Abstract
Gamma rays are rapidly absorbed by water. Therefore, the instrument including gamma ray detector needs to maintain proper contact with sea bed. But, it is difficult for the instrument to maintain proper contact on soft sediments. That is why geometry is important factor, especially in underwater gamma ray survey. It is for geometry to keep constant that the thick hemispheric shield surround a gamma ray detector, theoretically. Practically, the tank that is filled with water or liquid fuel surround the instrument including gamma ray detector. The liquid shield is very useful and economical. This idea will cause a wonderful revolution in underwater gamma ray survey.
