Exploration Geophysics: Data Acquisition and Interpretation
Data Acquisition
Exploration geophysics involves collecting data according to a defined survey pattern. This may be along a line, around a polygon, or over an area.
The type of data is determined by the purpose of the survey and by the expected underground structure.
Typical data are:
- Arrival times of seismic waves
- Arrival times of high-frequency electrical signals
- Variations in the local magnetic field
- Variations in local ground resistance
Physical properties that are commonly used are:
- 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 the surface measurement is determined by the contrast in the relevant property (hence material type) and by the three-dimensional structure.
Factors to Consider
Some factors to be considered are:
- What type and shape of feature is being imaged?
- Is an area or line survey the better?
- What physical properties will show the best contrast?
- Are there any strong but irrelevant contrasts that will mask the results?
- To what depth must the survey penetrate?
- What spatial resolution is needed?
- What are the time or cost constraints?
- Are there any special restrictions, e.g., on access or damage?
Contacting vs. Non-Contacting Methods
Contacting methods have the advantage that the power input can be greater and the reception more sensitive. Thus, the results may be more detailed or provide greater penetration.
Non-contacting methods are more rapid and therefore cheaper. Despite their generally lower resolution, the results may still be adequate for the purpose in hand.
Data Interpretation
Geophysical data suffer from two generic problems:
- Active methods: There is a trade-off between image resolution and depth of penetration into the ground.
- Passive methods: There is an inherent ambiguity in field strength data, since a wide shallow object generates the same field anomaly as a compact, deeper object.
However, there is always a loss of energy (attenuation) during propagation, and the loss is usually a constant ratio per wavelength.
Thus, a higher frequency (shorter wavelength) signal will lose more energy over a given distance than will a lower frequency signal.
The loss is thus cumulative with distance. This is an example of the Beer-Lambert absorption law.
Ambiguity arises in the interpretation of interface depths if the velocity of propagation is not accurately known.
The recorded data are the two-way arrival times. These are converted to depths using the velocity. Thus, if the seismic velocity is in error by, say, 10%, the depth will be in error by 10% also.
This is a particular problem with GPR data, since the velocity of the electrical signal in soil is sensitive to water content, which is highly variable and unlikely to be known accurately in advance.
Ambiguity also arises in the interpretation of field strength data.
It can be shown mathematically that an identical field anomaly can be produced by differing structures at differing depths.
Ground Truth
In the geophysics context, ground truth means:
- Physical property measurements on samples
- Direct determination of the depth or extent of an underground structure
Ground truth is necessary to overcome both inherent ambiguity and to establish the correct property values needed for quantitative interpretation.
The relevant observations may be:
- Measurements of density, seismic velocity, magnetic susceptibility, etc., made on samples recovered from boreholes
- Field measurements of velocity made from one or two boreholes
- Determination of the geological sequence at various positions, probably linked to property measurements
Typical Geophysical Survey Pattern
Most geophysical surveys on land follow a typical pattern:
- Basic survey of the site using aerial photos and ground stations
- Geophysical traverses between key locations, usually borehole positions
- Infill traverses to provide detail, perhaps after primary interpretation
- Ground truth from field or laboratory measurements
Keys to Successful Use of Geophysical Methods
Four keys to the successful use of all geophysical methods are:
- Using a method based on properties that have good contrasts across subsurface boundaries
- Applying a processing technique that will minimize the inevitable background noise in the signal
- Being able to relate the results to known subsurface materials and structure, usually via nearby boreholes
- If possible, using two or more methods in combination