Borehole Geophysics
Borehole geophysical investigations typically utilize a suite of logs to determine structural, geologic, hydrogeologic, and well construction characteristics within a borehole; however, the suite of logs is not always necessary to meet the objectives our clients may have. AGS has the knowledge and background to design a well-planned, and cost effective borehole geophysical survey to meet the many objectives our clients request. AGS also has the both the experience and ability to log under both static and pumping conditions.
The following instruments determine respective parameters:
Multi Tool
The Multi Tool is a multi-parameter resistivity tool that is typically used in groundwater monitoring wells to identify changes in lithology and can be an indicator of water producing or receiving zones. This instrument measures the following parameters simultaneously:
- Fluid Resistivity
- Natural Gamma
- 16 inch Normal Resistivity
- 64 inch Normal Resistivity
- Lateral Resistivity
- Spontaneous Potential
- Single Point Resistance
- Temperature
- Differential Temperature
Three-Arm Caliper
The Three-Arm Caliper is a mechanical instrument that is designed to measure changes in borehole diameter. The changes in borehole diameter, as detected by the three-arm caliper, are typically related to fractures (both vertical and horizontal) and well construction (i.e. casing, drill-bit size, or drill bit gouges).
Heat Pulse Flow Meter
The heat pulse flow meter (HPFM) is an instrument that measures the vertical direction and flow rate of the fluids that are present within a borehole. This instrument is lowered to a desired depth (typically above and below a known fracture), and a heat grid is released from the instrument into the water. The travel time of the heat grid to the sensor above or below the heat source is calculated and converted to a flow rate in gallons per minute.
Acoustic Televiewer
The acoustic televiewer (ACTV) generates an image of the borehole wall by transmitting acoustic pulses from a rotating sensor and records the subsequent amplitudes and travel times reflected at the interface (borehole wall). The final product is an unwrapped and continuous acoustic image of the borehole.
Optical Televiewer
The optical televiewer (OPTV) log generates a continuous (360 degree) image of the borehole using a downhole CCD camera. This instrument requires a low sediment environment.
Sonic Tool
The full wave form sonic tool (FWST) emits a series of high frequency pulses into the formation. The pulses are detected by receivers at varying distances from the transmitter. Each receiver samples the arriving waveform that is predefined by the tool configuration.
Borehole Video Log
The borehole video log provides a real-time recording of the borehole environment, and are typically used for well inspection.
Advantages
Geophysical well logs provide a continuous profile of the in-situ response versus depth in the well. Typically, direct soil/rock sampling is undertaken at intermittent intervals. A substantial amount of information can be obtained from a few logging runs in a well. In addition, the data can be correlated easily between adjacent wells.
Limitations
The data collected in a borehole represents a single point in the plan view of a site. Also, the radius of investigation may be small, and not completely representative of the bulk formation.