Geophysical Survey Methods: Principles and Selection Criteria

Physical Properties in Geophysical Surveys

Commonly used physical properties include:

• Elastic wave velocity (elasticity, density): Seismic methods
• Electric pulse velocity (dielectric constant): Georadar (GPR)
• Electrical DC resistance (resistivity): DC resistivity methods
• Electrical AC conductivity: EM conductivity methods
• Magnetic field strength (susceptibility): Magnetic methods
• Gravity field strength (density): Gravity methods

The value of surface measurements is determined by the contrast in the relevant property (material type) and the three-dimensional structure.

Selecting the Right Geophysical Method

When planning a survey, consider the following factors:

• What type and shape of feature is being imaged?
• Is an area or line survey more effective?
• Which physical properties will show the best contrast?
• Are there strong but irrelevant contrasts that will mask results?
• To what depth must the survey penetrate?
• What spatial resolution is required?
• What are the time or cost constraints?
• Are there special restrictions, such as access or potential damage?

Passive vs. Active Geophysical Methods

Passive methods are non-destructive and require less equipment. They are often based on anomalies in the strength of a potential field (e.g., magnetics), though results are inherently ambiguous.

Geophysical methods are further subdivided into contacting or non-contacting methods based on ground interaction:

• Contacting methods:
  • Seismics
  • Electrical resistivity
• Non-contacting methods:
  • Gravity
  • Magnetics
  • Ground penetrating radar
  • EM conductance methods

Common Challenges in Geophysical Data

Geophysical data typically face two generic problems:

• Active methods: There is a trade-off between image resolution and depth of penetration.
• Passive methods: There is an inherent ambiguity in field strength data, as a wide, shallow object can generate the same field anomaly as a compact, deeper object.