlate closely with geologic contacts. Spectrum utilizes the Seistronix RAS-24 twenty-four channel signal enhancement seismograph along with geophones and associated cabling to collect seismic refraction data.

The seismic refraction method is based on the fact that when a wave reaches a boundary between two materials having different seismic velocities, that seismic wave will be refracted (or bent) either toward the normal to the interface or away from the normal to the interface, depending on whether the velocity increases or decreases at the boundary. In the special case where layer velocity increases with depth at the boundary, critical refraction occurs where seismic waves travel along the interface between the two materials. The angle at which the seismic waves are critically refracted (the critical angle) is uniquely determined by the ratio of the velocities of the two materials: θc= sin^-1(V1/V2), and because the critical angle is uniquely determined, the depth at which the boundary between the layers occurs can be calculated using geometry and the measured first arrival travel times.

In practice, a linear array (or spread) of geophones is established along the ground surface and connected to a multi-channel seismograph. The seismic source is then established at a certain location along the line, and a seismic “shot” is then made. The first arrivals at each geophone location on the seismic record from a given shot are plotted on a time vs. distance graph. The slope of the line segments created by the first arrivals is the inverse of the velocity of the material through which the waves have traveled, and the intercept time, crossover distance or critical distance can be used to calculate the depth to the target layer. During a typical seismic refraction survey, shots at several different locations on the spread are made in order to obtain a measure of the two-dimensional variation of seismic velocity with depth along the line. Several different software programs are currently available to process seismic refraction data, and the end product is generally a two-dimensional profile indicating the variation of seismic velocity along the line.

Spectrum uses the generalized reciprocal method (GRM) to acquire, process, and interpret seismic refraction data. GRM is a seismic refraction interpretation method designed to accurately map undulating refractor surfaces from in-line refraction data using both forward and reverse shots. The method is related to the Hales and the reciprocal methods of seismic refraction interpretation. In this method it is crucial to acquire at least 7 shots per spread: Assuming a 24-channel acquisition system, this requires one center shot between geophones 12 and 13, one shot between each of geophone pairs 6 and 7 and 18 and 19, a near-offset shot from each end geophone (geophones 1 and 24), and one far-offset shot off each end of the spread. In practice, Spectrum often uses 13 shots per spread for high resolution investigations. In order to map the bedrock surface, one condition that must be met is that of having an off-end shot at each end of the spread such that each first arrival on the geophone spread is a bedrock arrival.

Blind zones and hidden layers may be encountered. A hidden layer is an intermediate velocity, intermediate depth layer whose thickness or velocity is such that rays from a deeper, higher velocity layer arrive at the ground surface sooner than rays from the hidden layer. Standard seismic refraction interpretation schemes will yield significant errors in calculated refractor depths in the presence of blind zones or hidden layers. GRM is the only method capable of overcoming this problem.

Data processing includes the selection of first break arrival times, the generation of time-distance plots for each line, the assignment of selected portions of the travel time data to individual refractors, and the phantoming of travel time data for the target (lower) refractor. Once this preprocessing work has been done, GRM processing begins. Once this is done, layer thicknesses and velocities are calculated and a geophysical interpretation of the geological parameters may be made. The end product is a seismic refraction profile that indicates the seismic layers detected, the depths to the interfaces between layers as they vary along the line and the seismic velocities encountered.

Seismic reflection is a surface method that uses the properties of acoustic waves to obtain highly detailed images of subsurface layers and bedding planes to significant depths. The seismic reflection method has a long history with the oil industry where seismic sections are interpreted for subsurface structural features such as anticlines, faults, unconformities and other sources of oil traps. More recently, seismic reflection methods have been used for environmental and engineering investigations to delineate faults, aquifers, aquitards or other subsurface features of interest.

The seismic reflection method is based on the fact that when a wave reaches a boundary between two materials having different acoustic impedances (product of velocity and density) that wave will be reflected back to the surface. The angle at which the seismic waves are reflected is determined by the angle of incidence of the waves.

In practice, a linear array (or spread) of geophones is typically established along the ground surface and connected to a multi-channel seismograph. Once this is done, Spectrum uses a common midpoint (CMP) shooting configuration with a short-as-possible geophone and shot interval (5 or 10 feet for p-wave and 2.5 feet for s-wave) for high resolution, high quality reflection data. We use a 48- to 72-channel system and high frequency (40 Hz for p-wave and 28 Hz for s-wave) geophones. Seismic sources consist of a 20-lb sledgehammer on an aluminum plate or a weight drop for p-wave, or a 20-pound sledgehammer struck sideways on a wood plank or a steel “brick” for S-wave.

Because of the presence of unconsolidated and/or saturated sediments, we have found that shear wave reflection provides superior results to p-wave reflection in many areas of southern California. This is because the shear wave wavelength is generally 1/3 to 1/5 that of the p-wave wavelength in the presence of unconsolidated and/or saturated sediments and thereby can detect smaller changes in velocity and density along a line than p-waves. In addition, unconsolidated sediments often do not generate strong p-wave reflections; whereas a small change in grain size at the interface between two sediment layers (such as a silt versus a sand) will cause a measurable shear wave reflection. Shear wave surveys, although more expensive and time consuming than p-wave surveys, are absolutely necessary in the presence of a shallow water table because shear waves don’t propagate in water and therefore are capable of detecting changes in the velocity and density of sediments as if the water were not there at all. By contrast, P-wave surveys are heavily affected by the presence of the water table because the overlying dry sediments/saturated sediments interface generates very strong primary reflections, as well as multiples in the data, that can dominate the reflection section. The lateral and vertical resolution of p-waves in saturated sediments is also greatly reduced from that of shear waves because the p-wave velocity of saturated materials is much higher than the shear wave velocity of those materials (there is an inverse relation between velocity and resolution for the same frequency). Because vehicle and aircraft traffic are significant sources of noise to a seismic reflection survey, data are commonly collected during times of least amount of traffic.