P-WAVE LOGGER

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

the liner.

3. Split or damaged liners can affect the contact between the

transducers and the liner or between the liner and the

sediment.

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

signal.

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)>

<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"