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Hydrology

Lecture 16 Geophysical techniques

Reading assignment: Skim Chapter 13

Text in (parentheses) is taken from the USGS Introduction to Borehole Geophysics website at http://wwwdnyalb.er.usgs.gov/projects/bgag/intro.text.html.

During and after the drilling of wells, piezometers, and boreholes, a variety of well

loggingtechniques 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-geophysicallogging 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 logcan be recorded. This is simply a description of the

layers of material encountered during drilling, and is often called a drillers 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.

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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 inthe 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 anelectrode 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 thedetermination 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 logsrecord 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 logsrecord 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

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by bed thickness, borehole diameter, and borehole fluid and can only be collected in

water- or mud-filled open holes.)

Induction: Measures the strength of a secondary electric field induced in the aquifer by

the electrode. This is a good technique for locating saltwater wedges as the salinegroundwater is a good conductor of electricity.

(Electromagnetic-induction logsrecord the electrical conductivity or resistivity of the

rocks and water surrounding the borehole. Electrical conductivity and resistivity are

affected by the porosity, permeability, and clay content of the rocks and by the dissolved-

solids concentration of the water within the rocks. The electromagnetic-induction probe

is designed to maximize vertical resolution and depth of investigation and to minimize

the effects of the borehole fluid.)

Gamma logs: Uses a gieger counter-like probe to measure the natural gamma radiationemitted by aquifer materials. Clays are enriched in uranium minerals and clay-rich units

have high gamma counts.

(Gamma logs record the amount of natural gamma radiation emitted by the rocks

surrounding the borehole. The most significant naturally occurring sources of gamma

radiation are potassium-40 and daughter products of the uranium- and thorium-decay

series. Clay- and shale-bearing rocks commonly emit relatively high gamma radiation

because they include weathering products of potassium feldspar and mica and tend to

concentrate uranium and thorium by ion absorption and exchange.)