Marine geophysics

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Marine geophysics is the scientific discipline that employs methods of geophysics to study the world's ocean basins and continental margins, particularly the solid earth beneath the ocean. It shares objectives with marine geology, which uses sedimentological, paleontological, and geochemical methods. Marine geophysical data analyses led to the theories of seafloor spreading and plate tectonics.

Contents

Age of oceanic lithosphere Age of oceanic lithosphere.png
Age of oceanic lithosphere

Methods

Marine geophysics uses techniques largely employed on the continents, from fields including exploration geophysics and seismology, and methods unique to the ocean such as sonar. Most geophysical instruments are used from surface ships but some are towed near the seafloor or function autonomously, as with Autonomous Underwater Vehicles or AUVs.

Objectives of marine geophysics include determination of the depth and features of the seafloor, the seismic structure and earthquakes in the ocean basins, the mapping of gravity and magnetic anomalies over the basins and margins, the determination of heat flow through the seafloor, and electrical properties of the ocean crust and Earth's mantle.

Modern marine geophysics, as with most oceanographic surveying with research ships, use Global Positioning System satellites, either the U.S. GPS array or the Russian GLONASS for ship navigation. Geophysical instruments towed near the seafloor typically use acoustic transponder navigation sonar networks.

Ocean depth

The depth of the seafloor is measured using echo sounding, a sonar method developed during the 20th century and advanced during World War II. Common variations are based on the sonar beam width and number of sonar beams as is used in multibeam sonar or swath mapping that became more advanced toward the latter half of the 20th century. [1]

Multibeam sonar swath mapping Fis01334 (27555144884).jpg
Multibeam sonar swath mapping

Sedimentary cover of the seafloor

The thickness and type of sediments covering the ocean crust are estimated using the seismic reflection technique. This method was highly advanced by offshore oil exploration companies. The method employs a sound source at the ship with much lower frequencies than echo sounding, and an array of hydrophones towed by the ship, that record echoes from the internal structure of the sediment cover and the crust below the sediment. In some cases, reflections from the internal structure of the ocean crust can be detected. [2] Echo sounders that use lower frequencies near 3.5 kHz are used to detect both the seafloor and shallow structure below the seafloor.  Side-looking sonar, where the sonar beams are aimed just below horizontal, is used to map the seafloor bottom texture to ranges from tens of meters to a kilometer or more depending on the device.

Structure of the ocean crust and upper mantle

When the sound or energy source is separated from the recording devices by distances of several kilometers or more, then refracted seismic waves are measured. Their travel time can be used to determine the internal structure of the ocean crust, and from the seismic velocities determined by the method, an estimate can be made of the crustal rock type. [3]   Recording devices include hydrophones at the ocean surface and also ocean bottom seismographs. Refraction experiments have detected anisotropy of seismic wave speed in the oceanic upper mantle. [4]

Measuring Earth’s magnetic and gravity fields within the ocean basins

The usual method of measuring the Earth's magnetic field at the sea surface is by towing a total field proton precession magnetometer several hundred meters behind a survey ship. [5] In more limited surveys magnetometers have been towed at a depth close to the seafloor or attached to deep submersibles. [6]   Gravimeters using the zero-length spring technology are mounted in the most stable location on a ship; usually towards the center and low. They are specially designed to separate the acceleration of the ship from changes in the acceleration of Earth's gravity, or gravity anomalies, which are several thousand times less. In limited cases, gravity measurements have been made at the seafloor from deep submersibles. [7]

Determine the rate of heat flow from the Earth through the seafloor

The geothermal gradient is measured using a 2-meter long temperature probe or with thermistors attached to sediment core barrels. Measured temperatures combined with the thermal conductivity of the sediment give a measure of the conductive heat flow through the seafloor. [8]

Measure the electrical properties of the ocean crust and upper mantle

Electrical conductivity, or the converse resistivity, can be related to rock type, the presence of fluids within cracks and pores in rocks, the presence of magma, and mineral deposits like sulfides at the seafloor. [9] Surveys can be done at either the sea surface or seafloor or in combination, using active current sources or natural Earth electrical currents, known as telluric currents. [10]

In special cases, measurements of natural gamma radiation from seafloor mineral deposits have been made using scintillometers towed near the seafloor. [11]

Examples of the impact of marine geophysics

Evidence for seafloor spreading and plate tectonics

Echo sounding was used to refine the limits of the known mid-ocean ridges, and to discover new ones. [12] [13] Further sounding mapped linear seafloor fracture zones that are nearly orthogonal to the trends of the ridges. [14] [15]  Later, determining earthquake locations for the deep ocean discovered that quakes are restricted to the crests of the mid-ocean ridges and stretches of fracture zones that link one segment of a ridge to another. These are now known as transform faults, one of the three classes of plate boundaries. [16]  Echo sounding was used to map the deep trenches of the oceans and earthquake locations were noted to be located in and below the trenches. [17]

A spreading center (ridge segment) offset by a transform fault. Both are plate boundaries. A fracture zone is the scar of the active transform fault Fracture Zone - bstern3.png
A spreading center (ridge segment) offset by a transform fault. Both are plate boundaries. A fracture zone is the scar of the active transform fault

Data from marine seismic refraction experiments defined a thin ocean crust, approximately 6 to 8 kilometers in thickness, divided into three layers. [18] [19] Seismic reflection measurements made over the ocean ridges found they are devoid of sediments at the crest, but covered by increasingly thicker sediment layers with increasing distance from the ridge crest. [20] This observation implied that the ridge crests are younger than the ridge flanks.

Magnetic surveys discovered linear magnetic anomalies that in many areas ran parallel to an ocean ridge crest and showed a mirror-image symmetrical pattern centered on ridge crests. [21] Correlation of the anomalies to the history of Earth's magnetic field reversals allowed the age of the seafloor to be estimated. [22] This connection was interpreted as the spreading of the seafloor from the ridge crests. [23] [22] Linking spreading centers and transform faults to a common cause helped to develop the concept of plate tectonics. [24]

When the age of the ocean crust as determined by magnetic anomalies or drill hole samples was compared to the ocean depth it was observed that depth and age are directly related in a seafloor depth age relationship. [25] This relationship was explained by the cooling and contracting of an oceanic plate as it spreads away from a ridge crest. [26]

Evidence for paleoclimate

Seismic reflection data combined with deep-sea drilling at some locations have identified widespread unconformities and distinctive seismic reflectors in the deep sea sedimentary record. [27] [28] These have been interpreted as evidence of past global climate change events. Seismic reflection surveys made on polar continental selves have identified buried sedimentary features due to the advance and retreat of continental ice sheets. [29] [30] Swath sonar mapping has revealed the gouge tracks of ice sheets cut as they traversed polar continental shelves in the past. [31]

Evidence for hydrothermal vents

BlackSmoker.jpg

Heat flow measured in the ocean basins revealed that conductive heat flow decreased with the increased depth and crustal age of flanks of ocean ridges. [26] [25] On the ridge crest, however, conductive heat flow was found to be unexpectedly low for a location where active volcanism accompanies seafloor spreading. [32] This anomaly was explained by the possible heat transfer by hydrothermal venting of seawater circulating in deep fissures in the crust at the ridge crest spreading centers. This hypothesis was borne out in the late 20th century when investigations by deep submersibles discovered hydrothermal vents at spreading centers. [33] [34] [35]

Evidence for Mid-Ocean Ridge structure and properties

Marine gravity profiles made across Mid-Ocean Ridges showed a lack of a gravity anomaly, the Free-air anomaly is small or near zero when averaged over a broad area. [36] [37] This suggested that although ridges reached a height at their crest of two kilometers or more above the deep ocean basins, that extra mass was not related to an increase of gravity on the ridge of the magnitude that would be expected. The ridges are isostatically compensated, meaning the total mass below some reference depth in the mantle below the ridge is about the same everywhere. This requires a lower density mantle below the ridge crest and upper ridge flanks. [36] Data from seismic studies revealed lower velocities under the ridges suggesting parts of the mantle below the crests are lower density rock melt. [38] This is consistent with the theories of seafloor spreading and plate tectonics.

Centers of research conducting marine geophysics

See also

Related Research Articles

<span class="mw-page-title-main">Seafloor spreading</span> Geological process at mid-ocean ridges

Seafloor spreading, or seafloor spread, is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.

<span class="mw-page-title-main">Convergent boundary</span> Region of active deformation between colliding tectonic plates

A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Wadati–Benioff zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.

<span class="mw-page-title-main">Oceanic crust</span> Uppermost layer of the oceanic portion of a tectonic plate

Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumulates. The crust overlies the rigid uppermost layer of the mantle. The crust and the rigid upper mantle layer together constitute oceanic lithosphere.

Marine geology or geological oceanography is the study of the history and structure of the ocean floor. It involves geophysical, geochemical, sedimentological and paleontological investigations of the ocean floor and coastal zone. Marine geology has strong ties to geophysics and to physical oceanography.

<span class="mw-page-title-main">Mid-ocean ridge</span> Basaltic underwater mountain system formed by plate tectonic spreading

A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about 2,600 meters (8,500 ft) and rises about 2,000 meters (6,600 ft) above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a divergent plate boundary. The rate of seafloor spreading determines the morphology of the crest of the mid-ocean ridge and its width in an ocean basin.

<span class="mw-page-title-main">Back-arc basin</span> Submarine features associated with island arcs and subduction zones

A back-arc basin is a type of geologic basin, found at some convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found in the western Pacific Ocean. Most of them result from tensional forces, caused by a process known as oceanic trench rollback, where a subduction zone moves towards the subducting plate. Back-arc basins were initially an unexpected phenomenon in plate tectonics, as convergent boundaries were expected to universally be zones of compression. However, in 1970, Dan Karig published a model of back-arc basins consistent with plate tectonics.

<span class="mw-page-title-main">Fred Spiess</span> American marine biologist

Dr. Fred Noel Spiess was a naval officer, oceanographer and marine explorer. His work created new advances in marine technology including the FLIP Floating Instrument Platform, the Deep Tow vehicle for study of the seafloor, and the use of acoustics for underwater navigation and geodetic positioning.

<span class="mw-page-title-main">Izu–Bonin–Mariana Arc</span> Convergent boundary in Micronesia

The Izu–Bonin–Mariana (IBM) arc system is a tectonic plate convergent boundary in Micronesia. The IBM arc system extends over 2800 km south from Tokyo, Japan, to beyond Guam, and includes the Izu Islands, the Bonin Islands, and the Mariana Islands; much more of the IBM arc system is submerged below sealevel. The IBM arc system lies along the eastern margin of the Philippine Sea Plate in the Western Pacific Ocean. It is the site of the deepest gash in Earth's solid surface, the Challenger Deep in the Mariana Trench.

<span class="mw-page-title-main">Vine–Matthews–Morley hypothesis</span> First key scientific test of the seafloor spreading theory of continental drift and plate tectonics

The Vine–Matthews–Morley hypothesis, also known as the Morley–Vine–Matthews hypothesis, was the first key scientific test of the seafloor spreading theory of continental drift and plate tectonics. Its key impact was that it allowed the rates of plate motions at mid-ocean ridges to be computed. It states that the Earth's oceanic crust acts as a recorder of reversals in the geomagnetic field direction as seafloor spreading takes place.

Non-volcanic passive margins (NVPM) constitute one end member of the transitional crustal types that lie beneath passive continental margins; the other end member being volcanic passive margins (VPM). Transitional crust welds continental crust to oceanic crust along the lines of continental break-up. Both VPM and NVPM form during rifting, when a continent rifts to form a new ocean basin. NVPM are different from VPM because of a lack of volcanism. Instead of intrusive magmatic structures, the transitional crust is composed of stretched continental crust and exhumed upper mantle. NVPM are typically submerged and buried beneath thick sediments, so they must be studied using geophysical techniques or drilling. NVPM have diagnostic seismic, gravity, and magnetic characteristics that can be used to distinguish them from VPM and for demarcating the transition between continental and oceanic crust.

<span class="mw-page-title-main">Adare Basin</span>

The Adare Basin is a geologic structural basin located north-east of Cape Adare of Antarctica, for which its named, and north of the western Ross Sea. The Adare Basin is an extensional rift basin located along a seafloor spreading center that forms the failed arm of the Tertiary spreading ridge separating East and West Antarctica, known as the West Antarctic Rift System and similar in structure to the East Africa Rift System. Centrally located in the Adare Basin is the Adare Trough. The extension of this rift system is recorded in a series of magnetic anomalies which run along the seafloor at the extinct, north–south trending, Adare spreading axis. The Adare spreading system continues unbroken into the Northern Basin underlying the adjacent Ross Sea continental shelf.

<span class="mw-page-title-main">Gravity of Mars</span> Gravitational force exerted by the planet Mars

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<span class="mw-page-title-main">RISE project</span> 1979 international marine research project

The RISE Project (Rivera Submersible Experiments) was a 1979 international marine research project which mapped and investigated seafloor spreading in the Pacific Ocean, at the crest of the East Pacific Rise (EPR) at 21° north latitude. Using a deep sea submersible (ALVIN) to search for hydrothermal activity at depths around 2600 meters, the project discovered a series of vents emitting dark mineral particles at extremely high temperatures which gave rise to the popular name, "black smokers". Biologic communities found at 21° N vents, based on chemosynthesis and similar to those found at the Galapagos spreading center, established that these communities are not unique. Discovery of a deep-sea ecosystem not based on sunlight spurred theories of the origin of life on Earth.

<span class="mw-page-title-main">Project FAMOUS</span> Marine scientific exploration by manned submersibles of a diverging tectonic plate boundary

Project FAMOUS was the first-ever marine scientific exploration by manned submersibles of a diverging tectonic plate boundary on a mid-ocean ridge. It took place between 1971 and 1974, with a multi-national team of scientists concentrating numerous underwater surveys on an area of the Mid-Atlantic Ridge about 700 kilometers west of the Azores. By deploying new methods and specialized equipment, scientists were able to look at the sea floor in far greater detail than ever before. The project succeeded in defining the main mechanisms of creation of the median rift valley on the Mid-Atlantic Ridge, and in locating and mapping the zone of oceanic crustal accretion.

The depth of the seafloor on the flanks of a mid-ocean ridge is determined mainly by the age of the oceanic lithosphere; older seafloor is deeper. During seafloor spreading, lithosphere and mantle cooling, contraction, and isostatic adjustment with age cause seafloor deepening. This relationship has come to be better understood since around 1969 with significant updates in 1974 and 1977. Two main theories have been put forward to explain this observation: one where the mantle including the lithosphere is cooling; the cooling mantle model, and a second where a lithosphere plate cools above a mantle at a constant temperature; the cooling plate model. The cooling mantle model explains the age-depth observations for seafloor younger than 80 million years. The cooling plate model explains the age-depth observations best for seafloor older that 20 million years. In addition, the cooling plate model explains the almost constant depth and heat flow observed in very old seafloor and lithosphere. In practice it is convenient to use the solution for the cooling mantle model for an age-depth relationship younger than 20 million years. Older than this the cooling plate model fits data as well. Beyond 80 million years the plate model fits better than the mantle model.

Suzanne Carbotte is a marine geophysicist known for her research on the formation of new oceanic crust.

Anne Sheehan is a geologist known for her research using seismometer data to examine changes in the Earth's crust and mantle.

Mathilde Cannat is a French geologist known for her research on the formation of oceanic crust and the tectonic and magmatic changes of mid-ocean ridges.

Margo Helen Edwards is a marine geologist known for mapping of the seafloor and hydrothermal vents. She led the 1999 SCICEX and was the first women to live aboard a United States' Navy submarine while doing under-ice research.

Roger Clive Searle is an English geophysicist, known for using sonar imaging in research on the geology and geophysics of the ocean floor. In particular, he has made important contributions to understanding the oceanic spreading system and the mid-ocean spreading centres.

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