Seismic reflection profiling is accomplished by towing or mounting to the vessel, a sound source that emits acoustic energy in timed intervals behind a research vessel. The transmitted acoustic energy is reflected from boundaries between various layers with different acoustic impedances (i.e. the water-sediment interface or between geologic units). Acoustic impedance is defined by the bulk density of the medium times the velocity of the sound within that medium. The reflected acoustic signal is received either by a ship-towed hydrophone (or array of hydrophones) or with some chirp systems, by the same tuned transducer array that generates the outgoing source signal. The receiver converts the reflected signal to an analog signal. The analog signal is digitized, displayed, and logged with high-speed computers. The data can then be processed and plotted on paper or imported to computer mapping programs for interpretation.
The WHSC operates a variety of seismic sources in order to carry out operations in a variety of environments. The operating frequency ranges for the equipment we currently use are:
|Water gun||20-1500 Hz|
|Air Gun||100-1500 Hz|
|500 Hz-12 kHz|
2 - 7 kHz
4 - 24 kHz
3.5 kHz and 200 kHz
The higher frequencies of operation provide the highest resolution, but are limited in amount of penetration below the sea floor. The lower frequencies yield more penetration, but less resolution. This effect is displayed graphically below.
Relative comparison of power and frequencies for various seismic sound sources. (Derived from Trabant, 1984)
When selecting a system or systems to use in a prospective study, we try to determine the geologic setting if possible; for example -- What are potential sediment types and geologic units? What is the maximum stratigraphic depth that is relevant to the particular study? This aids in determining the appropriate system to use. If little information is available to us, we will operate multiple seismic-reflection systems simultaneously. The research objectives and survey environment will dictate system choice. One consideration is always the trade-off between range, or penetration, and resolution. In the marine, lacustrine, or estuarine environments, the best source is determined primarily by the water depth and the type of sediments/rocks in the substrate. Additionally, logistical parameters (e.g. cost, boat size, noise, time available, number of crew available, weather, environmental factors (ambient noise, ship traffic, etc.) enter into the decision as to which system(s) will be utilized for a given survey. The Woods Hole Science Center has water gun, air gun, boomer, and sparker as "'impulse' type sources. We also operate four chirp, or swept frequency, sources whose operating-frequencies are listed in the table above.
As examples of the capabilities of the different systems, data from research programs that involved the use of multiple seismic systems are shown. One study was run offshore South Carolina (http://woodshole.er.usgs.gov/project-pages/scarolina/). Data from two chirp systems (left and center image), and a boomer system (image on right) are displayed. The vertical scale in each is 30 milliseconds two-way travel time. (Baldwin et al USGS Open File Report 2004-1013).
Another example is shown from a study that was carried out in Bear Lake, Idaho-Utah in which chirp, 3.5 kHz, and boomer systems were used as part of a comprehensive research program. The lake floor is best defined with the highest operating frequency system, the 3.5 kHz (center image). The uppermost layers are best resolved with the Chirp (left image), while the boomer (right image) clearly defines the deeper structure. This demonstrates how a variety of sources can complement one another.
Baldwin, W.E., Morton, R.A., Denny, J.F., Dadisman, S.V., Schwab, W.C., Gayes, P.T., and Driscoll, N.W., 2004, Maps showing the stratigraphic framework of South Carolina's Long Bay from Little River to Winyah Bay, U.S. Geological Survey Open-File Report 2004-1013.
Denny, J.F. and S.C. Colman, 2002, Geophysical surveys of Bear Lake, Utah-Idaho, September, 2002, U.S. G.S. Open File Report, 03-150, online.
Trabant, P.K., 1984, Applied High-Resolution Geophysical Methods: Boston, Ma., International Human Resources Development Corporation, p.103.