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The geologic and topographic structures of the ocean floor primarily reflect plate tectonic activity that has occurred over the past 150 million years of the 4
Civil Engineering (BSCE 01)
Ateneo de Davao University
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The geologic and topographic structures of the ocean floor primarily reflect plate tectonic activity that has occurred over the past 150 million years of the 4-billion-year age of the Earth. Despite their youth and geologic simplicity, most of this deep seafloor has remained poorly understood because it is masked by 3-5 km of seawater. For example, the Pacific-Antarctic rise, which has an area about equal to South America, is a broad rise of the ocean floor caused by sea floor spreading between two major tectonic plates (see Poster southeast of New Zealand). To the west of the ridge lies the Louisville seamount chain which is a chain of large undersea volcanoes having a length equal to the distance between New York and Los Angeles. Recently, high density data collected by the Geosat (US Navy) and ERS-1 (European Space Agency) spacecraft data show the Pacific-Antarctic Rise and the Louisville Ridge in unprecedented detail.
The reason that the ocean floor, especially the southern hemisphere oceans, is so poorly charted is that electromagnetic waves cannot penetrate the deep ocean. However, because research vessels travel quite slowly, it would take approximately 125 years to chart the ocean basins using the latest swath-mapping tools. To date, only a small fraction of the sea floor has been charted by ships. Fortunately, such a major mapping program is largely unnecessary because the ocean surface has broad bumps and dips which mimic the topography of the ocean floor. These bumps and dips can be mapped using a very accurate gravity mapping mounted on a satellite.
According to the laws of physics, the surface of the ocean is an "equipotential surface" of the earth's gravity field. ( Basically this means that if one could place balls everywhere on the surface of the ocean, none of the balls would roll down hill because they are all on the same "level". To a first approximation, this equipotential surface of the earth is a sphere. However because the earth is rotating, the equipotential ocean surface is more nearly matched by an ellipsoid of revolution where the polar diameter is 43 km less than the equatorial diameter. These bumps and dips in the ocean surface are caused by minute variations in the earth's gravitational field.
These tiny bumps and dips in the geoid height can be measured using a very accurate radar mounted on a satellite (Figure). For example, the Geosat satellite was launched by the US Navy in 1985 to map the geoid height at a horizontal resolution of 10-15 km (6 - 10 mi) and a vertical resolution of 0 m (1 in). Geosat was placed in a nearly polar orbit to obtain high latitude coverage (+- 72 deg latitude). The Geosat altimeter orbits the earth 14 times per day resulting in an ocean track speed of about 7 km per second (4 mi/sec). The earth rotates beneath the fixed plane of the satellite orbit, so over a period of 1 years, the satellite maps the topography of the surface of the earth with an ground track spacing of about 6 km (4 mi).
Two very precise distance measurements must be made in order to establish the topography of the ocean surface to an accuracy of 0 m (1 in) (Figure). First, the height of the satellite above the ellipsoid h* is measured by tracking the satellite from a globally-distributed network of lasers and/or doppler stations. Second, the height of the satellite above the closest ocean surface h is measured with a microwave radar operating in a pulse-limited mode on a carrier frequency of 13 GHz. ( The spherical wavefront of the pulse ensures that the altitude is measured to the closest ocean surface. The difference between the height above the ellipsoid and the altitude above the ocean surface is approximately equal to the geoid height N = h* - h.
The Poster shows gravity anomaly derived from geoid height measurements from 4 years of Geosat measurements and 2 years of ERS-1 measurements. We have developed a new method to convert these raw geoid height measurements, which have a variety of accuracies, track spacings and data densities, into images (or grids) of gravity anomaly. This conversion is done to enhance the small-scale features of the seafloor. Moreover, after the conversion, the satellite-derived gravity measurements can be compared and combined with gravity anomaly measurements made by ships. The algorithms of the conversion are based on laws of physics, geometry and statistics.
The Geosat data were collected by the US Navy to fulfill their navigational and mapping requirements. Consider measuring accelerations in a moving submarine or aircraft in order to determine your position as a function of time. ( Thus the gravity data are needed for correction of inertial navigation/guidance systems. The military applications are obvious and provided the rationale for the 80 million dollar cost of the Geosat mission as well as the classification of these data, especially during the cold war when nuclear submarines were more active than they are today. On the commercial side, Honeywell Inc. is using these data to update their inertial navigation systems in commercial aircraft.
We are using these dense satellite altimeter measurements in combination with sparse measurements of seafloor depth to construct a uniform resolution map of the seafloor topography. These maps do not have sufficient accuracy and resolution to be used to assess navigational hazards but they are useful for such diverse applications as locating the obstructions/constrictions to the major ocean currents and locating shallow seamounts where fish and lobster are abundant.
On a broad scale the topography of the ocean floor reflects the cooling and subsidence of the plates as they move away from the spreading center.
Using these data we are exploring the internal deformations of the plates, especially outboard of trenches where the forces generated by the slab-pull force of the subducted plates is greatest
All of the major petroleum exploration companies use satellite altimeter gravity data from Geosat and ERS-1 to locate offshore sedimentary basins in remote areas. This information is combined with other reconnaissance survey information to determine where to collect or purchase multi-channel seismic survey data. Currently, the regions of most intense exploration interest are the continental shelves of Australia and the former Soviet Union; recently companies have expressed interest in the Caspian Sea. Dr. Mark Odegard of UNOCAL Inc. says "We routinely use satellite gravity data in any exploration effort in the oceans outside of the Gulf of Mexico. This obviously will not be done, but we are beginning to collect high-resolution (shipboard) data over selected targets outside the U. The other companies that use the satellite data, that I know of, are: Exxon, Mobil, and Texaco.
There are numerous other scientific applications that cannot be described in a short report. One of the traditional uses of marine gravity measurements is to estimate the thickness of the elastic portion of the tectonic plates. This deformation is appears in the gravity field as a donut-shaped gravity low surrounding the gravity high associated with the volcano itself. By measuring the amplitude and width of the gravity low and relating this to the size of the volcano as measured my a ship with an echo sounder, one can establish the thickness and strength of the elastic plate. The new satellite-derived gravity data enable researchers to perform this type of analysis everywhere in the oceans.
The geologic and topographic structures of the ocean floor primarily reflect plate tectonic activity that has occurred over the past 150 million years of the 4
Course: Civil Engineering (BSCE 01)
University: Ateneo de Davao University
