4.3 Magnetic surveying and data interpretation
4 min read•august 14, 2024
Magnetic surveying measures variations in Earth's to detect subsurface features and mineral deposits. It uses magnetometers on the ground or in aircraft to collect data, which is then processed and interpreted to understand geology and find resources.
Interpreting magnetic data involves analyzing anomaly maps and profiles. Positive anomalies often indicate magnetic rocks or minerals, while negative ones suggest less magnetic materials. The shape and intensity of anomalies provide clues about subsurface structures and compositions.
Magnetic Surveying Principles and Techniques
Principles and Instrumentation
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- Magnetic surveying measures variations in the Earth's magnetic field detects subsurface geological features and mineral deposits
- The magnetic field is a vector quantity has both magnitude and direction
- Ground magnetic surveys are conducted using portable magnetometers along traverses or in a grid pattern
- Proton precession and fluxgate magnetometers are commonly used instruments for ground surveys
- Airborne magnetic surveys are conducted using magnetometers mounted on aircraft allows for rapid coverage of large areas
- Airborne surveys are typically flown along parallel lines at a constant elevation
Survey Design and Data Collection
- Magnetic data are recorded as total magnetic intensity (TMI) or as individual components of the magnetic field vector (horizontal, vertical, or total field)
- Magnetic survey design considerations include line spacing, sampling interval, and survey altitude
- These factors are determined based on the desired resolution and target depth
- Closer line spacing and shorter sampling intervals provide higher resolution but increase survey cost and time
- Survey altitude affects the sensitivity to shallow sources lower altitudes enhance the detection of near-surface features
Magnetic Anomalies and Subsurface Geology
Magnetic Properties of Rocks and Minerals
- Magnetic anomalies are local variations in the Earth's magnetic field caused by the presence of magnetic minerals (primarily magnetite) in the subsurface
- These anomalies can be positive (higher magnetic intensity) or negative (lower magnetic intensity) relative to the background field
- The of rocks and minerals determines their contribution to magnetic anomalies
- Ferromagnetic minerals, such as magnetite, have high magnetic susceptibility are the primary sources of magnetic anomalies
- Igneous and metamorphic rocks often have higher magnetic susceptibility than sedimentary rocks makes them more likely to generate magnetic anomalies
Geologic Structures and Mineral Deposits
- The shape, amplitude, and wavelength of magnetic anomalies provide information about the geometry, depth, and magnetic properties of the causative geological features
- Mineral deposits containing magnetic minerals, such as iron ore, can produce distinct magnetic anomalies aids in their detection and delineation
- Geologic structures, such as faults, folds, and intrusions, can also create magnetic anomalies due to the juxtaposition of rocks with different magnetic properties
- Faults may appear as linear features or offsets in the magnetic data
- Folds may produce characteristic "bulls-eye" or arcuate anomalies
Data Processing for Magnetic Surveys
Diurnal Correction and Reduction-to-Pole
- Diurnal correction removes the effect of daily variations in the Earth's magnetic field caused by solar activity
- Diurnal variations are monitored using a base station magnetometer the corrections are applied to the survey data
- Reduction-to-pole (RTP) is a data processing technique transforms magnetic anomalies to the form they would have if the magnetic field were vertical (as if the survey were conducted at the magnetic pole)
- RTP simplifies the interpretation of magnetic anomalies by centering them over their causative bodies
Additional Processing Techniques
- Other data processing techniques include leveling, gridding, and filtering (low-pass, high-pass, and band-pass filters) enhances specific anomaly characteristics and removes noise
- Magnetic data are often integrated with other geophysical datasets (gravity, electromagnetic) and geological information improves interpretation and reduces ambiguity
- Integration helps to constrain the interpretation and reduce uncertainty
- Examples of complementary data include drill hole information, geologic maps, and seismic data
Interpreting Magnetic Data for Geology and Minerals
Magnetic Anomaly Maps and Profiles
- Magnetic anomaly maps display the spatial distribution of magnetic field variations allows for the identification of geological features and patterns
- Color scales or contour lines are used to represent the intensity of the magnetic field
- Magnetic profiles show the variation of the magnetic field along a specific survey line provides a cross-sectional view of the subsurface
- Profiles help identify the shape, amplitude, and wavelength of magnetic anomalies
Interpretation Techniques and Considerations
- Positive magnetic anomalies may indicate the presence of highly magnetic rocks or minerals, such as mafic intrusions (gabbro) or iron-rich ore bodies (magnetite)
- Negative anomalies may suggest the presence of less magnetic rocks (sedimentary) or alteration zones
- The shape of magnetic anomalies provides clues about the geometry of the causative bodies
- Symmetric, circular anomalies often indicate vertical or steeply dipping structures (kimberlite pipes)
- Elongated or asymmetric anomalies may suggest dipping or fault-bounded bodies (dipping dikes)
- The depth to the causative bodies can be estimated using techniques such as the half-width rule or Euler deconvolution relates the anomaly shape to the depth of the source
- Integration of magnetic interpretation with other geological and geophysical data helps to constrain the interpretation and reduce uncertainty
Key Terms to Review (18)
3D inversion: 3D inversion is a geophysical modeling technique that reconstructs the subsurface geological structures and properties from surface measurements, especially in magnetic surveying. This method converts raw data into three-dimensional models, helping to visualize and interpret the distribution of various materials beneath the Earth's surface. By effectively incorporating spatial relationships, 3D inversion enhances the understanding of geological features and aids in decision-making for exploration and resource management.
Aeromagnetic survey: An aeromagnetic survey is a geophysical method used to measure the magnetic field of the Earth from an aircraft. This technique allows for the collection of magnetic data over large areas efficiently, helping to map geological structures and identify mineral deposits by analyzing variations in the Earth's magnetic field caused by subsurface rocks.
Anomaly Detection: Anomaly detection is the process of identifying patterns in data that do not conform to expected behavior, often referred to as outliers or anomalies. This technique is essential in geophysics for recognizing unusual geological features or measurement errors in data collected from various surveying methods, ensuring accurate interpretation and analysis of subsurface conditions.
Archaeological investigations: Archaeological investigations are systematic studies aimed at uncovering, analyzing, and interpreting material remains from past human activities. These investigations can reveal information about ancient cultures, technologies, and environments, often employing various techniques, including non-invasive methods like magnetic surveying to detect subsurface features without disturbing the site.
Basaltic intrusions: Basaltic intrusions are bodies of molten basalt that penetrate into pre-existing rocks during volcanic activity, solidifying underground. These intrusions can create various geological structures such as sills, dikes, and laccoliths, which can provide important insights into the volcanic processes and the composition of the Earth's crust.
Data filtering: Data filtering is the process of removing or isolating specific datasets from a larger collection based on defined criteria. This technique is crucial in data analysis, allowing researchers to focus on relevant information while minimizing noise and enhancing the quality of data interpretation.
Fault zones: Fault zones are regions where the Earth's crust has fractured, allowing for movement along faults due to tectonic forces. These areas are characterized by a complex network of fractures and can significantly impact geological features and mineral deposits. Understanding fault zones is crucial for assessing seismic risks and exploring natural resources.
Fluxgate magnetometer: A fluxgate magnetometer is a sensitive instrument used to measure the strength and direction of magnetic fields, primarily Earth's magnetic field. This device employs a core material that becomes magnetized when exposed to external magnetic fields, allowing it to provide precise measurements crucial for various geophysical applications, including magnetic surveying, core dynamics analysis, and data acquisition systems in geophysics.
Gradient Mapping: Gradient mapping is a technique used in geophysical surveys to visualize the spatial variation of a property, such as magnetic intensity, by representing the gradient or rate of change of that property. This method allows geophysicists to identify anomalies and trends in the data more effectively, as it highlights areas where there are significant changes in magnetic fields, which may indicate the presence of mineral deposits or geological structures.
Ground magnetic survey: A ground magnetic survey is a geophysical method used to measure variations in the Earth's magnetic field caused by subsurface geological structures. This technique helps geophysicists locate and map features like mineral deposits, faults, and archaeological sites by detecting changes in magnetic intensity, which can reveal information about the composition and orientation of subsurface materials.
Inverse modeling: Inverse modeling is a mathematical approach used to deduce the properties or parameters of a system by analyzing the observed data generated from that system. This technique is essential in various geophysical applications, allowing researchers to interpret complex datasets and estimate subsurface characteristics based on surface measurements. By applying inverse modeling, scientists can generate models that best fit the observed data, which is crucial for understanding subsurface structures and processes.
Magnetic field: A magnetic field is a vector field that describes the magnetic influence exerted by electric currents and magnetic materials in space. It plays a crucial role in various geophysical processes, allowing scientists to interpret subsurface features and identify anomalies during magnetic surveying.
Magnetic modeling: Magnetic modeling refers to the process of creating representations of the Earth's magnetic field and its variations in response to subsurface structures. This involves the interpretation of magnetic survey data to understand geological features such as mineral deposits, fault lines, or volcanic activity. By using mathematical techniques and software, geophysicists can simulate magnetic field anomalies, which helps in visualizing and analyzing subsurface geology.
Magnetic susceptibility: Magnetic susceptibility is a measure of how much a material will become magnetized in an applied magnetic field. It indicates the degree to which a substance can be magnetized and is crucial for understanding how different materials interact with magnetic fields in geological surveying and data interpretation.
Mineral exploration: Mineral exploration is the process of searching for and assessing mineral resources in the Earth's crust, which includes identifying potential locations for mining operations. This process involves various methods and techniques to gather data about geological formations, mineral compositions, and the economic viability of extracting these resources.
Paleomagnetism: Paleomagnetism is the study of the magnetic properties of rocks and sediments, particularly the record of Earth's magnetic field preserved in them over geological time. This concept connects various geophysical aspects, such as understanding the historical movement of tectonic plates, the behavior of Earth’s magnetic field, and how these factors can be utilized in applications like magnetic surveying and interpreting data related to the Earth’s core dynamics.
Remanent magnetization: Remanent magnetization is the magnetization that a material retains after an external magnetic field is removed. This property is crucial for understanding the magnetic history of geological formations and contributes to the interpretation of magnetic data, especially in the exploration of natural resources and understanding Earth’s magnetic field history.
Total field magnetometer: A total field magnetometer is an instrument used to measure the total strength of the Earth's magnetic field at a specific location. It provides valuable data for various applications, including mineral exploration, archaeological investigations, and environmental studies. The readings from a total field magnetometer help in identifying subsurface structures and anomalies that are linked to different geological formations or human-made features.