Page 20 - Acquisition and Processing of Marine Seismic Data

1.2 MARINE ACOUSTIC METHODS 11

scientific purposes worldwide, and are not 10 and 200 Hz) or spark arrays (between 50 and
included in the site surveys. 800 Hz), while higher-frequency acoustic signals
The marine geophysical methods for high- from approximately 3 to 800 kHz are produced in
resolution site surveys use different sensors to the water column by specific instruments called
produce acoustic signals within a significantly transducers. These instruments convert one type
wide frequency spectrum. Table 1.2 shows gen- of energy (e.g., the electric signal) into another
eral information on the frequency band of the (e.g., the acoustic signal), or vice versa. Trans-
most common marine acoustic systems. The most ducers use piezoelectric crystals made by ceramic
suitable method with an appropriate frequency is material, which vibrate when they are excited by
selected by factors such as required information, an electric pulse. These vibrations are transmitted
resolution and penetration purposes. Relatively into the water column as pressure (P)waves to
low-frequency signals used in seismic surveys generate the acoustic signal. The same transducer
are produced by air guns (generally between is also used to receive the reflected signals, and

TABLE 1.2 Frequency Ranges of Major Marine Geophysical Survey Systems