Gravity Method in Geophysics

Gravity Method

• Non-invasive, non-destructive, and relatively inexpensive
• Passive, meaning no energy needs to be put into the ground to acquire data
• Studies subsurface geology using changes in the Earth's gravitational field caused by differences in density between subsurface rocks

Physical Property

• Density is the controlling physical property
• Different types of rock have different densities, and denser rocks have greater gravitational attraction
• Higher-density formations arched upward in a structural high, such as an anticline, increase the Earth's gravitational field over the axis of the structure compared to its flanks

Uses

• Mainly used for oil exploration
• Sometimes used in mineral and groundwater prospecting
• Effective for mapping sedimentary basins where the basement rocks have a higher density than the sediments
• Suitable for locating and mapping salt bodies due to the low density of salt compared to surrounding formations
• Used for direct detection of heavy minerals such as chromite

Types of Gravity Geophysical Methods

Microgravity Profiling

• Passive technique that measures relative shifts in the Earth's gravitational field with extreme precision
• Uses a gravity meter to take measurements
• Can identify voids or submerged features like underground storage tanks due to subtle changes in gravity caused by density differences

Magnetic Method

• Obtains information related to the direction, gradient, or intensity of the Earth's magnetic field
• Measures distortions in the Earth's magnetic field caused by ferrous materials
• Used for large-scale geology mapping, detecting buried metallic objects, and detecting some geologic features like lava tubes

Ground-Based Surveys

Seismic Surveys

• Measure variations in reflected ground vibrations as they pass through the earth
• Use an energy source to create high-frequency vibrations
• Provide information about rocks down to depths of several kilometers
• Particularly suited to flat-lying sedimentary basins
• Most often used in petroleum and coal exploration

Magnetic Surveys

• Measure variations of the Earth's magnetic field due to the presence of magnetic minerals
• Subtle variations in magnetic minerals are used to interpret rock types and identify resources
• Most often used in metallic mineral exploration

Radiometric Surveys

• Measure gamma rays continuously emitted from the Earth by natural decomposition of radiogenic minerals
• Focus on recording naturally occurring radiation from potassium, thorium, and uranium
• Most often used in metallic and industrial mineral exploration

Gravity Surveys

• Measure the gravity field with a gravimeter to determine variations in rock density in the Earth's crust