abem Terraloc Pro 2 User Manual
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ABEM Terraloc Pro 295The seismic reflection method has mainly been used for deep investigations (depth >30 m) in oil prospecting. During recent years however, shallow reflection investigationshave become common for engineering and environmental purposes. It is now animportant complement to refraction investigations, and has even sometimes replacedrefraction. The main reasons for the increase in use of the reflection method is thedevelopment of lightweight, high-performance seismographs and the possibility ofadvanced data processing on inexpensive personal computers. Thus, the cost forreflection investigations has decreased considerably.Both acquisition and processing of reflection data are more complex and timeconsuming than they are for refraction data.Optimum OffsetThis is a special case of the seismic reflection method, in which data are recorded witha fixed source-receiver offset. It is a method for shallow investigations. The offset ischosen to be an optimum value (hence the name), and typically, it is a window wherethe reflection from the target is located between the refracted first arrivals and theground roll in the seismogram.TomographyThe general idea for tomography is that information about the properties of the interiorof a region can be obtained through measurements at the boundary. Thus, this is amethod for finding the (2-dimensional) distribution of some physical property (e.g.velocity, reflectivity, bulk modulus, etc.). It can involve borehole-to-borehole, surface-to-borehole, or surface-to-surface measurements. The main restriction is that the sourceand receiver positions, and hence any boreholes, must be confined to the same plane.This plane can have any orientation.Usually the travel times for a large number of ray paths through the rock volume ismeasured and, sometimes, even amplitudes (direct or reflected) are analyzed. Then thedataset goes through an inversion process where the spatial distribution of the physicalproperty is estimated. The technique is very computational intensive and is costlybecause of the need for boreholes.The final results are usually presented as maps or plots where the values of the physicalproperty are coded in color or grayscale.An introduction to this method can be found in Worthington (1984).VSPVSP is short for Vertical Seismic Profiling, i.e. measurements with the receivers locatedin a borehole and the source located on the ground. If the source is moved away fromthe head of the borehole, it is called "offset VSP". In "Reversed VSP", the receivers arelocated on the ground and the source is located in the borehole.The VSP technique is seldom used alone, but is rather used to provide betterinterpretation of seismic reflection data. VSP allows accurate determination of one-waytravel time to various geologic units and analysis of attenuation and acousticimpedances, which are needed for construction of synthetic seismograms.A brief introduction to this method is given in Cassel (1984). |
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