Magnetic surveying measures variations in Earth's magnetic field 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 magnetic susceptibility 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