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How the self-floating ocean bottom seismograph (OBS) works

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Self-floating ocean bottom seismograph (OBS)

  The self-floating ocean bottom seismograph (OBS) is a device capable of recording minute vibrations transmitted from the seafloor. It is used to record natural earthquakes and perform a seismic refraction survey of the sea-bottom structure. The following is a brief explanation of this survey method.

Self-floating ocean bottom seismograph (OBS)
Self-floating ocean bottom seismograph (OBS)
Maximum pressure-withstand operating depth: 6,000 m
Total weight: approximately 80 kg

Hard hat Hard hat
A device specially created to protect the internal glass ball from shocks and to house other equipment. The internal glass ball itself actually houses the data-recording equipment.
Transponder and transducer Transponder and transducer
These are a control circuit inside the pressure capsule, a battery, and the transducer (the transmitter/receiver for the sound waves). The OBS, which is deployed on the seafloor, receives an acoustic signal originated from the transmitter/receiver at the bottom of the vessel via the transducer and activates the separator. It is also possible to estimate the position of the OBS on the seafloor by having the vessel receive the sound wave transmitted from the transducer.
Separator Separator
The OBS and the anchor are connected by two thin stainless steel sheets. The transponder applies an electric current to these sheets to separate the OBS from the anchor by forced electric corrosion, a method of artificially producing corrosion for fusion. When the process is completed, the OBS begins to float toward the sea surface.
Anchor Anchor
The purpose of the anchor, which weighs about 40 kg, is to anchor the OBS firmly to the seafloor. The anchor usually remains connected to the OBS via the separator, and is disconnected only when the OBS is to be recovered.
Hydrophone Hydrophone
The hydrophone receives sound waves (direct waves) originated by an air gun or the like. It remains connected to the data recording section inside the glass ball and records any sound waves received.
Flasher Flasher
Equipped with a battery, the flasher lights when the OBS has floated to the sea surface. The flasher makes it possible to recover the OBS at night.
Radio beacon Radio beacon
Equipped with a battery, the radio beacon originates a radio wave when the OBS has floated to the sea surface. The radio wave is received by the on-board direction finder for OBS recovery.
Seismic sensor Seismic sensor
A built-in seismic sensor consisting of three components (two horizontal axes and one vertical axis) is used to send a vibration occurring at the seafloor to the recording device. In addition to a speed sensor capable of detecting relatively slow movements, the sensor also contains a built-in gimbal mechanism that maintains the OBS's horizontal attitude even when a slope is encountered.
Recording device Recording device
Four-component observation data, which is the three-component vibration recorded by the seismic sensor and the sound waves recorded by the hydrophone outside the glass ball, is converted to digital form and recorded on a hard disk.
Battery Battery
The capacity of the battery determines the duration of observation for the ocean bottom seismograph, which operates independently on the seafloor. Options include a lithium-ion storage battery for a three-week observation period or a lithium-ion battery for a three-month observation period.
   
 

Flow of a seismic refraction survey

 
1. Where OBSs are deployed
When the vessel arrives at the point where an OBS is to be deployed, the OBS is allowed to fall freely from the sea surface to the seafloor. The position where the OBS is to be seated is estimated from the data regarding the communication distance and direction obtained by an exchange of signals between the transponder and the vessel. Usually, 110 OBSs are arranged in a line at intervals of 5 km.
2. Air-gun oscillation
At a water depth of roughly 10 m, compressed air is released instantaneously to produce a strong sound (vibration). The device that does this is called an air gun, and is the same as the air gun used in an MCS system. The vibration passes through the geologic stratum at the seafloor surface to the intersection with another stratum (at a maximum depth of about 30 km). It then returns to the seismic sensor housed in the OBS and is recorded. Usually, the air gun is oscillated at an interval of 200 m on a course set so that the oscillation runs directly above all deployed OBSs. Because vessel speed is 3 to 4.5 knots (5 to 8 km/h), it takes 4 to 6 days for the air gun to complete its oscillations.
3. OBS recovery
The vessel moves to a position directly above the OBS to be recovered, at which point the transmitter/receiver at the bottom of the vessel sends a separation signal to the transponder. When it receives the separation signal, the transponder applies a current to the separator (stainless steel sheets) to disconnect the anchor from the OBS (yellow hard hat) by forced electric corrosion, which is a method of artificially producing corrosion for fusion. Once the separation is complete, the OBS floats to the sea surface for on-board recovery. Usually, 6 to 8 days are required to recover all 110 OBSs.
Nippon Marine Enterprises, Ltd.
Nippon Marine Enterprises, Ltd.
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