Geoscience Reference
In-Depth Information
Consequently, capabilities presently exist for obtaining continuous seismic measurements along a
transect, thereby dramatically increasing the speed at which a seismic survey can be conducted.
Seismic data analysis products include an assortment of different depth sections, maps, and
three-dimensional displays that depict various seismic wave attributes. Complex computer-process-
ing procedures are often required to generate these different seismic data analysis products. A
detailed description of seismic theory, data collection, and data analysis are beyond the scope of
this topic, and readers requiring more information on these topics are again referred to a number
of excellent texts that cover seismic methods (Coffeen, 1978; Dobrin and Savit, 1988; Lines and
Newrick, 2004; Pelton, 2005a, 2005b; Telford et al., 1976).
Equation (8.1) and Equation (8.2) show P-wave and S-wave velocities to be dependent on the
elastic moduli and density of the soil and rock material through which the waves travel. These elas-
tic moduli and density may be significantly correlated with soil properties and conditions. Conse-
quently, shallow investigation (<2 m) seismic methods that obtain information on spatial variations
of P-wave and S-wave velocities could provide important insight on spatial patterns of soil proper-
ties or conditions, such as horizontal and vertical changes in soil texture or compaction. In a similar
manner, the seismic wave adsorption coefficient in Equation (8.3) is a property that may also cor-
relate with soil properties and conditions; therefore, seismic methods capable of measuring spatial
changes in the adsorption coefficient could additionally supply information on soil property and
condition spatial patterns.
Up to this point, there has been no discussion regarding seismic wave frequency. Earthquake
seismic waves have frequencies ranging from 0.03 to 20 Hz (Cleveland Museum of Natural History,
2006). Traditional seismic methods, especially those used for exploration purposes in the petro-
leum industry, typically employ seismic waves with frequencies less than 100 Hz (Coffeen, 1978).
Sharma (1997) has a much more inclusive definition of seismic waves, which incorporates elastic
waves with frequencies up to 10 kHz. Based on this broader definition, a large percentage of acous-
tic waves would be considered seismic waves. Acoustic waves are simply P-waves that are audible
to the average human and have a frequency range from 20 Hz to 20 kHz. High-frequency seismic
waves, as indicated previously, attenuate more rapidly with distance traveled than do lower-fre-
quency seismic waves. Consequently, where investigation depths are large, as with petroleum explo-
ration, geophysical methods employing low-frequency seismic waves are appropriate; however, for
agricultural applications having extremely shallow investigation depths, geophysical methods using
higher-frequency seismic waves may provide useful information. Traditional seismic methods have
rarely been utilized for agricultural purposes, but laboratory studies employing 2 to 7 kHz acoustic
waves have provided evidence that acoustic wave velocities correlate significantly with soil compac-
tion, soil porosity, and soil water content, and acoustic wave adorption coefficients exhibit signifi-
cant correlation with soil bulk density and soil water content (Lu et al., 2004; Oelze et al., 2002).
Seismic tomographic imaging techniques have been proven effective for differentiating healthy and
decayed woods in tree trunks (al Hagrey, 2007). Therefore, seismic methods have promise in regard
to agricultural applications and will likely find greater use within the near future.
RefeRenCeS
al Hagrey, S.A. 2007. Geophysical imaging of root-zone, trunk, and moisture heterogeneity.
Journal of Exper-
imental Botany
. v. 58, pp. 839-854.
Clark, A. 1996.
Seeing Beneath the Soil: Prospecting Methods in Archaeology
. B.T. Batsford. London, UK.
Cleveland Museum of Natural History. 2006. Earthshaking terminology. www/cmnh.org/site/researchand-
collections_SeismicObservatory_EarthquakeTerminology.aspx.
Coffeen, J. A. 1978.
Seismic Exploration Fundamentals
. PennWell. Tulsa, OK.
Dobrin, M. B., and C. H. Savit. 1988.
Introduction of Geophysical Prospecting
, 4th edition. McGraw-Hill.
New York.
Gaffney, C., and J. Gater. 2003.
Revealing the Buried Past: Geophysics for Archaeologists
. Tempus. Glouces-
tershire, UK.
