Grav/Mag Data Processing
Processing of Gravity Data
Gravity method is a geophysical technique that measures variations in the Earth's gravitational field to infer subsurface geological structures.
Applications:
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The gravity method is used in a wide range of applications, including:
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Mineral exploration: Locating ore bodies with density contrasts.
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Oil and gas exploration: Mapping subsurface structures and sedimentary basins.
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Engineering and environmental studies: Assessing bedrock depth, identifying sinkholes, and mapping subsurface voids.
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Geothermal exploration: Mapping subsurface heat sources.
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Archaeological investigations: Identifying buried structures.
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The data collected from a gravity survey is processed and interpreted to create a map of gravity anomalies.
Data Corrections:
Instrument Drift Correction:
Gravimeters can drift over time, so corrections are applied to account for these changes.
Elevation Correction (Free-Air Correction):
This correction accounts for the change in gravity due to the elevation difference between the measurement point and a reference level (usually sea level).
Bouguer Correction:
This correction accounts for the gravitational attraction of the rock mass between the measurement point and the reference level.
Latitude Correction:
The Earth's shape and rotation cause the gravitational field to vary with latitude, requiring a correction.
Tidal Correction:
The gravitational pull of the moon and sun can cause tidal variations in gravity measurements.
Terrain Correction:
The presence of hills and valleys can affect gravity measurements, requiring corrections based on surrounding topography.
Processing and Interpretation of Magnetic Data
A magnetic survey in geophysics is a method used to map variations in the Earth's magnetic field to identify subsurface geology. Ferrous materials (like iron) and rocks with varying magnetic properties create distortions in the magnetic field, resulting in anomalies that can be detected by the magnetometer.
Applications:
Mineral Exploration:
Identifying deposits of magnetic minerals like iron ore, or locating other minerals associated with magnetic anomalies (e.g., nickel, chromium).
Archaeology:
Detecting buried structures, such as walls, foundations, hearths, and ditches, by identifying anomalies caused by burnt materials or changes in soil magnetism.
Geological Mapping:
Mapping the boundaries of rock formations and identifying faults, folds, and other geological structures.
Engineering and Environmental Studies:
Locating buried metallic objects like pipelines, storage tanks, and drums, as well as identifying landfills and other environmental hazards.
Types of magnetic surveys:
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Ground Surveys: Magnetometers are carried or towed across the surface, often using vehicles or backpacks.
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Airborne Surveys: Magnetometers are mounted on airplanes or helicopters to cover large areas quickly.
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Marine Surveys: Magnetometers are towed behind ships to survey underwater areas
The processing of magnetic data depends on the type of magnetic survey and includes removal of time variations, of regional trends and reduction to the pole.
To remove time variations the general practise is to establish a base-station which continually measures the magnetic field, because Earth's magnetic field changes over time, by natural sources.
The second step is to remove the regional field -anomalies caused by geologic features larger than the survey area.
Reduction to the pole
RTP correction takes the anomaly, as measured and transforms it into that which would have been measured if the body had been laid at the magnetic pole.