**1. Introduction**

54 Gamma Radiation

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Deep-Sea *in-situ* radioactivity measurements were initiated in 1964 for investigation of sunken USN atomic power submarine Thresher by using Geiger counters (Wakelin, 1964). Since then, *in-situ* radioactivity monitoring utilizing a towed-fish and/or remote sea bottom stations has been carried out in various countries. But they are mainly used for surveys of artificial radio activities to evaluate potential risks around atomic power plants, radioactive waste disposal sites and sunken nuclear objects etc. (Jones et al., 1988). Thus, underwater gamma ray measurement has been rather limitedly performed compared to the sub aerial ones, which has been widely utilized for explorations of uranium or other valuable minerals for mining or much wider environmental analysis (Bristow, 1983), due to very effective shielding by seawater. In underwater environment, Yoshida and Tsukahara (1987) reported environmental gamma ray characteristics around an active cold seepage associating with an active fault in Sagami Bay, southern coast of mainland Japan. Besides that, deep-sea gamma ray surveys for scientific use are intensively carried out by ODP-IODP logging for determination of rock types.

This chapter intends to demonstrate some usage of environmental gamma-ray measurement, through the studies in the deep sea environment, e.g. deep sea hydrothermal vent, cold seeps, active faults. Further, this chapter also tries to document relationship with tectonic settings and geological events. For this purpose, the specification and assemblage of the gamma ray sensor systems of JAMSTEC are introduced firstly. Next, an examples of the field measurement will be shown as a temporal variation that reflecting the uniqueness of deep sea. Although the sensitivities of the current model of sensor are significantly reduced by the thick Al or Ti pressure hull to endure high pressure in deep sea, the collected gamma ray spectra and distributions of intensities indicated some linkages with tectonic settings to date.
