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>

<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"