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

DATA FILE FORMAT

Header Records (5)

Record 1 - <Run Number> <Run Type> Sensor Name> <Date> <Time> <Name of Datafile> <Comment> <Sampling Interval (cm)> <Scan Velocity (cm/s)> <Sample Temperature (degree centigrade)> <Sample in Liner>

Explanation of these items is the same as described in the GRAPE data Header Record 1.

Record 2 - <Sensor Header Data>

Record 3 - "Header:" <Sample exposure time (s)> <Energy Range 1 (first channel number - number of channels)> <Energy Range 2 (first channel number - number of channels)> <Energy Range 3 (first channel number - number of channels)> <Energy Range 4 (first channel number - number of channels)> <Energy Range 5(first channel number - number of channels)>

Record 4 - "backgrnd:" <number of background counts in energy range 1> <number of background counts in energy range 2> <number of background counts in energy range 3> <number of background counts in energy range 4> <number of background counts in energy range 5>

Record 5 - "START OF DATA"

Data Records (N)

Each data record has the following data columns:

(1 through 9) STANDARD ATTRIBUTES - the 9th column corresponding to sub-bottom depth is always empty.

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

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

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

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

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

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

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

(17) RUN NUMBER - the run number added by the MST program.Trailer Records (2)

Record 1 - "END OF DATA"

Record 2 - "trailer"