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Determining underground geology and
physical properties—

geophysical methods used to answer questions for solving problems in drilling, engineering and construction

How to determine…

Density

What is the density of the rock? Where are the low density portions of the stratigraphy? How does the density vary spatially? Where are the faults? Where are the abandoned-mine’s air shafts? How can I find sinkholes or underground voids? Where is the gas in my well?

Best method: Gravity
May also be applicable: Seismic Refraction

Depth to Bedrock

How far from the surface is a rock layer? Where is a postulated fault? What are the subsurface elastic properties of a zone or layer? Where is the water table? Where is a channel cutting into the bedrock? Why don’t my borehole picks of top of rock make sense? Where is the location of a weak zone, a soft layer or fault gouge zone?

Best method: Seismic Refraction
May also be applicable: Electromagnetics for geologic structure, DC Resistivity, Gravity

Elastic Properties of Earth Materials

What are the IN SITU values of the elastic constants-Young’s modulus, shear modulus, bulk modulus, Poisson’s ratio? How strong is the alluvium? How strong is the rock? What is the shear wave (S-wave) velocity and compressional wave (P-wave) velocity of specific subsurface layers? Is liquefaction and collapse a risk for the dam? What are the design parameters for this wind farm generator foundation?

Best method: Cross-hole Seismic
May also be applicable: Surface-wave Analysis, P And S Wave Refraction

Electric Properties of Earth Materials

What is the ground resistivity for grounding mat design? What is a Wenner sounding? How conductive are the soils along my pipeline route? How do I do a four pin resistivity measurement? What are the design specifications for the cathodic protection system? Where are the geologic formation boundaries?

Best method: DC Resistivity
May also be applicable: Electromagnetics for Geologic Structure

Fault Location

Determining underground geology related to fault location: Is there a fault and where? What is its direction of displacement? How big is the fractured zone around the fault?

Best method: Seismic Refraction
May also be applicable: DC Resistivity, Seismic Reflection

Ground Motions (vibration analysis or blast monitoring)

How do I determine ground motions due to the industrial blast? What are ‘safe’ limits for blasting? Can I build a microchip factory next to that drop forge? How high are the vibrations from that conveyor belt?

Best method: Seismic Recording or Monitoring

Permeability

What is the permeability of the rocks? Which way is the water flowing? Where do I drill my water well for the best flow rate? Where are zones of increased water production?

Permeability is qualitatively interpreted or interpolated with the help of adequate ground-truth data  (i.e., correlated to a sandstone bed or shear-zone that can be mapped using remote sensing). There is no known way of directly measuring permeability using remote sensing. Various pump-tests and borehole methods get us closest.

Best method: CSAMT, Borehole Methods
May also be applicable: IP

Rippability

How strong is the bedrock? How big a bulldozer do I need to rip (rather than blast) a highway cut? Are there hard patches in the subsurface that I cannot rip?

Best method: Seismic Refraction
May also be applicable: Ground Penetrating Radar, Cross-hole Seismic, Seismic Reflection

Site Classification / Seismic Hazard Analysis

What is the probabilistic seismic hazard analysis for this proposed dam? What is the acceleration that my structure will experience during its useful lifetime? What acceleration should my structure be designed for to prevent catastrophic collapse? What is the UBC site classification (A,B,C,D,E) that I should use in foundation design? Why do I care in this state?

Best method: for Hazard Analysis – Long-Term Seismic Recording, for Site Classification – Seismic Refraction
May also be applicable: Surface Wave Analysis

Temperature

What is the temperature profile with depth? Can I heat this commercial greenhouse with geothermal waters? How does the heat flow vary across the earth’s surface? Where is the geothermal resource? Where do I put the steam plant?

Best method: Thermal Gradient
May also be applicable: DC Resistivity