principles and applications of microearthquake networks

2.4. Oceun- Bused Mic~roearthquake Systems 43
etry to a central recording site. Typically the central site is a nearby
ship, but for short-lived systems aircraft can be used. A land-based recording
site can be used if the sonobuoy array is sufficiently close to land.
A major problem with sonobuoy arrays is determining the location of
each unit so that precise hypocenters can be calculated. The drift of the
sonobuoy units relative to each other further complicates the problem.
Reid et (11. (1973) used an air gun aboard the recording ship to determine
the relative sonobuoy position. The time delay between firing the air gun
and the arrival of the direct water wave at the hydrophone gives the
distance between the ship and the hydrophone. If the air gun shots are
recorded with the ship at different positions, and if the sonobuoy has not
drifted too far in the meantime, then the position of each sonobuoy relative
to the ship can be determined by triangulation. Absolute position of
the ship can be determined by standard navigational methods.
Reid et cil. (1973) found that predominant frequencies of oceanic microearthquakes
were largely in the 20-Hz range. This required no modification
of the frequency response characteristics of most hydrophones. They
stated that their system could detect magnitude zero earthquakes at a
distance of about 10 km.
If the sonobuoy has a life of a few days, and if the array is close to land,
then the expense of a ship standing by just to record the data may be
avoided by anchoring sonobuoys to the ocean bottom and recording on
land. However, moored systems are much noisier, and special anchors,
cables, and other devices must be used.
2.4.3. Anchored Buoy OBS Systems
In the anchored buoy system, the instrument package that rests on the
sea floor contains everything necessary to detect and record the seismic
data. The package is lowered to the sea floor by means of a cable. The
cable usually is attached to an anchored buoy, although it may be attached
to a ship. The instrument package must be isolated from the buoy and the
cable to avoid vibrational noise. Rykunov and Sedov (1967) and Nagumo
and Kasahara (1976) described cable systems in which the buoy was held
in place by an anchor and ballast weights on the sea floor. The instrument
package was 150-250 m from the anchor and was connected to it by a
series of shackles and rope. The package was recovered by pulling it up
with the rope. The anchored buoy method allows the instrument package
to be self-contained, but operation in deep ocean is difficult (Francis,
1977).
Rykunov and Sedov ( 1967) used a vertical-component seismometer
with 3.5-Hz natural frequency. Seismic data were recorded on analog