NATURAL GAMMA

INTRODUCTION

The sensor to record Natural Gamma radiations from the cores was added to the MST on Leg 150 (Hoppie, Blum et. al., 1994). Currently gamma radiations are recorded as counts in five non-overlapping energy ranges. The total counts are determined by summing the counts in the five individual energy ranges. The five energy ranges are 200 - 500, 500 - 1100, 1100 - 1590, 1590 - 2000, and 2000 - 3000 KeV. The energy ranges are the same as those in the Schlumberger Natural Gamma Log according to the Schlumberger logging manual. It is necessary to observe background counts in each energy range at least once at the start of the leg, and that these background counts are expected to be recorded in each data file together with the time for which the background counts were observed. Approximately 10 counts/sec are expected as the background noise. The background counts are expected to be higher for the energy range 1 and are likely to be reduced asymptotically for the higher energy ranges. The observed background counts should be subtracted from the natural gamma counts of the cores.

The instrument was calibrated on Leg 150 using the natural gamma peaks of Europium and Potassium sources, and the channel numbers were assigned so that the 2000 available channels would cover energy ranges from 200 to 3000 KeV. Channel number 95 was used for the Europium (Eu-152) peak at 121.78 KeV and channel number 980 was used for the Potassium (K-40) peak at 1460.75 KeV. This resulted into the following linear relationship between channel number and energy:

Energy (KeV) = 1.51296 * channel number - 21.95

EXPLANATION OF DATA ITEMS

LEG - Number identifying the cruise.

SITE - Number identifying the site. A site is the position of a beacon around which holes are drilled.

HOLE - Letter identifying the hole.

CORE - Number identifying the core. Cores are numbered serially from the top of the hole downward.

CORETYPE - A letter code identifying the drill bit/coring method used to retrieve the core. The coretype codes are:

B - Drill Bit
C - Center Bit
D - Positive Displacement
H - Hydraulic Piston Core
M - Miscellaneous
N - Downhole Mud Motor (Navidrill)
P - Pressure Core Barrel
R - Rotary
S - Side Wall Core
W - Wash Core
X - Extended Core Barrel

SECTION - A core is cut into 1.5 meter long sections that are numbered serially from the top of the core. A core with full recovery will have 7 sections and a core catcher and be approximately 9.8 meters long.

TOP_INTERVAL - Distance in centimeters from the top of the section to the top of the sample interval.

SUB_BOTTOM_DEPTH - Distance in meters from the seafloor to the top of the sample interval.

TOTAL_COUNTS - total number of natural gamma counts obtained by summing the counts for the five individual energy ranges described below.

SAMPLE TIME - time in seconds for which the core was exposed to the natural gamma sensor.

SEGMENT 1 COUNTS - number of counts in the energy range 1 - for the energy range see Header Record 3.

SEGMENT 2 COUNTS - number of counts in the energy range 2 - for the energy range see Header Record 3.

SEGMENT 3 COUNTS - number of counts in the energy range 3 - for the energy range see Header Record 3.

SEGMENT 4 COUNTS - number of counts in the energy range 4 - for the energy range see Header Record 3.

SEGMENT 5 COUNTS - number of counts in the energy range 5 - for the energy range see Header Record 3.

RUN NUMBER - the run number added by the MST program.