Borehole Methods

Borehole Video

SAIC applies borehole geophysics to measure the physical properties of the earth within or near a well. This figure shows a solution cavity located at a depth of 220 feet below the ground surface on the color borehole video. View Video Image.

Borehole Wireline Measurements

Additional properties typically measured by SAIC using wireline geophysical techniques are water temperature, borehole diameter (caliper), natural gamma radiation, resistance, resistivity, and spontaneous potential. SAIC also performs borehole video surveys to evaluate the subsurface by lowering a video camera down a borehole to videotape the walls of a well. The resulting geophysical logs provide valuable information on stratigraphy, water-bearing zones, and well construction.

Temperature
Usually the first instrument used in water wells, it is primarily used to identify temperature changes due to water-bearing zones or non-static water conditions. Temperature changes are normally related to water bearing zones in a borehole. View Example.

Caliper
The caliper log is used to measure the overall diameter of the borehole to identify washed-out zones or fractures. This tool can be used to develop an idea of borehole rugosity and may be useful in identifying the quality of the casing/rock interface. View Example.

Natural Gamma Radiation
This stratigraphic identification tool is primarily used to differentiate between sands and clays in an unconsolidated environment or sandstone and shale in a consolidate environment. Natural gamma instruments can also be used for radioactive material investigations. View Example.

Resistance
This measures the electrical properties of the borehole and rock pore fluids present in and around the borehole. This tool is useful in identifying water-bearing zones, stratigraphic changes, and fractures. This measurement is performed using a referencing electrode at the ground surface resulting in a bulk resistance measurement between the surface electrode and the in-well measuring tool.

Resistivity
This measures the electrical properties around the borehole. This tool is useful in identifying stratigraphic changes. This tool is differs from the single point resistance tool as it uses multiple electrodes spaced typically at 8, 16, 32 or 64-inches apart. Longer electrode spacing "looks" further into the formation into areas that may not have been influenced by drilling activities.

Fluid Resistivity
Similar to the resistivity tools, this tool is not deep sensing, and usually related to the borehole fluids. Frequently fluid conductivities are measured using electromagnetic methods and converted to fluid resistivity. Fluid resistivity is frequently used to evaluate groundwater quality.

Spontaneous Potential
When used in conjunction with the resistance log, this tool is used to determine stratigraphy. This tool is normally used to infer rock grain size, permeability, and flowing (moving) groundwater.

Deviation
Measuring the deviation of a borehole is performed using a sonde with three axis magnetometers and accelerometers. Knowing borehole deviation is a requirement to correct for interpreted bedrock strike and dip in televiewer logs. Abrupt borehole deviations can pose problems for long pump settings in water supply wells. View Example.

Heat Pulse Flow Meter
Using a small heater surrounded by high sensitivity temperature sensors, this tool is used to identify very low flow (less than 0.5 gallons per minute) through boreholes. Flow direction can be lateral into or out of a borehole, or vertical within the borehole. Unlike most other tools, measurements are performed while this tool is stationary within the borehole at a discrete depth. View Example.

Impeller Flow Meter
Using a propeller this tool measures the amount of water passing the tool as it is moved along the borehole. This tool is used to identify moderate borehole flow, typically in excess of 3 gallons per minute.

Spectral Gamma
This tool measures the radioactive spectrum in the borehole enabling the evaluation of specific radioactive isotopes. View Example.

Velocity
This measures ultrasonic velocities of the material around the borehole. Shear and compressional wave velocities are useful for the development of engineering properties of the bedrock. Stonley wave amplitudes can be used to indicate permeability and sonic logs can be used to assess non-vertical fractures. Cement bonds (both to casing and to formation) can be assessed using velocities.

Acoustic Televiewer
This ultrasonic tool requires a fluid-filled borehole to effectively evaluate borehole fractures and rugosity. The output of this tool is frequently presented as a virtual core. Interpretation typically includes the location and orientation of borehole fractures. View Example.

Optical Televiewer
Uses a rotating mirror to image air or water filled boreholes. Typical resolution is 2 millimeters with fracture resolution down to 0.1 millimeter, and radial resolution to 1° depending upon borehole clarity. The output of this tool is frequently presented as a virtual core. Interpretation typically includes the location and orientation of borehole fractures. View Example.