Detecting Physical Properties (Fieldwork)

Geophysical field surveys are used to investigate the subsurface. Geophysics may utilize passive techniques, measuring and mapping changes in the ambient field, or as active techniques that measures how the geological environment responds to an input signal. In either instance, the geophysical signal is processed and interpreted to indicate particular physical properties of geological materials. Those physical properties are used to determine a geological interpretation for subsurface materials and structures that would plausibly result in the observed geophysical signals.

The advantage of geophysics surveying is that passive techniques are non-destructive, that measurements are taken close (or at) the area of interest, and that an appropriate survey can provide a wealth of information about the subsurface in a cost-effective manner. The disadvantage of field surveying is that active techniques can be destructive (depending on the type of source providing an active signal), that environmental conditions (weather, access, injury) can increase survey time beyond estimates, and that in-situ properties do not always match properties determined in controlled laboratory settings.

Passive techniques include any survey where sensors measure the ambient field. Examples include gravity and magnetic methods, and some resistivity techniques. Active techniques include any survey where a source provides an active signal, and sensors measure how that signal is altered by the environment. Examples include induced polarization and artificial-source seismic techniques.

Borehole Logging

Boreholes are a circular cross-section made in soil or rock, that is either cased with piping or left bare. A borehole log is the record of the depth, geologic units, sample recovery (if any), water level, and any other significant facts related to the drilling including geophysical data collected through borehole sondes. A sonde is a long, tubular object containing instruments that is attached to an armoured cable, lowered into the drill hole with a pulley and winch system. The sonde is lowered to depth, then data is collected during a slow pull up the hole.

The advantages of a borehole logging with geophysical probes is that we can obtain in-situ measurements close to the target area, can use core or drill chipping to guide geological interpretation of the geophysical data, and, for some types of surveys, obtain measurements for the undisturbed surface away from the borehole. The disadvantages of borehole logging is that drilling is expensive (particularly depending on the location, subsurface characteristics, depth, and if core is preserved), destructive, and may induce fracturing or breach impermeable layers spreading contamination plumes.

A sonde may contain one or more geophysical systems, most commonly sensors to measure resistivity, acoustic velocity, ground penetrating radar, spontaneous potential, neutron-neutron, natural gamma, gamma-gamma, temperature, fluid flow, gravity, or magnetic properties. A survey may consist of in individual hole in isolation, or a cross hole survey with the source in one hole and receivers in another.

Data extracted from geophysical borehole probes is interpreted in conduction with other information from the site. Two additional borehole probes are commonly used in conjunction with the geophysical sondes are calipers (or callipers for those across the ocean), providing a record of borehole diameter, and an optical televiewer, creating an oriented digital image of the borehole wall. Additional information on site conditions, elevation, diameter and angle of hole, material used during drilling or casing, observations on the extracted core or chip pings, and the people involved in the project are all part of standard field logs that can aid geophysical interpretation.

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