Geophysical Survey Methods

A well-designed reconnaissance program will give you intelligence about the subsurface. The success of a geophysical survey depends on the selection of the best methods and well-planned data acquisition parameters. Geophysics utilizes remote measurements of the earth to conduct non-destructive surveys of the subsurface. Our staff has designed, collected and interpreted geophysical data for many applications using a variety of methods.

Geophysical Methods Table – 2015

Click the methods below for more information.

Geophysical

Ground Penetrating Radar (GPR)

Ground penetrating radar is extensively used to map wide range of shallow features, such as voids and cavities, rebar in concrete, utilities, underground tank locations, concrete thickness, historical riverbed scouring, earthen dams, buried foundations, geologic structures, missing persons investigations and archaeological sites. Radar data can be compiled into 3-D data volumes, allowing the creation of times slices, providing map view images that can be correlated with site drawings and aerial photographs.

 

Limitations: Depth of penetration limited by clay and moisture content.

Surface Wave Seismic

Non-invasive surface seismic method is capable of measuring shear wave velocities in the upper 100 ft of soil for seismic site class applications. Testing is Surface Wave Seismic Exampleperformed on the ground surface, allowing for less costly measurements and faster results than with traditional borehole methods. This method compares favorably with downhole and CPT measurement systems.

Limitations: Horizontal resolution can be poor; decreasing resolution of velocity structure with depth.

Seismic Reflection

Seismic reflection detects subsurface boundaries, assisting in the mapping of geologic structures. It is a robust geophysical method that has been increasingly used in shallow applications, such as groundwater exploration and civil engineering evaluations. Typical applications include subsurface mapping and bedrock void detection.

Limitations: Vibrational noise car impair results; poor resolution for shallow reflections.

Seismic Refraction

The seismic refraction technique is an established, proven method for mapping buried bedrock surfaces. The method is particularly well suited for measuring the depths to bedrock in connection with the construction of large buildings, dams, tunnels, and highways. It is a favored method for mapping alluvial cover (i.e. buried bedrock valleys) and the depth to bedrock.

 


Limitations:
Seismic velocities required to increase with depth; generally limited to a depth of 100 ft with standard equipment.

Parallel Seismic

Borehole seismic measurements provide an indirect means of estimating pile tip elevations in otherwise Parallel Seismicinaccessible deep foundation elements. Seismic energy can by delivered to the pile cap or exposed pile surfaces, and measured in a nearby borehole that is drilled parallel to the pile. The method is relatively robust, and can often be employed without interruption to traffic or activities within a structure.

Limitations: Vibrational noise can impair results; requires cased boreholes next to structure.

Gravity / Microgravity

Gravity surveys have been extensively used to map buried valleys and cave features with demonstrated success. Changes in the gravity measurements can reflect bedrock structures, buried bedrock valleys, voids and caves, tunnels, or other features that exhibit contrasting material densities.

Limitations: Vibrational noise can impair results; poor resolution; non-unique solutions.

Electromagnetics (E/M)

Electromagnetic surveys map variations in the electrical conductivity of geologic materials. Buried Electromagnetics Data Exampleobjects and geologic discontinuities can be detected by mapping electromagnetic anomalies. The method is useful for detecting metallic objects such as pipes, drums, cables, and tanks, as well as determining the extent of landfill areas. The method is also used extensively in unexploded ordnance (UXO) surveying.

Limitations: Interpretation is qualitative; able to define only one to two layers with simple stratigraphy.

Magnetics

Magnetometer Example 1Measurements of the magnetic field of the earth, and local disruptions to the field, can be used to map man-made objects such as buried drums, tanks, or ordnance; geologic contacts and structures; and the presence of naturally occurring ore bodies. In ground magnetic surveys, the earth’s magnetic field is measured at closely spaced stations. The data can be filtered and plotted to define localized magnetic anomalies attributable to the presence of buried structures.

Limitations: Ferrous objects cause unwanted interference; non-unique solutions.

Electrical Conductivity & Resistivity

Electrical resistivity is a proven method for mapping contrasting geological materials – no other surface geophysical method has been more widely used in groundwater studies. The method can effectively determine both vertical and horizontal extent of geologic units. Resistivity data may be collected relatively rapidly and inexpensively. In comparison to seismic surveys, this method has no noise or impact on surrounding community or environment.Resistivity Data Example_edit

 

Limitations: Limited vertical and horizontal resolution; non-unique solutions; data impaired by electrical noise.

Natural Gamma Logging

Natural gamma logging is a borehole tool used to map soil and rock lithologies through an installed well Gamma Logcasing. This method is useful when continuous lithologic data are needed for an existing well but continuous well logs are not available. Gamma logging can also be employed as a well is augured to identify sand and gravel zones, guiding well screen placement.

Limitations: Interpretation is qualitative.