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Hydrology Lecture 16 Geophysical techniques
Reading assignment: Skim Chapter 13
During and after the drilling of wells, piezometers, and boreholes, a variety of well logging techniques can be used to gain information about the subsurface at relatively little expense.
Reasons for logging wells include:
Delineation of hydrogeologic units (The different hydrogeologic units found in the subsurface display a wide range of capabilities to store and transmit ground water and contaminants. Borehole-geophysical logging provides a highly efficient means to determine the character and thickness of the different geologic materials penetrated by wells and test holes. This information is essential for proper placement of casing and screens in water-supply wells and for characterizing and remediating ground-water contamination.
Definition of ground-water quality (The quality of ground water is highly variable and ground-water contamination may be caused by man-made or natural sources. Integration of borehole-geophysics logging with water-quality sampling provides a more complete picture, whether the objective is to develop a water-supply well or remediate a contaminated aquifer.)
Determination of well construction and conditions (Wells are the access points to the ground-water system, and knowledge of their construction and condition are important whether they are being used for ground-water supply, monitoring, or remediation. The location and condition of casing and screen can be rapidly evaluated with geophysical logging.
Types of Well Logs If samples of the material being drilled are brought to the surface and described by a geologist, then a lithologic well log can be recorded. This is simply a description of the layers of material encountered during drilling, and is often called a driller’s log.
After a well is drilled, several geophysical techniques can be employed to learn something of the properties of the geologic material and fluids in the subsurface. These techniques involve lowering some kind of a probe into the hole and measuring a physical property as the probe descends and ascends.
Caliper log - because many of the geophysical logging methods are sensitive to the diameter of the borehole, often a log of borehole width is made against which the results of other analyses can be compared.
Borehole diameter is affected by the swelling of clays in the sides of the hole, by differential resistance of the drill bit to different lithologies, and by dissolution cavities in the rock. Caliper logs alone can often be used to identify both productive and confining units in an aquifer.
(Caliper logs record borehole diameter. Changes in borehole diameter are related to well construction, such as casing or drilling-bit size, and to fracturing or caving along the borehole wall. Because borehole diameter commonly affects log response, the caliper log is useful in the analysis of other geophysical logs, including interpretation of flowmeter logs.)
Electric logs - there are a variety of geophysical techniques that involve lowering an electrode down the well while continuously measuring the change in current flow (electrical potential) through the electrode.
Spontaneous Potential : measures the difference in natural voltage between the probe and a surface grounded electrode. This method is sensitive to pore water chemistry, which is greatly influenced by the lithology of the subsuface. Generally, coarser grained sandy units induce a positive voltage difference while fine-grained, clay units induce a negative potential.
(Spontaneous-potential logs record potentials or voltages developed between the borehole fluid and the surrounding rock and fluids. Spontaneous-potential logs can be used in the determination of lithology and water quality. Collection of spontaneous-potential logs is limited to water- or mud-filled open holes.)
Resistivity : Measures the resistence to an introduced current flow between two electrodes. Coarse-grained units have high resistence, fine-grained units have low resistence.
( Single-point resistance logs record the electrical resistance from points within the borehole to an electrical ground at land surface. In general, resistance increases with increasing grain size and decreases with increasing borehole diameter, fracture density, and dissolved-solids concentration of the water. Single-point resistance logs are useful in the determination of lithology, water quality, and location of fracture zones.)
( Normal-resistivity logs record the electrical resistivity of the borehole environment and surrounding rocks and water as measured by variably spaced potential electrodes on the logging probe. Typical spacing for potential electrodes are 16 inches for short-normal resistivity and 64 inches for long-normal resistivity. Normal-resistivity logs are affected
