Geoscience Reference
In-Depth Information
S 1
G 2
G 3
G 4
G 5
G 6
G 7
a
b
c
c
c
c
Layer 1
Layer 2
fIGURe 8.9 Three main seismic wave travel paths from source ( S ) to geophone sensors ( G 1 , G 2 , G 3 , G 4 , G 5 ,
G 6 , and G 7 ): (a) direct path, (b) reflected path, and (c) refracted path.
ρ
ρ ρ
V
VV
2
T
C =
11
(8.9)
+
11
22
where V 1 is the P-wave or S-wave velocity of Layer 1, V 2 is the P-wave or S-wave velocity of Layer 2,
and all other quantities have been previously defined. The ρ V product term in Equation (8.7) through
Equation (8.9) is referred to as the “acoustic impedance” for a particular soil and rock material.
The three main seismic wave travel paths from source to geophones are depicted in Figure 8.9.
The direct travel path (a) along the surface from source to geophones can be taken by all seismic
wave types including Rayleigh and Love surface waves along with P and S body waves. Only seis-
mic body waves take the reflected (b) and refracted (c) travel paths. Furthermore, as illustrated in
Figure 8.8, conversions between P-waves and S-waves can occur along these reflected and refracted
seismic wave travel paths. Figure 8.9 represents source-to-geophone travel paths based on a subsur-
face having two layers. As the number of distinct subsurface layers increases, so too do the number
of possible source-to-geophone reflected and refracted seismic body wave travel paths.
8.3.4
s e i is M i c M e t h o d d a t a c o l l e c t i o n , d a t a a n a l y s i s , a n d P o t e n t i a l
a g R i c u l t u R a l a P P l i c at i o n s
Seismic survey data are typically collected as the energy source and an array of geophone sensors
are moved along a transect or series of transects. Explosive, impact, vibratory, and acoustic energy
are commonly employed to generate seismic waves. Geophones convert seismic ground vibrations
into an electrical potential difference (voltage) that is then recorded (Sheriff, 2002). As previously
mentioned, the energy source is ordinarily positioned on the surface or at a shallow depth, and the
array of geophones is normally inserted at the ground surface. Seismic reflection surveys measure
the travel times and amplitudes associated with seismic waves taking reflected travel paths (b in
Figure 8.9). Relative to the depth of investigation, the source and the geophone array are located
fairly close together when conducting a seismic reflection survey. Seismic refraction surveys are
focused predominantly on measuring travel times for seismic waves that take refracted travel paths
(c in Figure 8.9). For a seismic refraction survey, the source and the geophone array are located a
considerable distance apart relative to the depth of investigation. Some seismic surveys concentrate
only on seismic waves traveling directly from source to geophone (a in Figure 8.9), particularly in
regard to measurement of body wave travel times and amplitudes or surface wave travel times and
dispersion characteristics (especially for Rayleigh waves).
There is continuous development of new and improved seismic equipment for field data acquisi-
tion. Safe, portable seismic energy sources are now available for shallow subsurface investigations.
Land-based geophone array streamers have been recently produced that can be pulled along the
ground surface without the inconvenience of having to detach and then reinsert individual geophones.
Search WWH ::




Custom Search