7.1 Gravity and magnetic potential fields
4 min read•august 14, 2024
Gravity and magnetic fields are invisible forces shaping our planet. They're like Earth's fingerprints, revealing hidden structures beneath our feet. Understanding these fields helps geophysicists uncover secrets about our planet's composition and structure.
Potential field methods use gravity and magnetism to explore the Earth's subsurface. By measuring tiny variations in these fields, scientists can map out underground features like mineral deposits, oil reservoirs, and even ancient buried landscapes.
Gravity and Magnetic Fields: Fundamental Principles
Vector Fields and Potential Theory
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- Gravity and magnetic fields are both potential fields that vary in strength and direction in three-dimensional space
- Represented by field lines indicating the direction of the force at any given point
- Gravitational and magnetic fields can be described mathematically using potential theory
- The and are scalar fields that decrease with distance from the source
Gravitational Field Characteristics
- The gravitational field is a conservative field produced by the mass of the Earth
- Always points towards the center of the Earth
- Strength decreases with distance from the center according to the inverse square law
- The strength of the gravitational field is measured in milligals (mGal)
Magnetic Field Characteristics
- The magnetic field is produced by the motion of charges, such as electric currents in the Earth's outer core
- Dipole field with north and south poles
- Field lines connect the poles
- The strength of the magnetic field is measured in teslas (T) or nanoteslas (nT)
Sources and Characteristics of Anomalies
Gravitational Anomalies
- Gravitational anomalies are variations in the Earth's gravitational field caused by lateral variations in the of subsurface rocks
- Positive anomalies indicate the presence of high-density rocks (e.g., igneous intrusions)
- Negative anomalies indicate low-density rocks (e.g., sedimentary basins)
- Gravitational anomalies can be caused by geological structures such as sedimentary basins, igneous intrusions, and ore bodies
- The shape and amplitude of gravitational anomalies depend on the geometry, depth, and physical properties of the subsurface sources
- Shallow sources produce narrow, high-amplitude anomalies
- Deep sources produce broad, low-amplitude anomalies
Magnetic Anomalies
- Magnetic anomalies are variations in the Earth's magnetic field caused by lateral variations in the of subsurface rocks
- Positive anomalies indicate the presence of highly magnetic rocks (e.g., igneous intrusions)
- Negative anomalies indicate weakly magnetic or non-magnetic rocks (e.g., sedimentary rocks)
- Magnetic anomalies can be caused by geological structures such as igneous intrusions, metamorphic rocks, and mineralized zones
- The shape and amplitude of magnetic anomalies depend on the geometry, depth, and physical properties of the subsurface sources
- Shallow sources produce narrow, high-amplitude anomalies
- Deep sources produce broad, low-amplitude anomalies
Regional and Local Anomalies
- Gravitational and magnetic anomalies can be regional or local in scale
- Regional anomalies reflect large-scale geological features (e.g., sedimentary basins)
- Local anomalies reflect smaller-scale features (e.g., ore bodies)
Potential Fields and Subsurface Structures
Sedimentary Basins
- Sedimentary basins typically produce negative gravitational anomalies due to the low density of sedimentary rocks compared to the surrounding basement rocks
- The shape of the anomaly reflects the geometry of the basin
Igneous Intrusions
- Igneous intrusions often produce positive gravitational and magnetic anomalies due to their high density and magnetic susceptibility
- The shape of the anomalies can provide information about the geometry and depth of the intrusion
Faults and Folds
- Faults and folds can produce linear or curved gravitational and magnetic anomalies
- Depends on the contrast in physical properties across the structure
- Depends on the orientation of the structure relative to the potential field
Mineralized Zones and Ore Bodies
- Mineralized zones and ore bodies can produce local positive or negative anomalies
- Depends on their density and magnetic susceptibility relative to the host rocks
Depth Estimation
- The amplitude and wavelength of potential field anomalies can be used to estimate the depth to the source
- Deeper sources produce broader, lower-amplitude anomalies
- Shallower sources produce narrower, higher-amplitude anomalies
Density vs Magnetic Susceptibility: Effects on Potential Fields
Density and Gravitational Fields
- Density is a measure of the mass per unit volume of a material
- Controls the gravitational field and gravitational anomalies
- Rocks with higher density produce positive gravitational anomalies (e.g., igneous and metamorphic rocks)
- Rocks with lower density produce negative anomalies (e.g., sedimentary rocks)
- The density of rocks depends on their composition and porosity
- The density contrast between different rock types determines the amplitude of gravitational anomalies
- A larger density contrast produces a larger anomaly
Magnetic Susceptibility and Magnetic Fields
- Magnetic susceptibility is a measure of the extent to which a material can be magnetized in the presence of an external magnetic field
- Controls the magnetic field and magnetic anomalies
- Rocks with higher magnetic susceptibility produce positive magnetic anomalies (e.g., igneous and metamorphic rocks)
- Rocks with lower susceptibility produce negative anomalies or no anomalies (e.g., sedimentary rocks)
- The magnetic susceptibility of rocks depends on their content of magnetic minerals, such as magnetite, pyrrhotite, and ilmenite
- The magnetic susceptibility contrast between different rock types determines the amplitude of magnetic anomalies
- A larger susceptibility contrast produces a larger anomaly
Comparison of Gravitational and Magnetic Anomalies
- Gravitational and magnetic anomalies do not always coincide, as they are controlled by different physical properties
- A rock unit may have a high density but low magnetic susceptibility, or vice versa
- Results in different patterns of gravitational and magnetic anomalies
Key Terms to Review (17)
Density: Density is a physical property defined as mass per unit volume, often expressed in grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³). It plays a crucial role in understanding the composition and behavior of Earth's layers, influences gravitational fields, and is essential in the exploration of natural resources and the analysis of seismic waves.
Filtering: Filtering is a process used to isolate specific frequency components of a signal while removing unwanted noise or irrelevant data. This technique is crucial in geophysical data analysis as it helps enhance the clarity and interpretability of gravity and magnetic measurements, allowing for a better understanding of subsurface structures and anomalies.
Gauss's Law for Magnetism: Gauss's Law for Magnetism states that the total magnetic flux through a closed surface is zero, meaning that magnetic monopoles do not exist; instead, magnetic field lines are continuous loops. This principle connects with the idea of magnetic fields and their sources, showing that every magnetic field line that enters a closed surface must also exit it, thus reinforcing the understanding of how magnetic fields behave in space.
Geological mapping: Geological mapping is the process of creating a visual representation of the distribution, nature, and age of rock formations at the Earth's surface. This practice helps geologists understand the geological history of an area, identify resources, and assess geological hazards. Through techniques such as gravity and magnetic surveys, geological mapping can reveal subsurface features and provide insights into the Earth's structure and composition.
Gravimeter: A gravimeter is an instrument used to measure the acceleration due to gravity at a specific location, which helps in determining variations in the Earth's gravitational field. By detecting these variations, gravimeters can reveal insights about subsurface structures, such as mineral deposits and geological formations. This capability makes them crucial in studies related to gravity and magnetic potential fields, as well as for data acquisition and processing in geophysical surveys.
Gravitational Potential: Gravitational potential is the potential energy per unit mass at a point in a gravitational field, which reflects the work done to move a mass from infinity to that point. It is closely tied to the gravitational force and influences how objects move under gravity's influence. Understanding gravitational potential helps in analyzing gravitational fields and how they affect objects, particularly in contexts like planetary motion and the shape of Earth's gravitational field.
Gravity anomaly: A gravity anomaly is a variation in the gravitational field from what is expected based on a standard reference model of Earth's gravity. These anomalies indicate deviations caused by the presence of mass distributions, such as mountains, ocean trenches, or geological structures. Understanding gravity anomalies helps geophysicists interpret subsurface features and assess the geological composition of areas, playing a crucial role in studies related to Earth's structure and its potential resources.
Gravity surveying: Gravity surveying is a geophysical method used to measure variations in the Earth's gravitational field caused by differences in subsurface density. It provides valuable information about geological structures, mineral deposits, and can even assist in archaeological investigations. By analyzing gravity anomalies, geophysicists can infer the composition and distribution of materials beneath the Earth's surface.
Inversion: Inversion refers to a process in geophysics where data collected from the Earth's subsurface is transformed into a model that represents the physical properties of the subsurface materials. This process is essential for interpreting various geophysical datasets, allowing scientists to extract meaningful information about geological formations, their composition, and structural characteristics. Inversion techniques are crucial in many areas such as electromagnetic induction, gravity and magnetic fields, and seismic data processing, where they help to convert complex data into simpler models that can aid in understanding the Earth's interior.
Magnetic anomaly: A magnetic anomaly refers to a variation in the Earth's magnetic field strength at a specific location compared to the expected magnetic field strength based on a standard model. These anomalies can be caused by various geological features, such as mineral deposits or tectonic structures, and are essential in understanding subsurface geology and resource exploration.
Magnetic potential: Magnetic potential is a scalar quantity that represents the potential energy per unit magnetic pole strength at a given point in a magnetic field. It is used to describe how a magnetic field influences the movement of magnetic materials and charged particles, providing insight into the underlying physics of magnetic fields and their interactions with matter.
Magnetic surveying: Magnetic surveying is a geophysical method that involves measuring the Earth's magnetic field at various locations to identify subsurface geological structures. This technique is used to detect variations in magnetic properties caused by different rock types or mineral deposits, aiding in resource exploration and geological mapping.
Magnetic susceptibility: Magnetic susceptibility is a measure of how much a material will become magnetized in response to an external magnetic field. It quantifies the degree to which a rock or mineral can be magnetized, and it plays a crucial role in understanding the magnetic properties of geological materials, the behavior of the Earth's magnetic field, and the interpretation of geophysical data.
Magnetization: Magnetization refers to the process by which a material becomes magnetized, resulting in the alignment of its magnetic dipoles. This phenomenon is essential for understanding how geological materials can record the Earth's magnetic field and is crucial in the study of potential fields, especially when interpreting magnetic anomalies and their implications for subsurface structures.
Magnetometer: A magnetometer is an instrument used to measure the strength and direction of magnetic fields. This device plays a crucial role in geophysical surveys, allowing scientists to detect variations in the Earth's magnetic field, which can reveal important information about geological structures and processes below the surface. By understanding these magnetic properties, researchers can gain insights into the Earth's crust and its history, as well as applications in mineral exploration and environmental studies.
Mineral Exploration: Mineral exploration is the process of searching for and discovering mineral resources, including metals and other valuable materials, beneath the Earth's surface. This process involves various geophysical and geochemical methods to identify potential deposits, assess their economic viability, and inform extraction strategies.
Newton's Law of Universal Gravitation: Newton's Law of Universal Gravitation states that every mass attracts every other mass in the universe with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. This principle provides a foundational understanding of gravitational forces, which are crucial for analyzing both gravity and magnetic potential fields in geophysics.