Downhole logging is the process of measuring physical, chemical,
and structural properties of penetrated geological formations using
logging tools that are either lowered into the borehole on a wireline
cable (wireline logging) or placed just behind the drill bit as
part of the drill pipe itself (logging-while-drilling). The tools
employ various acoustic, nuclear, and electrical measurement techniques
to acquire downhole logs of properties such as sonic velocity, density,
and electrical resistivity. The wireline cable provides real-time
communication between the tools and the surface; logging-while-drilling
tools typically record the logs in downhole memory devices, which
are subsequently downloaded when the tool returns to the ship.
The downhole logs are rapidly collected, are continuous with depth,
and measure in situ properties. They can be interpreted in terms
of the formation’s stratigraphy, lithology, and mineralogy.
The sampling interval is typically 15 cm, with a vertical resolution
of about 35 cm. Some tools have a higher sampling interval and resolution;
for example, the FMS can electrically image sub-cm-scale features.
Logging tools are generally designed to measure formation properties
some distance into the formation, in order to minimize the effects
of variable borehole diameter and roughness. Logs also provide the
major link between borehole and seismic section: sonic velocity
logs and checkshots improve depth to travel-time conversion, and
synthetic seismograms may be compared directly to the seismic section.
While downhole logs are complementary to core measurements, they
also offer certain advantages. In a hole where there is only limited
core recovery, the depth location of the incomplete cores can be
uncertain; logs provide a continuous depth record of formation properties.
Where there is preferential recovery of a certain rock type - for
example, basalt pillows can be more easily recovered than breccia
-- the logs can reveal a more realistic stratigraphy. The in-situ
nature of the downhole measurements is in contrast to measurements
on recovered core: when material is no longer under the high-pressure
conditions that exist at depth, it can physically expand and gas
hydrates can dissociate. The core may also be degraded by the coring
process: rotary coring can grind up sediment, resulting in "biscuits"
of coherent sediment in a ground-up matrix.
Third-party tools are designed and developed by investigators
at other institutions involved with IODP and are reviewed by the
relevant IODP committees and panels for deployment on the JOIDES Resolution.
IODP Logging Services provides support for broad aspects of third-party
downhole tool deployment, and to third party investigators in the
areas of data acquisition systems and software, tool design and
manufacturing assistance, and tool testing.
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