Geophysical Techniques - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 213

Cement Bond Logs
The cement bond log is obtained with a sonic tool to determine how good
the cement seal is on the outside of the casing. The highest amplitude is related
to poor or nonexistent bonds between the casing and the cement. Low
amplitude indicates good bonding, which allows the sound energy to penetrate
away from the casing. A good bond ensures against leaks of water or pollutants
from one rock layer to another.
Borehole Acoustic Televiewer Logs
The borehole acoustic televiewer log is used to develop acoustical analog
images of the rock face around the borehole. This is particularly useful for
determining fracture patterns and directions. The fracture directions are needed
when designing horizontal drilling programs where the bit must cross fractures
to drain oil reservoirs efficiently. It can be used in drilling mud. Also, this tool
can be used in conjunction with hydraulic fracturing to determine in situ stress
orientation of the principal tectonic stresses. The hole is first surveyed with a
televiewer. Then packers are set at the top and bottom of the interval of interest
and the formation is hydraulically fractured by injecting water into the interval
between the packers. Finally, the packers are removed and the hole is inspected
again by televiewer to determine the direction the fracture orientations.
Fracturing occurs perpendicular to the direction of least principal stress.
Borehole Television Camera
The borehole television camera is used to develop a visual image of the
borehole walls. It is used for many of the same things as the acoustic televiewer,
but the water in the hole must be relatively clear for it to work. When it works,
it can provide a more detailed image of fractures and even of lithology than the
acoustic televiewer.
Drill Penetration Logs
The rate of drill penetration into the ground is measured with a drill
penetration log. It is used in conjunction with logs of mud-pump pressure, rate
of spin of the bit, and weight on the bit. In areas where previous