Fiveable
Fiveable

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

Top images from around the web for Principles and Instrumentation
Top images from around the web for Principles and Instrumentation
  • 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


© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
Glossary
Glossary