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2.4 Geophysics 67
Wave

l

y
Displacement

l = wavelength
y = amplitude
Distance
Figure 2.10 Seismic wave parameters: wavelength λ = v/f ,
where v is speed of propagation and f is frequency. The
period T is the time for one complete cycle for an oscilla-
tion of a wave. The frequency f is the number of periods per
unit time (for example, 1 second) and is measured in hertz.
f = 1/T.

Seismic methods determine subsurface elastic properties influencing the prop-
agation velocity of elastic waves: as the waves travel through the subsurface, wave
velocities change depending on the density of the rock, and wave paths are reflected
and refracted by elastic discontinuities such as sedimentary layering, boundaries
between different rock units, and fractures. Because fractures present a consider-
able elastic discontinuity affecting the path and velocity of a wave, seismic methods
have the potential to identify not only their presence but also fracture attributes
such as orientation, density, aperture, and filling. This potential makes them par-
ticularly interesting for EGS exploration, where the orientation and distribution of
faults, fractures, fissures, and cracks is of utmost importance for the access to and
exploitation of the desired resource.
In terms of resolution, seismic methods provide the most detailed structural
information at depth. The maximum possible resolution is between one quarter
and on eight of the dominant wavelength if recent advances in the incorporation
of amplitude information are applicable. For a porous rock with a velocity of
2000 m s −1 and a frequency of 100 Hz resulting in a wavelength of 20 m, the
resolution limit would be 2.5–5 m. Seismic resolution decreases with depth as the
velocities normally increase and high frequencies are lost due to absorption. So,
the smallest features to be seen on a seismic diagram are still large at the surface
outcrop scale.

2.4.2.1 Active Seismic Sources
The physical phenomenon measured with geophones during seismic surveys is
the ground motion due to the elastic waves generated by explosions (shots) or
weight drop (Vibroseis). These geophones record the arrival of the waves that travel
along varying paths in the subsurface and thus arrive at different times and with