Page 94 - Geothermal Energy Systems Exploration, Development, and Utilization

70 2 Exploration Methods
The travel time T between shot point S and geophone G is given by the length of
the ray path L and the seismic velocity v within the subsurface layer, such that
L
T = (2.12)
v
From Figure 2.12, we see that
2
L 2
2
x
= D + (2.13)
2 2
2 1/2
2
Such that T = 1/v(4D + x )
This equation contains D and v as unknowns, which can be constrained if
measurements of T are available for many geophones and shot points. Depending
on the shot point layout and the spacing of geophones, there is usually considerable
overlap of measurements over a common point on the reflector, which is referred
to as fold. Assuming a horizontal layer as the reflector, the travel times for
reflection events from a common point vary with offset (x in Figure 2.12). This
variation in travel time depends only on the velocity of the subsurface layer, thus
the subsurface velocity can be derived, assuming this velocity does not change
horizontally.
Incidence elastic waves reflected at a single reflector and then detected at the
surface are called primary reflections. However, in reality, many waves are reflected at
multiple interfaces before they are detected and are therefore referred to as multiple
reflections or multiples. Multiples generally have lower amplitudes than primary
reflections as energies are split at every reflection. The correct identification of
multiples is a crucial step in the interpretation of seismic traces. Travel times
of multiples can be calculated from the corresponding primary reflections and
can be identified and filtered by appropriate processing techniques. As signal
strength decreases significantly with depth, processing will always need to involve
improvement of the signal-to-noise ratio.
Each shot from the source generates not only multiple reflections but also
several different primary reflections from different boundaries at various depths.
The arrival times of the reflected waves vary with the depth of the reflector and
with the velocities of the different layers crossed by the wave. The seismic trace
resulting from a single shot at one receiver is thus composed of a series of
arrivals. These ‘‘spikes’’ will vary considerably in amplitude depending on the
attenuation within the subsurface. Generally, amplitude decreases rapidly with
depth. In addition to the reflections generated at different depths and the arrival of
multiples, seismic traces contain a lot of seismic noise and signals from surface
waves and air waves. All these signals result in a rather complex diagram, which
requires extensive processing before reflections can be recognized and interpreted.
The traces recorded by all receivers resulting from an individual shot are assembled
in shot gathers. Usually the traces are plotted side by side, allowing an alignment
of reflection events and their correlation from trace to trace.
Reflection profiles are taken with shot points and geophones aligned and
moved along lines, resulting in a 2D seismic survey. Such surveys are very
common, also for geothermal exploration. They supply sufficient information of