MOISTURE AND DENSITY

Moisture content and mineral density are basic physical properties that are determined most accurately through mass and volume determinations. The results of MAD measurements provide a direct estimate of porosity, void ratio, and the average density of the samples. Porosity variations are controlled by consolidation and lithification, composition, alteration, and deformation of the rocks or sediments. The physical properties can be used with other types of data to study processes in the ocean crust (e.g., fluid migration studies or analysis of seismic survey data).

The MAD data set, sometimes called "index properties," is one of the most complete sets of data collected by ODP. More than 92,000 samples were taken from cores recovered during 105 of the 110 ODP legs. MAD properties have been determined from samples starting on the first leg of DSDP, as described by Boyce (1973). Although the method for calculating MAD properties has changed since the beginning of DSDP, these data still represent a significant data set. During ODP, MAD properties were calculated using wet and dry mass measurements taken with electronic balances and wet and dry volume measurements taken with helium displacement pycnometers. Properties that were calculated from these data are wet bulk density, dry bulk density, grain density, bulk and dry water content, percent porosity, and void ratio.

Data Acquisition

Mass and volume measurements were needed in order to calculate MAD properties. Four methods of calculating the physical properties were used during ODP. Methods A, B, and C assumed the sample was saturated and all pore spaces filled with water. Method D was developed to analyze unsaturated samples. Over the course of the program, Method C was determined to more accurately estimate the MAD properties for saturated samples. A comprehensive discussion of the methods and calculations can be found in Chapter 2 of Technical Note 26 (Blum, 1997).

Mass measurements were made using two high-precision electronic balances. The ship environment, with constant motion and cyclically changing gravity, made mass determinations more difficult. The two-balance system allowed a reference mass to be used at the same time as the unknown sample mass in order to obtain as accurate a measurement as possible.

Volume measurements were made using a helium pycnometer. A pycnometer works on the principle that a sample displaces an amount of fluid equal to its volume. The pycnometer used had five measurement cells, and usually a sphere standard was run with four unknowns to provide a control measurement. The standard was moved through the cells to check the drift. When the drift was >0.02 cm3, the cell was recalibrated.

Different methods of removing water from the samples were used. Freeze-drying and heating in a microwave had been used, but the most common method was drying samples in a convection oven at temperatures between 100 and 110C for 24 hr.

Changes in MAD data acquisition procedures were due to improvements in the data acquisition software. The first programs were written in BASIC and required much operator entry. The data acquisition program used at the end of ODP required little manual entry from the operator and automated the procedures for data collection.

Standard Operating Procedures

Core samples of approximately 8 cm3 were collected from the working half after the sections had been split. Two samples per section were usually taken for physical properties determination, but sampling density was highly variable depending on core recovery or the scientific requirements of the Shipboard Scientific Party.

The samples were put into calibrated beakers for analyses. The beakers had been previously measured to determine mass and volume. Sample mass and volume measurements were dependent on the method that would be used to calculate the properties. Wet samples were weighed; for much of the ODP, volume measurements were also taken on the wet samples. The samples were then dried for 24 hr at temperatures between 100 and 110C in a convection oven. Drying was intended to remove interstitial water and was the most critical part of the procedure. Using this method to dry samples may have also removed a substantial portion of the interlayer water from clays, so samples with high clay content could have errors of as high as 20% in calculated porosity. After drying, the samples were weighed again and the volume determined using the helium pycnometer.

Calibration

The balances and pycnometer required periodic calibration, usually at the beginning of each leg and during the leg if there was a problem. Calibration information was not archived.

Beaker calibrations were also done, though not for every leg. Beaker volume was difficult to measure because the low volume to void ratio in the pycnometer cell gave inaccurate values. Instead, a beaker's mass was determined and the volume calculated based on the density of the material. Aluminum beakers (density = 2.78 g/cm3) were used from Leg 101 until they were replaced with glass beakers (density = 2.2 g/cm3) during Leg 169.

Calculations

Table T13 is a summary of the measured and calculated parameters used to calculate MAD properties with the four methods during ODP. Table T14 are other properties and formulas used for MAD density determination.

Archive

Pre-Janus Archive

Early in ODP, MAD data were collected on log sheets that were sent back to ODP/TAMU at the end of each cruise. The data were entered into the S1032 database and the log sheets were microfilmed for archival storage. Data entry routines were implemented so that data entry could be done on the ship and the practice of collecting data on log sheets ended. MAD data were stored in the S1032 database through Leg 149. For Legs 150–166, MAD data were stored in a Macintosh 4D database. After Leg 166, data were stored in text files or Excel spreadsheets. All files were archived on ODP/TAMU servers.

Migration of MAD Data to Janus

The data model for MAD data can be found in "Janus MAD Data Model" in "Appendix F." The relational diagram and list of the tables that contain data pertinent to MAD analyses, column names, and definition of each column attribute are included. ODP Information Services Database Group was responsible for the migration of pre-Leg 171 data to Janus.

One of the difficulties with the migration of MAD data to Janus was the migration of the beakers and their mass and volume data. Most of the data entries recorded the beaker number that was used for each analysis; however, very often the mass and volume of the beaker were not saved in that entry. Beakers were not calibrated on each leg, so it was not always clear which beaker mass and volume file was used during the leg when no beaker file was archived at ODP/TAMU with the leg's MAD data.

Janus MAD Data Format

MAD analyses can be retrieved from Janus Web using a predefined query. The MAD 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, depth, or latitude and longitude ranges. In addition, the MAD query gives the user options to retrieve the raw data, retrieve data for a single method, and filter records based on a range of calculated values of one of the properties (e.g., bulk density, porosity, etc.).

Most of the calculated values are not stored in Janus. They are calculated from the raw data when the Web query is run. When Janus first started operations, the method used to calculate MAD properties was determined by which measurements were in the database. The method itself was not explicitly stored. After migration of MAD data started, changes to the data model became necessary because some legs were missing raw data, and some beaker mass and volume values were missing. The data model was changed to store calculated values, and a column that explicitly defined the method was added. Samples with missing raw data or beaker information were added to Janus using the calculated values reported in the Initial Reports volumes or calculations from the log sheets.

Table T15 contains the data fields retrieved from the Janus database using the Janus Web predefined query with the output raw data option. 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 calculations used to produce the value. Calculations for some of the parameters differ depending on the method used for analysis (see Table T13). "Description of Data Items from MAD Query with Output Raw Data Option" in "Appendix F" contains additional information about the fields retrieved using the Janus Web MAD query and the data format for the archived ASCII files.

Data Quality

The MAD data set is one of the most complete sets of data collected by ODP. There are few known instances where there was a major problem with data collection. The MAD properties values are calculated from the raw data stored in the MAD tables. For this reason it was very important to link the correct beaker information with each analysis record. Verification of the MAD data included (1) determining the correct beaker mass and volume set that should be associated with each leg, (2) verifying that all samples had been entered, (3) verifying that each analysis record was associated with the correct beaker number and date, and (4) verifying that each record was associated with the correct method used to calculate MAD properties.

Much of the data collected before Leg 171 used Method B to calculate bulk density, dry density, porosity, and void ratio and Method C to calculate grain density. Bulk water content and dry water content calculations are the same for both Methods B and C. In many cases, the raw data exist to use both Methods B and C to calculate MAD properties. As part of the verification, an additional record was entered so that MAD properties would be calculated by both methods. Even though the original reports may have published the Method B calculations, the Method C calculations may be better estimates.

There were two common sources of errors found during the migration and verification of the MAD data: missing samples and a generic operator error. Missing samples were a common problem, as there were no constraints that required a sample record to exist in order to analyze and save data. In those instances, a sample was entered into the database so that the data could be migrated. Anything that was written or typed was subject to operator error. Analytical results were written on log sheets. These data were then typed into the S1032 database. Data entry programs were implemented to add the data to the S1032 database, but it still required manual entry. Data acquisition programs were later implemented to collect the raw MAD measurements data, but the operator manually entered the sample information. Writing or typing incorrect information occasionally happened and some mistakes were not identified. Often, the Scientific Party found errors and corrected them for the data included in the Initial Reports volume, but data sent back to ODP/TAMU were not corrected.

Verification of added samples and the entire MAD properties data set were 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.

Janus does not contain any calibration information for the MAD instrumentation. Procedures for storing calibration information were not implemented during ODP.

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