A new technique using a radioisotope (RI) density log with gamma rays is proposed for investigation of sediment environments. A series of field investigations was carried out by penetrating an RI cone probe by its own weight from a small boat. Main conclusions are obtained as follows: The wet density of sediments comprising suspension, fluid mud, and bed mud can be measured with sufficient accuracy using an RI-density log. The lower limit of water, which is the sediment surface, is definable based on a pore water pressure distribution obtained from a cone penetration test.