Applied Geophysical Surveying
Gravity, magnetic, and borehole surveys complement seismic and electrical methods, mapping density and magnetization contrasts at the surface or from the air and measuring rock properties directly down a borehole.
Definition
Applied geophysical surveying is the practical acquisition and interpretation of geophysical measurements, especially gravity, magnetic, and borehole logging methods, to map subsurface density, magnetization, and other rock properties for exploration and site characterization.
Scope
This topic covers the potential-field and borehole methods of applied geophysics: ground, airborne, and marine gravity and magnetic surveys, their corrections and the interpretation of anomalies, and the use of magnetic surveying for mineral and structural mapping. It treats well logging, the suite of borehole measurements of porosity, resistivity, density, and lithology that calibrate surface geophysics and characterize reservoirs. It also addresses survey design, data reduction, and integrated interpretation. The emphasis is on the practical methods of mapping physical properties for exploration and characterization.
Core questions
- How are gravity and magnetic surveys conducted on the ground, in the air, and at sea?
- How are potential-field data corrected and reduced to anomalies?
- What rock properties do well logs measure, and how do they calibrate surface methods?
- How are multiple survey types integrated for interpretation?
Key concepts
- Gravity and magnetic survey acquisition
- Airborne and marine geophysical surveys
- Anomaly reduction and interpretation
- Well logging of porosity, resistivity, and density
- Integrated survey design and interpretation
Key theories
- Potential-field surveying
- Gravity and magnetic surveys map spatial variations in density and magnetization; because both are potential fields, anomalies are reduced and interpreted with potential theory to outline subsurface bodies and structures over wide areas, including from aircraft.
- Borehole logging
- Well logs record physical properties of the rocks penetrated by a borehole, providing direct, high-resolution measurements of porosity, resistivity, and lithology that calibrate and constrain the interpretation of surface geophysical surveys.
Mechanisms
Gravity and magnetic instruments measure tiny spatial variations in the gravitational and magnetic fields caused by density and magnetization contrasts at depth; after corrections for known effects these become anomalies outlining subsurface structures, while in a borehole, tools measuring electrical, nuclear, and acoustic responses sample the rock directly, tying the remote surface and airborne measurements to ground truth at the well.
Clinical relevance
Gravity and magnetic surveys are workhorses of regional mapping and mineral and petroleum exploration, airborne surveys cover large or inaccessible areas efficiently, and well logging is essential to reservoir evaluation, groundwater assessment, and the calibration of all other geophysical methods.
History
Torsion-balance and pendulum gravity surveys and magnetic prospecting developed in the early twentieth century, the Schlumberger brothers introduced electrical well logging in 1927, and airborne magnetic and gravity surveying expanded after mid-century, with modern instruments and positioning greatly increasing precision and coverage.
Key figures
- Conrad Schlumberger
- Marcel Schlumberger
- Robert Sheriff
Related topics
Seminal works
- telford1990
- kearey2002
- blakely1995
Frequently asked questions
- What is well logging?
- Well logging lowers instruments down a borehole to measure properties of the surrounding rock, such as its electrical resistivity, density, porosity, and natural radioactivity, providing a detailed depth profile that identifies rock types and fluids and calibrates surface geophysical surveys.
- Why survey gravity and magnetics from an aircraft?
- Airborne surveys can rapidly and uniformly cover large or rugged areas that would be slow or impossible to map on the ground, making them efficient for regional mineral and petroleum exploration and for geological mapping over remote terrain.