NATURAL GAMMA-RAY ACTIVITY COMPARED TO GEOTECHNICAL AND ENVIRONMENTAL CHARACTERISTICS OF NEAR SURFACE MARINE SEDIMENTS

Abstract

The main purpose of the present study is to investigate the use of natural gamma activity as a measure of geotechnical properties such as density, porosity, water content and shear strength of near surface marine sediments. Natural gamma activity data were acquired under in situ conditions at two locations in the Arkona basin of the Baltic Sea. Sedimentological parameters were determined at intervals of 10 cm in three gravity cores taken near the geophysical locations. The values of natural gamma radioactivity ranged from 5 API at the sea floor to 75 API at approximately 5 m below sea floor. High correlation coefficients (up to +/-0.79) were observed between natural gamma radioactivity and silt and clay content, wet bulk density, porosity, water content and shear strength. Moreover, most of the correlations were tested as significant by using the Student's t-test of significance. Since the intensity of natural gamma activity is proportional to the mass of radioactive substance (in this case, clay minerals), the increase of natural gamma activity in near surface marine sediments may be related to compaction processes. During compaction (cementation is not considered), as porosity is reduced, there would be an increase in the amount of clay minerals for the same volume of unconsolidated sediment, causing an increase in the natural gamma activity.