This article comes from a PPT for out-of-office exchanges. First help top 2 articles. Due to the recent busy project acceptance, the basic knowledge of gravity has not yet been written.
Marine geophysical exploration is mainly used for seabed scientific research and seabed mineral exploration. Ocean geophysical prospecting includes ocean gravity, ocean magnetic survey, ocean electromagnetic, seabed heat flow and ocean seismic methods. The working principle of marine geophysical prospecting is the same as that of terrestrial geophysical prospecting methods, but because the operation site is at sea, the medium of seawater is added, so there are special requirements for equipment and working methods.
High-precision navigation and positioning technology, ocean gravity measurement system, ocean geomagnetic measurement technology, seabed seismic detection and other detection technologies play an irreplaceable role in today's seabed resource exploration, marine scientific research, marine engineering and marine battlefield environment. As we all know, the ocean contains rich resources, such as mineral resources such as petroleum, natural gas hydrates, polymetallic nodules, hydrothermal sulfides, and deep-sea rare earths.
Marine geophysical exploration has been developed for a century and a half. As early as the early 1950s, Ewing and others used the newly-appearing precision echo sounder to conduct continuous water depth exploration and draw seabed topography and landform maps.
Geophysics is the use of physical theories and methods to study the internal structure, structure and dynamic processes of the earth, including potential field theory and wave theory.
According to specific detection means, equipment and purpose, it is usually divided into:
① Shipborne geophysical surveys, relying on scientific research ships (or manned submersibles, ROVs) to carry out a variety of geophysical surveys, such as ocean seismic survey methods (reflection, refraction), ocean gravity and magnetic measurement methods, ocean geothermal measurement methods, Ocean depth measurement method (side scan sonar technology, multi-beam), ocean electromagnetic measurement method, ocean deep towed gamma-ray spectrometer, etc.
②Undersea geophysical survey, such as undersea camera, undersea seismograph (OBS) and other advanced undersea exploration instruments.
③ Wellbore geophysical logging, such as acoustic logging, radioactive logging, resistivity logging, imaging logging, etc.
Polar
Submarine camera
Submarine Seismograph OBS
Ocean logging
6 conventional marine geophysical exploration methods
(1) Marine seismic exploration method
(2) Ocean gravity and magnetic measurement method
(3) Marine geophysical logging
(4) Ocean geothermal measurement method
(5) Ocean depth measurement method
(6) Marine electromagnetic measurement method
(1) Marine seismic exploration method
Marine seismic exploration is the use of the difference in elasticity and density of the ocean and underground media, by observing and analyzing the response of the ocean and the earth to natural or artificially excited seismic waves, studying the propagation law of seismic waves, and inferring the nature and shape of the underground rock layer and the structure of the ocean water mass A method of detection.
Choose a navigation and positioning system with higher accuracy, using satellite navigation and positioning, laser positioning and underwater sonar positioning, etc.
The characteristics of offshore seismic exploration are excitation in water and reception in water. Due to the particularity of the marine environment, non-explosive sources are mostly used for seismic sources (including air gun seismic sources, steam gun seismic sources, electric spark sources, etc., of which air gun seismic sources account for more than 95%), and piezoelectric geophones are used for reception.
"Single-channel seismic data processing method and its application in Pingtan sea area" Zheng Jianglong
(2) Ocean gravity and magnetic measurement method
Ocean gravity measurement is to place a gravimeter on a survey ship or place it on the seabed after being sealed for observation to determine the uneven distribution of various rock layers in the seabed crust. Since there are stratum interfaces with different densities on the seafloor, the fluctuation of this interface will lead to changes in surface gravity. Through the interpretation of various gravity anomalies, including the analysis and extension of gravity anomalies, it is possible to obtain data on the shape of the earth, crustal structure, and interface anomalies at certain interfaces in sedimentary rock formations, and then solve the tasks of tectonic and regional geology. Provide evidence for finding metal deposits.
Gravity data processing and regional gravity field characteristics from ships in Bransfield Strait, Antarctica_Malong
Gravity anomalies can be divided into pure gravity anomalies and mixed gravity anomalies. Pure gravity anomaly is the difference between the earth's gravity value and the normal gravity value at the same point, also known as disturbing gravity. The mixed gravity anomaly is the difference between the gravity value of a point on one surface and the normal gravity value of the corresponding point on the other surface.
The pure gravity anomaly cannot be obtained directly, and needs to be indirectly inferred through the disturbance position. The mixed gravity anomaly can be directly inferred. If the ground mixed gravity anomaly is obtained, the gravity of a point on the ground can be obtained through actual measurement, and the normal gravity of the corresponding point on the earth surface is similar to the normal gravity of the corresponding point on the earth surface. Then it is reduced to the surface of the earth.
Calculating the measured gravity value on the ground to the geoid is called gravity correction. It contains two aspects: one is to remove the influence of the elevation from the observation point to the geoid on the observed value of gravity; the other is to completely eliminate the influence of masses other than the geoid in some way. The corrected value is the gravity value on the geoid without any external mass. Depending on the impact to be corrected, different corrections will be added to the gravity observations.
Observed gravity value minus normal gravity value, plus space correction, is called spatial anomaly, and if local terrain correction is added, it is called Faye anomaly. Observed gravity value minus normal gravity value, plus Bouguer correction, called Bouguer anomaly; plus local terrain correction and equilibrium correction, called equilibrium anomaly.
Spatial correction: Consider the correction of the vertical gradient of the normal gravity field according to the elevation of the ground gravity observation point.
Interlayer correction: correction added to eliminate the influence of the mass layer between the horizontal plane of the observation point and the geoid on the observed gravity. This correction is equivalent to removing all the mass higher than the geoid as an infinite plane thick layer.
The space correction and interlayer correction of the gravity observation value is equivalent to moving the ground gravity observation point to the geoid. The sum of these two corrections is called Bouguer correction.
Equilibrium correction: It is a correction that takes into account the excess (or insufficient) quality of the mountains (or ocean) and the insufficient (or excess) below the geoid to compensate each other according to the equilibrium hypothesis of the crust.
After the height is corrected and calculated to the geoid , the gravity difference obtained by subtracting the normal gravity value
Space gravity anomaly
Bouguer gravity anomaly
Equilibrium gravity anomaly
Ocean magnetic measurement is to use the proton precessing cesium optical pump magnetometer or magnetic gradiometer behind the towed work ship to collect the geomagnetic field intensity data in the ocean area and conduct ocean magnetic observation. The observation value is subtracted from the normal magnetic field value and corrected for the diurnal variation of the geomagnetic field to obtain the magnetic anomaly. By analyzing the magnetic anomaly field generated by the magnetic difference of seabed rocks and ore, explore regional geological characteristics, such as the undulation of crystalline basement, the thickness of sedimentation, the distribution of large faults, and the range of volcanic rocks. Use seabed geological mapping to find magnetic minerals.
Study on the Characteristics of Regional Geophysical Fields of Nantong Sheet_杨慧良
Magnetic value to interfere
For more inquiries, see xiaok marine surveying and mapping network and the public account of the same name