The P-wave logger data file contains information
generated onboard the JOIDES Resolution by the shipboard
physical properties scientists and technicians. The P-wave
logger is an automated system used to measure and record the
compressional wave velocity (VP) of soft sediments within a
plastic core liner. A detailed velocity log can be obtained
by combining the data from a sequence of adjacent core
sections. These logs can be used to provide detailed cross-
correlation between different holes at the same site, or can
be used to identify sedimentary structures rapidly (such as
slump structures or turbidites). The detailed velocity
profiles provided by the logger, when used in conjunction
with the density profiles provided by the Gamma Ray
Attenuation Porosity Evaluator (GRAPE), can be used to
generate synthetic seismograms that can be correlated with
seismic profiles. The P-wave logger can be used on
conventional cores obtained by piston- or gravity-driven
drilling systems that use any type of cylindrical plastic
liners, although it was developed specifically for use with
cores obtained with the hydraulic piston corer.
Velocity measurements are automatically taken, with a
precision of +/- 1.5 m/sec, at regular intervals throughout
the core as the core travels between a pair of ultrasonic
transducers, one serving as a transmitter and the other
serving as a receiver. The P-wave logger measures the speed
of the compressional waves by timing pulses between the two
transducers, which are spring loaded and mounted
diametrically across the core. A good ultrasonic coupling is
maintained between the transducers and the core liner by
spraying the core liner with water.
A good "acoustic" coupling is necessary between the
transducers and the liner and between the liner and the
sediment in order to allow proper transmission of the
compressional wave between the transducers. Bad couplings,
resulting in data loss, can occur in several ways:
1. The core liner is dry. The liner must be kept wet in
order to assure a good coupling with the transducers.
2. The transducers act like spring loaded pistons and can
occasionally become stuck, resulting in loss of contact with
3. Split or damaged liners can affect the contact between the
transducers and the liner or between the liner and the
4. Since the cutting shoe is ~4mm smaller than the inside
diameter of the liner, it is necessary for the sediment to
swell to form a good contact with the liner. If this does
not occur, some water can leak out when the core is cut into
1.5 meter sections, and a thin air gap can be left between
the sediment and the liner. This can strongly attenuate the
5. Useful logs are impossible to attain with sediments that
contain even small amounts of free gas when the core reaches
the deck. The signal can be completely attenuated by small
amounts of gas inside the liner.
The inherent disadvantage of the P-wave logger is that
it will return good quality data on only high quality cores
that completely fill the liner. This effectively means only
HPC cores taken from unlithified, non-gassy sediments are
suitable for use with the logger, although some reliable data
can be returned from XCB cores with minimal disturbance. The
P-wave logger can return detailed velocity profiles through
the length of good quality cores at sampling intervals as
small as 1mm (although intervals of around 2cm are more
practical, due to the ~2.5cm diameter of the transducers).
The sampling interval itself can be chosen by the operator.
The data is fed from the transducers directly into a computer
for processing, real-time display, and storage. The only
procedure necessary for obtaining accurate velocity
calculations is proper calibration of the instrument, which
is described fully in the instrument manual.
DATA FILE FORMAT
Header Records (4):
Record 1 - <Run Number> <Run Type:> <Sensor Name> <Date>
<Time> <Name of Datafile> <Ten Character Comment> <Sampling
Interval (cm)> <Scan Velocity (cm/s)> <Sample Temperature
(degree centigrade)> <Sample in Liner>
Explanation of these items is the same as described in the
GRAPE data Header Record 1.
Record 2 - <Sensor Header Data>
Record 3 - "Header:" <Pulse Time Correction (usec)>
<Transducer Frequecy (KHz)> <Liner Thickness (x2) (mm)>
<Liner + Electronic Delay (usec)> <Displacement Zero
Calibration> <Displacement Scale Calibration (mm/step)>
<Displacement Calibration (test block, mm)> <Core Temperature
(deg. celsius)> <Data Mode (0=normal, 1=simulated)>
Record 4 - "START OF DATA"
Data Records (N)
Each data record has the following data columns:
(1 through 9) STANDARD ATTRIBUTES - the 9th column
corresponding to sub-bottom depth is always empty.
(10) P-WAVE VELOCITY (cm/s)
(11) SIGNAL STRENGTH - It indicates the quality of
observation - if this value is greater than 40, the observed
velocity is considered reliable.
(12) TRAVEL TIME
(13) DISPLACEMENT (steps) - This value ranges between 0 and
255, and can be used to calculate the thickness of the core
at each sample location using the constants given in Header
Record 3. This value is expected to be close to the
'Displacement Zero Calibration' value. It defaults to 255 if
the obervation is not reliable.
(14) RUN NUMBER - the run number added by the MST program.
Trailer Records (2)
Record 1 - "END OF DATA"
Record 2 - "trailer"