Compressional or P-wave velocity (primary wave) measurements are a measure of the velocity of sound waves through Earth materials with distance vs. time. P-wave velocity varies with the lithology, porosity, and bulk density of the material; state of stress, such as lithostatic pressure; and fabric or degree of fracturing. In marine sediments and rocks, velocity values are also controlled by the degree of consolidation and lithification, fracturing, and occurrence and abundance of free gas and gas hydrate. Together with density measurements, sonic velocity can be used to calculate acoustic impedance or reflection coefficients, which can be used to estimate the depth of reflectors observed in seismic profiles.
P-wave velocity data were collected on marine cores during DSDP; however, these measurements were taken at discrete locations. During Leg 108, a prototype PWL system, developed by the Institute of Oceanographic Sciences in the United Kingdom, was deployed (Schulthiess et al., 1988). For the first time, higher density sampling of velocity allowed scientists to create fine-scale velocity profiles that could be used to correlate similar horizons in adjacent holes, reveal the nature of sedimentary features, and provide high-quality data for seismic interpretation.
The PWL system measures the speed of compressional waves in sediments by timing the pulses traveling across the diameter of a totally full core liner. The basic parts of the PWL system include (1) a pair of spring-loaded transducers and a transmitter and receiver mounted on opposite sides of the core and perpendicular to the core axis; (2) transducers to measure the displacement between the transmitter and receiver; (3) a track system that moves core past the sensors or moves the sensors along the core; and (4) computer control of data acquisition and data capture.
The prototype PWL system was mounted on the GRAPE track during Leg 108. The basic system consisted of two 500-kHz transducers, displacement transducers with 0.04-mm resolution, and an A/D converter which digitized the output of the peak detector. The computer data acquisition and capture programs were modified throughout ODP as newer technology and better data acquisition programs became available. There have also been some major upgrades to the PWL system during ODP. During Leg 124E the Geotek MST, an automated core conveying and positioning system, was installed. During Leg 187, a new A/D converter was installed that significantly changed the output recorded by the data acquisition program. A brief summary of changes to the PWL system is shown in Table T23. A comprehensive report on the first PWL system installed during Leg 108 can be found in Schultheiss and McPhail (1989).
The basic velocity calculation is
For laboratory measurements, the liner and characteristics of the electronics can be sources of error in the measured velocity of the cored material. A constant liner thickness of dliner = 2.54 mm (2dliner = 5.08 mm) was subtracted from the measured diameter, though the liner thickness could vary between 2.35 and 2.82 mm. There are three types of time delays that are subtracted to correct the traveltime:
For routine measurements on whole cores in liners, the calculation for the velocity is
where
vcore = corrected velocity through core (km/s),
d'core = measured diameter of core and liner (mm),
dliner = liner wall thickness (mm), and
t0 = measured total travel time (µs).
The cores were stored on a rack to allow them to equilibrate to room temperature before analyzing them with the PWL. P-wave velocities are sensitive to the temperature of the core material. The highest quality velocity measurements were made on core liners that were completely full, APC cores, or longer continuous hard rock cores. In order to maintain close coupling of the transducers to the core liner, the outside of the liner was sprayed with water.
After the core was placed in the track, spring-loaded transducers measured the diameter of the core. A 500-kHz pulse was produced at a repetition rate of 1 kHz. The pulse was sent to the transmitter transducer which generated an ultrasonic compressional pulse. The P-wave propagated through the core and was received by the receiver transducer. The amplified signal was analyzed by an automatic peak detection algorithm and generated a traveltime.
Calibration of the P-wave system was usually performed at the beginning of leg, but would also be done after changing equipment, when transducers were showing signs of wear, or if problems were suspected. Pulse detection settings did not usually require any adjustment unless equipment was replaced or different measurement geometry was required. Pulse time was a time constant included in the total time measurement as a result of the peak detection procedure. This constant was subtracted from raw time measurements because it allowed more precise monitoring of system performance and gave measured time values that were independent of the peak detection procedure.
Transducer displacement and travel time delay calibrations were done simultaneously. This procedure was performed at least once per leg. Displacement was measured in volts. For the displacement calibration, three to four acrylic cylinder standards were measured, and a linear least-squares regression was run to determine the coefficients that relate the voltage readings to distance. A section of liner filled with distilled water (known velocity) was measured to verify that the calculated coefficients with the traveltime delays return the correct velocity. After the hardware and software upgrade during Leg 187, the raw calibration data for displacement and time delay were stored in separate tables, in recognition that these are two different calibrations.
Most of the original PWL data files were archived on the ODP servers. There was no interim database for PWL data. In a few instances, the files for a hole were concatenated into a single file. Some of these original files are no longer available, either because the scientists who concatenated the hole file deleted them, or they were not moved onto the ODP servers.
The data model for P-wave velocity can be found in "Janus P-Wave Velocity Data Model" in "Appendix J." Included are the relational diagram and the list of the tables that contain data pertinent to PWL, column names, and the definition of each column attribute. ODP Information Services Database Group was responsible for the migration of pre-Leg 171 data to Janus. The migration of PWL velocities was done in conjunction with the other MST data sets (GRA, MSL, and NGR). Each change in format was documented and added to the MST migration program. Additional information about the migration of PWL data or original file formats can be requested from the IODP/TAMU Data Librarian.
The PWL data can be retrieved from Janus Web using a predefined query. The P-wave velocity (PWL whole-core system) query Web page allows the user to extract data using the following variables to restrict the amount of data retrieved: leg, site, hole, core, section, specific run numbers, range in velocity values, or latitude and longitude range. In addition, the user can use the output raw data option in the query to extract the raw measurements and calibration parameters used to calculate the velocity values. Because there are ~2.6 million PWL data records in Janus, a user must restrict the amount of data requested.
Table T24 lists the data fields retrieved from the Janus database for the predefined PWL query with output raw data option turned on. The first column contains the data item, the second column indicates the Janus table or tables in which the data were stored, and the third column is the Janus column name or the calculation used to produce the value. "Description of Data Items from PWL Query" in "Appendix J" contains additional information about the fields retrieved using the Janus Web PWL query and the data format for the archived ASCII files.
Several things can affect the quality of PWL data. Type of material and drilling method used to recover the core are major factors. In addition to the requirement for good acoustic coupling between the core liner and the transducers, good coupling between the core and core liner is critical for quality measurements. Soft sediment found in the uppermost 50 m of a hole often yields good data. Below 50 m, the signal is often strongly attenuated. Less cohesion of the sediments and microcracks or gas voids make good measurements impossible. The sensitivity of PWL measurements to the quality of core material means that less of the recovered core was analyzed. Table T25 summarizes how much of the different types of core were analyzed on the PWL systems.
One other source of error to consider is operator error. Throughout ODP, the operator manually entered core information into the data acquisition program. Typographical errors will occasionally happen, and some mistakes will not be identified. Often, the Scientific Party found errors and corrected them for the data included in the Initial Reports volume, but the original files were not corrected. A significant amount of effort expended during verification of the PWL data has gone into finding sections that may have been misidentified. Some runs have been renamed to different sections, but the evidence for misidentification had to be conclusive before the runs were changed. Listed below are some of the clues used to find incorrectly identified analyses: