Borehole geophysics provide a means of evaluating a great variety of physical properties of subsurface rocks, sediments, and fluids.  Borehole logging offers an economical alternative to continuous sampling or coring, furnishing continuous, objective data sets in a graphical form that can be rapidly interpreted in the field.  Additionally, geophysical logging provides information about the subsurface that cannot be obtained through standard drilling and sampling techniques.  Some of the more commonly used techniques for downhole geophysical methods are listed below:

Natural Gamma 
Measures the level of gamma radiation emitted by  radioisotopes present in subsurface materials.  Maps lithology and provides relative porosity of soil and rock  based on clay content.

Resistivity 
Measures the resistivity of soil, rock, and pore fluid.  Maps lithology and provides for contaminant identification based on conductivity of pore fluids.

Electromagnetic Induction (EM)  
Measures the conductivity of soil, rock, and pore fluid.  Provides similar information to resistivity with the advantage of logging capability through PVC casing.

Spontaneous Potential  (SP)  
Records voltages caused by differences in physical and chemical properties of various rocks and differing fluids.  The differences permit identification of bed thickness, lithology, and changes in formation water quality.

Fluid Temperature & Resistivity
Measures changes in fluid temperature, and/or resistivity related to fluid flow in the borehole.  Fluid resistivity provides a measurement of water quality.

Borehole VideoEmployed to visually inspect boreholes, well casings and screens.  Depth encoded images from the borehole camera are recorded for later review, and provide a permanent record of open borehole conditions, fractures, well screen placement, cracks in casing, etc.

Field Procedures

Downhole probes are lowered on an armored cable into the borehole.  Measurements for most probes are recorded on the way up the borehole.  Data are digitally recorded allowing for computer processing at a later time.  A number of combination probes (i.e., fluid temperature / resistivity, spontaneous potential / single point-resistance / natural gamma) are available, performing several measurements in a single pass through the borehole.
 

A single operator can easily manage the MGX borehole logging system. Equipment can be driven or hand-carried to remote sites.

Information provided by: http://www.naevageophysics.com