abem Terraloc Pro 2, Руководство пользователя

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ABEM Terraloc Pro 2
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The seismic reflection method has mainly been used for deep investigations (depth >
30 m) in oil prospecting. During recent years however, shallow reflection investigations
have become common for engineering and environmental purposes. It is now an
important complement to refraction investigations, and has even sometimes replaced
refraction. The main reasons for the increase in use of the reflection method is the
development of lightweight, high-performance seismographs and the possibility of
advanced data processing on inexpensive personal computers. Thus, the cost for
reflection investigations has decreased considerably.
Both acquisition and processing of reflection data are more complex and time
consuming than they are for refraction data.
Optimum Offset
This is a special case of the seismic reflection method, in which data are recorded with
a fixed source-receiver offset. It is a method for shallow investigations. The offset is
chosen to be an optimum value (hence the name), and typically, it is a window where
the reflection from the target is located between the refracted first arrivals and the
ground roll in the seismogram.
Tomography
The general idea for tomography is that information about the properties of the interior
of a region can be obtained through measurements at the boundary. Thus, this is a
method 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 source
and 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 is
measured and, sometimes, even amplitudes (direct or reflected) are analyzed. Then the
dataset goes through an inversion process where the spatial distribution of the physical
property is estimated. The technique is very computational intensive and is costly
because of the need for boreholes.
The final results are usually presented as maps or plots where the values of the physical
property are coded in color or grayscale.
An introduction to this method can be found in Worthington (1984).
VSP
VSP is short for Vertical Seismic Profiling, i.e. measurements with the receivers located
in a borehole and the source located on the ground. If the source is moved away from
the head of the borehole, it is called "offset VSP". In "Reversed VSP", the receivers are
located on the ground and the source is located in the borehole.
The VSP technique is seldom used alone, but is rather used to provide better
interpretation of seismic reflection data. VSP allows accurate determination of one-way
travel time to various geologic units and analysis of attenuation and acoustic
impedances, which are needed for construction of synthetic seismograms.
A brief introduction to this method is given in Cassel (1984).