Marine geophysics

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Marine geophysics is a scientific discipline that uses the quantitative observation of physical properties to understand the seafloor and sub-seafloor geology. Surveying of the oceans using gravity, magnetic, swath bathymetry, and seismic reflection , seafloor spreading, continental drift, and plate tectonics. The history of the ocean circulation and climate change preserved in seafood sediments. Each of the principal branches of geophysical knowledge is involved: heat-flow data are obtained from ocean floors and from the mid-oceanic. Marine geophysics is intimately associated with the concepts and problems of seafloor spreading, continental drift, and plate tectonics.

Source of Sediments
Terrigenous

Terrigenous is derived from lands. Dominated by volume, covered by 45% of seafloor. Thus, terrigenous sediments are in the deep ocean entail turbidites and debris near margins. However, the turbidites can travel about 1000km from the continental shelf.

Biogenic

Derived from shells/ skeletons of organisms - siliceous, calcareous ( second most important by volume covered 55% of the seafloor. Consist of two main oozes in particular. Siliceous oozes consist of diatoms and radiolarians. Diatoms are single celled algae, found at high latitudes. Radiolarians are amoeba like that are found in equatorial regions. Calcerous oozes consist of coccoliths, foraminifera and pteropods. Coccoliths are algae that are most resistant to dissolution. Foraminifera are amoeba like and pteropods are drifting mollusks, that are most easily dissolved.

Hydrogenous (authigenic)

Manganese and phosphorite nodules, hydrothermal sediments, evaporites (salt).

Cosmogenic

Derived from meteorites, tektites, cosmic spherules.

Methods to Survey the Oceans

Gravity / Magnetic Process

The gravity process is the force of gravitational attractions, ‘g’, that is exerted by the earth on an object the surface is not constant but it all varies from place to place. This variation is due to the shape of the earth: oblate spheroid. The purpose of gravity measurements is to discover these variations and then ascribe them to reasonable geological structures. There can be many different interpretations to gravity measurements that can conclude to important features in the data. Large negative anomalies are associated with deep-sea trenches must be associated with corresponding deficiencies of mass.

Measurements at sea of the earth’s magnetic field have resulted in important information about the interior of the earth. All modern marine magnetic surveys emphasis the magnitude of the total intensity of the magnetic field. There are two kinds of instruments used to make these measurements: the proton-precision magnetometer and the flux gate magnetometer. The flux gate magnetometer is an instrument that utilizes the principle that if a rod of a material of high magnetic permeability is enclosed in a coil the alternating current passed through this coil, the asymmetrical saturation of the core generates even harmonics are proportional to the component of the magnetic field along the rod which can be obtained. The proton-precision magnetometer is an instrument in which a plastic bottle containing water is put within a coil. The assemblage of protons in the water act as tiny magnets. Executing random thermal motions that have the tendency to line themselves up along the earth’s field. Current is passed through the coil producing a field much stronger than that of the earth. The protons tend now to line up along the new field. The current is switched off (within seconds) the protons begin to process around the direction of the earth’s field.

Swath Bathymetry

Swath bathymetry is a system that is used to measure the depth in a line extending outwards from the sonar transducer. Systems obtain the data in the swath at right angles to the direction of the motion of the transducer [1] head. As the head continues to move forward the profile sweeps out in a ribbon shaped surface of depth measurement thus known as the swath as shown in the figure. There are currently two swath bathymetry systems that involves two different technologies to achieve bathymetry measurements across the seafloor: 1. Beam forming (Multi-beam echo sounders) as shown in Figure 2 below. Shows the interpretation of the multi-beam sonar swath bathymetry with the reflection intensity information of the regional deep waters. 2. Interferometric (as known as phase) discrimination sonars as shown in Figure 3a below.

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Figure 2: Multi-beam Sonar Boat: Interpretation of multibeam sonar swath bathymetry and reflection intensity information from regional deep waters.

In the interferometric sonar systems, the acoustic energy is propagated from the transducer downward in a beam that is narrow in the along-track dimension and wide in the across-track dimensions.Through this method it produces a line of depth measurements across-track and sets perpendicular to the research vessel's track line. As the vessels moves forward these profiles sweep out a ribbon-shaped surface of depth measurements. From interferometric systems the depth measurements are obtained by measuring the angle of the incoming sound signal in addition to the range. Interferometric sonar systems require three or more linear arrays of transducer staves,one to transmit acoustic energy and at least two to receive the returning signal.[1]

Measuring depth using both the travel time of the emitted pulse and the angle the returning sound energy makes with each linear array or stave.As shown in figure 3b how the interferometric sonar system is placed to obtain measurements. As shown in figure 3c shows the results from the data obtained from the interferometric sonar measurements. The lower the frequency transducer has greater range, but less resolution. The higher frequency transducer has less range, but greater resolution. 

Marquina3a.JPG

Figure 3a: Interferometric Sonar System diagram

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Figure 3b: Deployment of the interferometric sonar (offshore South Carolina USGS research cruise in 2001).

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Figure 3c: Swath bathymetry collected with an interferometric sonar (off the coast of South Carolina's Grand Strand)

The advantages and disadvantages comparing both of the swath bathymetry systems. These are the following advantages that the interferometric sonar systems have when comparison with the beam based sonar systems. Consist of high resolution that is helpful for detecting small targets in shallow water and for providing a better analysis of the deeper water interpretation.[1] Wider swath width, especially in shallow water aids to reduce the ship time and thus the survey costs. The most important advantage is the ability to differentiate several targets at the same angle. This is a powerful tool that can be very useful when there are targets to be resolved in the water columns. The disadvantages of interferometric sonar systems is the high data rates require a powerful processing system. The water column targets need to be filtered out in the data processing. It occurs where some types of targets can suffer from range ambiguity. Thus requires the target to be surveyed again just at a different range.[1]

Seismic Reflection Profiling

Seismic reflection profiling is a widely used technique for using sound waves to image underground rock strata (bedding). Thus plays an important role in oil and gas exploration. Thus, the sound wave is created by an air gun (definition) on the ship. The sound travels down through the water and penetrates into the layers of sediments and rocks of the ocean floor. Some travels back to the surface and is recorded by a hydrophone that is trailed behind the ship. Sharp pulses are detections that the pulses of reflected sound arrived. The sound intensity is a function of time. The air gun is shot again denote the same process occurs but the time of the reflection is different each time since the depth of the rock layers changes as the ship continuous to move. Thus, after several hours the seismic profile of the seafloor has been completed. In studies the seismic reflection profilers have shown that the active continental margins usually show undisturbed sediments in the bottoms of the trenches.[2] . Denoted above that seismic reflection profiling plays a huge role in locating potential oil and natural gas deposits

Marquina swath.jpg

Figure 3: Swath Bathymetry model

References

  1. 1.0 1.1 1.2 Geen, Matt. (1999). Sea Technology: Applications of Interferometric Swath Bathymetry.
  2. Drake, Charles L. (1970). Marine Geophysics.