An ocean-bottom seismometer (OBS) is a

seismometer A seismometer is an instrument that responds to ground noises and shaking such as caused by earthquakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The outpu ...

that is designed to record the earth motion under oceans and lakes from man-made sources and natural sources. Sensors at the sea floor are used to observe acoustic and

seismic Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

events. Seismic and acoustic signals may be caused by different sources, by

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

s and tremors as well as by artificial sources. Computing and analyzing the data yields information about the kind of source and, in case of natural seismic events, the

geophysics Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' so ...

and

geology Geology () is a branch of natural science concerned with Earth and other Astronomical object, astronomical objects, the features or rock (geology), rocks of which it is composed, and the processes by which they change over time. Modern geology ...

of the sea floor and the deeper crust. The deployment of OBS along a profile will give information about the deep structure of the Earth's crust and

upper mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle at . Temperatures range from appr ...

in offshore areas. OBS may be equipped with a maximum of a three-component

geophone A geophone is a device that converts ground movement (velocity) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analyzed for structure of ...

in addition to a

hydrophone A hydrophone ( grc, ὕδωρ + φωνή, , water + sound) is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potenti ...

, and thus it needs a capacity of more than 144 Mbytes, which would be the minimum for an adequate MCS profiling. In a typical survey, the instruments should be operational for several days (deployments can exceed 12 months), which requires a data storage capacity of more than 500 Mbyte. Other experiments, such as tomographic investigations within a 3D-survey or seismological monitoring, demand even larger capacities.

Instrument package

The OBS consists of an

aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

sphere which contains sensors, electronics, enough alkaline batteries to last 10 days on the ocean bottom, and an acoustic release. The two sphere halves are put together with an O-ring and a metal clamp to hold the halves together. A slight vacuum is placed on the sphere to better ensure a seal. The sphere by itself floats, so an anchor is needed to sink the instrument to the bottom. In this case, the anchor is flat metal plate 40 inches (1.02 meters) in diameter. The instrument has been designed to be able to deploy and recover off almost any vessel. All that is needed (for deployment and recovery) is enough deck space to hold the instruments and their anchors and a boom capable of lifting an OBS off the deck and swing it over to lower it into the water. The OBS is bolted to the anchor and then dropped (gently) over the side.

Working

Seismometers work using the principle of inertia. The seismometer body rests securely on the sea floor. Inside, a heavy mass hangs on a spring between two magnets. When the earth moves, so do the seismometer and its magnets, but the mass briefly stays where it is. As the mass oscillates through the magnetic field it produces an electric current which the instrument measures. The seismometer itself is a small metal cylinder; the rest of the footlocker-sized OBS consists of equipment to run the seismometer (a data logger and batteries), weight to sink it to the sea floor, a remote-controlled acoustic release and flotation to bring the instrument back to the surface.

Types of OBS

The

ground motion Ground motion is the movement of the earth's surface from earthquakes or explosions. Ground motion is produced by seismic waves that are generated by sudden slip on a fault or sudden pressure at the explosive source and travel through the earth a ...

caused by earthquakes can be extremely small (less than a millimeter) or large (several meters). Small motions have high frequencies, so monitoring them requires measuring movement many times per second and produces huge amounts of data. Large motions are much rarer, so instruments need to record data less frequently, to save memory space and battery power for longer deployments. Because of this variability, engineers have designed two basic kinds of seismometers:

Short-period OBSs

They record high-frequency motions (up to hundreds of times per second). They can record small, short-period earthquakes and are also useful for studying the outer tens of kilometers of the seafloor. Technical details for two models: WHOI D2 and Scripps L-CHEAPO.

Long-period OBSs

They record a much broader range of motions, with frequencies of about 10 per second to once or twice a minute. They are used for recording mid-sized earthquakes and seismic activity far from the instrument. Technical details for two models: WHOI long-deployment OBS and Scripps long-deployment OBS.

Custom OBSs

Custom OBS are beginning to be developed, as the need for expansion on coverage in the field of seismology increases Romanowicz, Barbara, et al. "MOISE: A pilot experiment towards long term sea-floor geophysical observatories." EARTH PLANETS AND SPACE 50 (1998): 927–938 and permanent deployments are necessary. One customizations to improve the data quality of the seismometers is to

borehole A borehole is a narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water ( drilled water well and tube well), other liquids (such as petrol ...

the seismometer in an aluminum casing into the surface (~1 m) to create stability in the soft sediment of the ocean floor. Another customization that is possible is to add a differential pressure gauge (DPG) and/or current meter, in order to understand how the pressure is changing around the seismometer. It can also be practical to store the datalogger and battery in
glass Benthos sphere
in order to be able to connect to the ship through the use of a remotely operated vehicle (ROV), which is a necessary advancement in order to have and maintain permanent OBS deployments.

Advantages

Very stable clocks make comparable the readings from many far-flung seismometers. (Without reliable time-stamps, data from different machines would be unusable.) Development of these clocks was a crucial advance for seismologists studying the Earth's interior. After recovering an ocean-bottom seismometer, scientists can offload the instrument's data by plugging in a data cable. This feature saves the task of gingerly disassembling the instrument's protective casing while aboard a rolling ship. The ability to connect a seismometer to a mooring or observatory makes the instrument's data instantly available. This is a huge advantage for geologists scrambling to respond to a major earthquake.

Disadvantages

The environment of these deployments complicates standard methods that are used in analyzing the data because of the ocean on top of the seismometer, as opposed to free-air above a typical land station. These seismometers also have a decreased

signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in de ...

because of noise created by the movement of the oceans due to wind driven tides, particularly at periods of 7 and 14 seconds. This long period motion and current flowing around the seismometer can create problems of long period noise on the horizontal components because the soft (saturated) sediment that the seismometer is resting on is more susceptible to allow the seismometer to tilt and ideally, the horizontal component will not move and be perpendicular to gravity to get the best results out of the seismometer. The saturated sediment also reduces signal-to-noise ratio significantly because the velocity of the P and

S-waves __NOTOC__ In seismology and other areas involving elastic waves, S waves, secondary waves, or shear waves (sometimes called elastic S waves) are a type of elastic wave and are one of the two main types of elastic body waves, so named because th ...

decreases and the seismic waves get trapped in the sediment layer creating a large amplitude ringing due to the

conservation of energy In physics and chemistry, the law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be ''conserved'' over time. This law, first proposed and tested by Émilie du Châtelet, means tha ...

Notable deployments

One of the largest OBS deployments ever was The Big Mantle Electromagnetic and Tomography (Big MELT) Experiment, involving almost 100 OBS in the

East Pacific Rise The East Pacific Rise is a mid-ocean rise (termed an oceanic rise and not a mid-ocean ridge due to its higher rate of spreading that results in less elevation increase and more regular terrain), a divergent tectonic plate boundary located alon ...

with the goal of understanding magma generation and

mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a div ...

development. The Cascadia Initiative is an offshore/onshore deployment to observe the deformation of the Juan de Fuca and Gorda Plates, as well as topics ranging from

megathrust earthquake Megathrust earthquakes occur at convergent plate boundaries, where one tectonic plate is forced underneath another. The earthquakes are caused by slip along the thrust fault that forms the contact between the two plates. These interplate earthqu ...

s to volcanic arc structure in the

Pacific Northwest The Pacific Northwest (sometimes Cascadia, or simply abbreviated as PNW) is a geographic region in western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Thou ...

. The Hawaiian PLUME (Plume-Lithosphere Undersea Melt Experiment) was an onshore/offshore (predominantly offshore) deployment to better understand what type of

mantle plume A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hot ...

is beneath Hawaii and to better understand the mantle upwelling in this region and its relationship to the lithosphere. The Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment (ALBACORE) deployment from 2010 to 2011 of 34 OBS to help better understand the tectonic interaction at the Pacific-North America plate boundary and deformation styles of the Pacific plate and the nearby microplates.

References

{{Reflist

External links

"French OBS"

"K.U.M. OBS"

Seismology instruments