P-wave or sonic velocity measurements are a measure of the velocity of seismic waves through Earth materials with distance versus 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 is used to calculate acoustic impedance or reflection coefficients, which can be used to estimate the depth of reflectors observed in seismic profiles and to construct synthetic seismic profiles.
P-wave velocity data was collected during DSDP. The Hamilton frame system was first used during DSDP Leg 15 and was part of the equipment that was transferred to ODP. Discrete velocity measurements were made on split sections or samples. After the PWL was installed to collect higher density velocity data, the discrete measurements were useful for studying the anisotropy of the cored material and to fill in when the PWL was no longer able to make good measurements.
ODP modified and updated the electronics for the Hamilton frame system, but the general data collection procedures were the same as described by Boyce (1973). Discrete samples had to be prepared carefully in order to ensure good contact with the transducers. Sometimes split sections would be measured in liners when the sediments were too weak to be handled without being destroyed. Measurements could be made in three directions: (1) A or z, parallel to the core axis; (2) B or y, perpendicular to core axis and parallel to split surface; and (3) C or x, perpendicular to core axis and perpendicular to split surface. Direction designation differed between legs.
During ODP Leg 130, the DSV system, developed by Dalhousie University and Bedford Institute of Oceanography, was brought aboard the JOIDES Resolution to demonstrate the system and to collect velocity data on unconsolidated sediments. This system was computer-controlled and collected not only the raw data but the full waveform that could be analyzed later. ODP installed a DSV system for Leg 138. Two transducer pairs were designed to be inserted into soft and semiconsolidated sediments and were mounted orthogonal to each other to measure along the core axis (z), perpendicular to the axis, and within the split plane (y).
Both the Hamilton frame and the DSV systems were replaced by new systems during Leg 169. Hardware and software had improved significantly and the new systems were designed to take advantage of those advancements. The PWS1 and PWS2 insertion probe system on a split-core track replaced the DSV. The Hamilton frame was replaced by the PWS3 contact probe system, which maintained the capability of measuring discrete samples or split core. A major upgrade to the PWS3 system occurred during Leg 191 when new data acquisition hardware was installed. This required modifications of both the data acquisition software and the Janus PWS3 tables. The changes to the PWS3 system were modeled after the PWL system. See Table T26 for a summary of discrete P-wave velocity systems used during ODP.
See "Standard Operating Procedures" in "P-Wave Velocity" for velocity calculations.
Explanatory Notes in the Initial Reports volumes refer to Boyce (1973), in which the operating procedures for the Hamilton frame system were described. It was important to prepare the sample correctly in order to get good contact between the transducers and the core material or core liner if measuring a split core. The time delay was determined by measuring the time with the transducers in contact with each other, at zero distance. Initially, the measurements were likely logged by hand and later entered into the S1032 database. The handwritten log sheets were returned to ODP/TAMU for archival. A computer data acquisition program was implemented on Leg 138, but there is little documentation about this or subsequent programs. Calibrations of the Hamilton frame system were not documented.
The DSV had two sets of piezoelectric transducers that were inserted into unconsolidated and semisoft material. One set was separated by ~7 cm along the core axis (z). The other set was separated by ~3.5 cm perpendicular to the core axis and parallel to the split surface. All functions of this system were controlled by a dedicated computer, including creating files with velocity measurements. Thermistors monitored the temperature of the core material during measurement. Periodically, the separation was checked by running a calibration procedure in distilled water. Time delays were estimated using a series of aluminum and lucite standards.
The principle behind PWS1 and PWS2 was the same as the DSV. In addition to the improvement in hardware and computer control of all data acquisition, calibration procedures were implemented, and all measurement and calibration data were uploaded to the Janus database. The distance between the transducers was measured with calipers at least once per leg, more often when being heavily used. The distance values were considered constant. The tdelay calibration was done by inserting the probes into a container filled with distilled water of known temperature and therefore of known velocity and calculating the time delay as the difference between the measured transit time and the known transit time in water. Control measurements as described in the data model were not implemented during ODP.
The PWS3 system was an upgraded Hamilton frame system. Improvements in hardware and computer control allowed the measurement and calibration procedures to be simplified. Rapid, precise measurement of sample thickness and pressure control on the transducers helped to ensure that the transducers contacted the split core or sample properly. Calibration procedures for the PWS3 system were equivalent to procedures for the PWL. Standards of different thickness were measured to obtain total transit times. Least-squares regression was run to determine the time delay. All data were stored in the Janus database. Control measurements as described in the data model were not implemented during ODP.
From the beginning of ODP, velocity data collected on the Hamilton frame were logged by hand on log sheets. Those data were later entered into the S1032 database. The completeness of the early archive is dependent upon what was written on the log sheets or transcribed from scientists' notes. A new data acquisition code was implemented for the Hamilton frame at the same time the new DSV system was installed. Both systems created data files that were archived on the ODP/TAMU servers.
The data models for the discrete velocity data can be found in "Janus PWS Data Model" in "Appendix K." Included are the relational diagram and the list of the tables that contain data pertinent to PWS, 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 data collected on the Hamilton frame system were migrated to the PWS3 tables. Data collected on the DSV system were migrated to PWS1 or PWS2, with all data collected in the z (or A) direction (parallel to core axis) migrated to the PWS1 tables, and data collected in the y (or B) direction (perpendicular to core axis, parallel to split surface) migrated to the PWS2 tables.
More detailed information about ODP P-wave velocity measurements can be found in Chapter 6 of Technical Note 26 (Blum, 1997).
PWS data can be retrieved from Janus Web using a predefined query. The Prove Velocity (PWS split-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, velocity type, depth range, or latitude and longitude ranges. 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.
Table T27 lists the data fields retrieved from the Janus database for the PWS1 and PWS2 predefined query with the output raw data option. The structure of the tables for PWS1 and PWS2 are identical, so the table names are interchangeable. Table T28 lists the data fields retrieved from the PWS3 tables. 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 PWS1 and PWS2 Queries" and "Description of Data Items from PSW3 Query," both in "Appendix K," contain additional information about the fields retrieved using the Janus Web PWS query and the data format for the archived ASCII files.
Several things can affect the quality of PWS discrete velocity data. Type of material and the drilling method used to recover the core are major factors. There must be good acoustic coupling between the core material and the transducers. When taking a measurement through the core liner, there must be good coupling not only between the transducer and the core liner, but also between the core material and the core liner. Even in soft sediment, less cohesion of the sediments, microcracks or gas voids can make good measurements impossible. PWS measurements were often used to augment the velocity data from the PWL system when the quality of core material prevented reliable measurements. Table T29 summarizes discrete velocity measurements taken on ODP cores.
One commonly occurring problem was error in the measurement location. This problem became more apparent after computer-controlled data acquisition programs were implemented. If the equipment was not properly zeroed before taking the measurement, the measurement location would not be recorded properly. Notation of the measurement interval on the log sheets could be used to correct the location error, but log sheets were not available for all legs, or locations were not always documented.
One other source of error was operator error. Anything written or typed was a potential source of error. Measurements were taken manually on the Hamilton frame with the results written on log sheets. These data were then typed into the S1032 database. Incomplete data on the log sheets prevent any verification of the older data. Typographical errors when transcribing the data into the S1032 database occasionally happened. Even when data acquisition programs were implemented to collect the velocity data, the operator manually entered the core information. Some mistakes were not identified. Often, the Scientific Party found errors and corrected the data included in the Initial Reports volume, but the data sent back to ODP/TAMU were not corrected.
The verification of the entire PWS data set was not completed because of time constraints. Most data collected after the Janus database was operational during Leg 171 were verified as part of the Janus data management and verification procedures (see "Janus Data Management and Verification"). Some verification was done on the pre-Leg 171 data; however, if there is a discrepancy between the database and data in the Initial Reports volumes, the published data should be considered more reliable.