Used to help determine the indirectly the extent and nature of geologic materials

beneath the surface of the earth.

Can be used to help locate/determine

Method with probably the greatest application to traditional hydrogeological

investigations.

These steps are part of the method

1. introduce a direct current of very low frequency,
2. introduce current through two electrodes,
3. measure voltage between two other electrodes,
4. compute the resistivity of the earth between the electrodes.

Electrical resistivity, R, is described by the following equation

The four electrodes used in a resistivity array are

A positive-current electrode

B negative-current electrode

M and N potential electrodes

 

If we call XY the distance between electrode X and Y, then we can express

the previous equation as

The resistivity calculated with the above equation is an apparent resistivity

because the resistivity of earth materials is rarely homogeneous or electrically

isotropic.

Electrodes can be placed in a number of different configurations depending on

what is being measured. The two most common spacings are the Wenner array

and Schlumberger array.

The dipole-dipole array is another spacing that is used to measure changes in

electrical properties with depth (soundings).

Horizontal profiling is another survey method that is used to

determine how electrical properties change laterally.

<insert figure 13.6 here>

After a number of soundings have been conducted, a plot of resistivity as a function

of electrode spacing is made on logarithmic paper to conduct interpretations.

<insert figure 13.7 here>

Measures the inverse of resistivity, the conductivity of earth materials.

Generally, pores saturated with fluid conduct current more easily than unsaturated

pores.

The method

Advantages of method

1. quick!
2. no electrode insertion
3. cheap
4. can detect contaminants, barrels, or salt-water

Seismic refraction method is best for determining the thickness of unconsolidated

material overlying bedrock.

1) Measure the arrival time of seismic waves at various distances

1. Calculate the depth to bedrock assuming a velocity of the wave

for the specific material

Can have depths of penetration from a few meters to 1,000's of meters

A relatively new method that uses pulses of electromagnetic waves in the

frequencies of 10 to 1,000 MHz.

Measures the difference in dielectric properties of different materials.

Used to locate barrels, contaminant plumes, ice thickness, overburden thickness

Measure the natural gravity or magnetic properties of earth materials.

A good companion to a lithologic well log.

Usually not cost effective for water-supply wells, but may be very cost

effective for monitoring wells in contaminated water or very deep water

wells.

Can help to meet the following specific hydrogeologic objectives

1. Site selection - porosity, permeability, lithology, fractures,

basic water quality information

2. Site design and well location - can help to plan location of pits

and trenches, locate water table(s)

3. Evaluating well construction - interpret well construction or understand

well failure

4. Monitoring contaminant distribution - can provide continuous

information on contaminants (not just from discrete depth samples)

5) Planning remediation program

1) Caliper logs - measures the diameter of an uncased borehole

In uncased, bedrock boreholes, can be used to find depth and width of fractures

2) Temperature logs - continuous log of fluid temperature with depth in a borehole

In uncased, fractured bedrock boreholes, can indicated different water

producing zones from different water-yielding fracture zones.

Use the electrical properties of the materials around the borehole to infer the type of

material.

Resistivity - use the resistance of a direct electrical current

Spontaneous potential - measures the natural electrical potential that develops

between the borehole and borehole fluids.

Measure of the natural radiation in a borehole.

Can be done in either a cased or an uncased hole.

Includes natural gamma, neutron and gamma-gamma radiation.

v = No Restrictions

† = Steel Casing Only

? = Possible Applications