Gamma radiation is electromagnetic waves with frequencies between 1019 and 1021 Hz. They are emitted spontaneously from an atomic nucleus during radioactive decay. NGR measurements are used for three purposes: (1) correlation of core to core and core-downhole log; (2) evaluation of the clay/shale content of a formation; and (3) abundance estimates for radioisotopes K, Th, and U. Minerals that fix K, Th, and U, such as clay minerals, are the principal source of naturally occurring gamma radiation. Other earth materials that emit gamma radiation include rockiest silt and sandstones, potassium salts, bituminous and alunitic schists, phosphates, certain carbonates, some coals, and acid or acido-basic igneous rocks.
A new NGR system was installed on the MST during Leg 149 and began data collection during Leg 150. This represented a return of NGR data collection to scientific ocean drilling. DSDP used a similar system early in its scientific program but removed the equipment because of the excessive time required to analyze cores. Early versions of the data acquisition program collected spectral data in five energy windows compatible with the Schlumberger natural gamma downhole logging tool. After advancements in sensor efficiency and data acquisition technology allowed the downhole tools to acquire 256-channel spectral data, the MST NGR did also.
In response to requests from the scientific community to address the need for additional capability to correlate cores between holes and integrate core and downhole logging data, the NGR system was added to the MST during Leg 149. There were four gamma ray scintillation detectors mounted at 90° angles from each other and in a plane orthogonal to the core track. Each scintillation counter contained a 3-in x 3-in doped sodium-iodide crystal and a photomultiplier to produce countable pulses. The detectors and sample chamber were mounted inside a lead housing recovered from the original NGR system used during DSDP.
The well-logging industry had used NGR logging tools for many years. Their reporting units were impractical for ODP use because the NGR apparatus could not be calibrated by the same method. For that reason, ODP NGR data are reported in counts per second (cps). This measurement unit was dependent on the device and volume of the material measured. Because one of the reasons for collecting NGR data was to facilitate the comparison of the core NGR data to downhole NGR logging data, early versions of the data acquisition program collected the spectral data in the following five energy windows compatible with the downhole tools:
A major MST upgrade during Leg 169 implemented the change to collecting and reporting the full 256-channel data. This was a major improvement, but in order to use NGR data for spectral analysis to determine elemental abundances of K, Th and U, significantly longer counting times were required. Data acquisition hardware continued to improve and made it possible to perform either longer counts or higher density of measurements, but not both. Also, with the need to keep the core moving through the MST, especially on legs with high core recovery, data acquisition speeds had not yet reached a point where it was practical to sample long enough for spectral analysis. Data analysis indicated there was a problem with the higher channels of the detectors; during Leg 189, the software was changed to report only 248 channels of data. See Table T19 for a summary of NGR systems used during ODP.
By the time NGR was added to the MST, procedures for analyzing sections were well established. After the cores were brought to the Physical Properties Laboratory, they were stored on a rack to allow them to equilibrate to room temperature before analyzing them on the MST (MSL and PWL measurements are sensitive to the temperature of the core). A zero background measurement would normally be taken once a day to check potential contamination within the laboratory. Because of the need for longer count times to achieve more accurate measurements, the sample interval was often set to 20 or 30 cm. As data acquisition hardware and software improved, a higher density sampling could be performed without decreasing the counting period significantly, but ODP did not reach the goal of routinely collecting high quality spectral data that could be used for elemental abundance.
The four scintillation counters must be tuned to return the same signal level for each emission energy. Amplification signals may drift; therefore, the counters were adjusted at the beginning of each leg. After the counters were tuned, an energy calibration was performed. K and Th standards were measured, and a linear regression returned the calibration coefficients that convert channel numbers to energy intervals. A full discussion of the NGR system and calibration procedures can be found in Chapter 5 of Technical Note 26 (Blum, 1997).
Most of the original NGR data files were archived on the ODP/TAMU servers. There was no interim database for these 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/TAMU servers.
The data model for NGR can be found in "Janus NGR Data Model" in "Appendix H." Included are the relational diagram and the list of the tables that contain data pertinent to NGR, 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 NGR data was done in conjunction with the other MST data sets (GRA bulk density estimation, MSL, and PWL). Each change in format was documented and added to the MST migration program. Additional information about the migration of NGR data or original file formats can be requested from the IODP/TAMU Data Librarian.
The structure of the NGR data tables was revised multiple times before the final version used at the end of ODP. Initially, the 256-channel spectral counts were stored in the NGR_SPECTRA_DATA table (described in the Technical Note 26 (Blum, 1997). This table structure rapidly became unusable. Instead, the spectral data were concatenated into a large text field that could be downloaded and the spectral counts extracted. The migration of older NGR data had already started with a table created to store the counts in the energy windows. It was decided not to reformat those data into the same field as the 256-channel data.
NGR data can be retrieved from Janus Web using a predefined query. The NGR 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, depth range, or latitude and longitude range. In addition, the user can use the output raw data option in the query to extract information relating to the core status, run parameters, and calibration data used to calculate the total counts and background-corrected counts. There is also the display spectra option that extracts the spectra collected at each sample location. There are more than one million NGR data records in Janus.
Table T20 lists the data fields retrieved from the Janus database for the predefined NGR query with output raw data and display spectra options 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 calculation used to produce the value. "Description of Data Items from NGR Query" in "Appendix H" contains additional information about the fields retrieved using the Janus Web NGR query and the data format for the archived ASCII files.
There are several factors that affect the quality of NGR measurements, including background radiation, sampling period and spacing, tool response, detector efficiency and energy response, sample volume, and operational characteristics.
Zero background is gamma radiation detected in the measurement area when no core material is present. Background measurements were done by measuring a core liner filled with distilled water. The background spectrum could then be subtracted from each sample spectrum. Studies over several years show that background values were relatively constant at 8–9 cps. A daily control measurement was done to check for potential contamination.
Counting statistics play an important role in the measurement of radioactive phenomena which are random and discrete. A longer time period at a sample location will give a better estimation of the amount of radioactive elements (K, Th, and U). For ODP purposes, this meant that there had to be a balance between longer counting periods and density of sampling. Because of the other sensors on the MST, high-density sampling and long counting periods were not usually possible. The ODP average total count rate was about 30 cps. With a sampling period of 30 s, the statistical error was ~3%, which gave data good enough for core to core correlations.
NGR measurements are dependent upon the sensitivity or efficiency of the system to detect when a gamma ray has been emitted. The sodium iodide crystals emit a single photon of light after being struck by a gamma ray. The photon then strikes a photocathode which releases a burst of electrons. The electrons are accelerated and a final electrode conducts a current through a resistor to produce the voltage pulse. Low detector efficiency or undetected electrical signals result in lower counts.
Radiation counts are directly proportional to the volume of material. APC systems recover softer, undisturbed sediments that routinely give the best results because the core liner is usually full. However, the sediments can also contain a lot of gas which creates voids in the cored material. XCB and RCB systems recover cores that are often biscuits surrounded by drilling mud or irregularly-shaped pieces. Voids, smaller diameter core, irregular pieces, and thin runny mud all result in less volume per measurement interval. Table T21 summarizes how much of each core type was analyzed for NGR.
The core sections were most often run through the MST system before the liners were opened and the core curated. During the curation process, core material was often shifted. In sedimentary cores, voids may have closed. Gassy cores may have small voids that continue to enlarge after analysis. Sections may not be completely full, and material may have spread throughout the liner. After curation, this material was shoved up to close voids and the section's curated length was less than what was originally analyzed. The effects can be seen when looking at the data for a section: (1) there are reasonable values beyond the curated length of the section (null depth values) and (2) there are lower values at an interval compared with adjacent measurements and GRA density values are low, indicating less volume.
Hard rock cores can be continuous cylinders with consistent diameter or can be broken into small irregular pieces. The curation process shifts hard rock pieces, sometimes even shifting core material from its original liner section to an adjacent section liner. Where the core material was in its liner during analysis and where it was eventually placed after curation can be very different. NGR data for these types of cores should be used with caution.
Another important factor to be considered is operator error. Throughout ODP, the operator manually entered core information into the data acquisition program. Typographical errors or typing in the wrong data occasionally happened, and some mistakes were not identified. Sometimes, the Scientific Party noticed the error and corrected it for the data included in the Initial Reports volume, but the original files were not corrected. During verification of the migrated NGR data, much effort was expended to find sections that may have been misidentified. Some runs have been renamed to different sections. The evidence for misidentification had to be conclusive. The following clues were used to find incorrectly identified analyses: