Water storage at Shi Men Reservoir started in May of 1963. After more than 40 years of service, the sediment in the reservoir has accumulated to 55,868,000 m3, amounts to 18.07% of the storage capacity. The Aerey typhoon of 2004 brought in large quantities of sediment. The area surrounding the intake of the power plant (elevation 173 m) has been covered by over 10 m of sediment. In order to facilitate the safety evaluation of the intake valve operation, Northern Region Water Resources Office (NRWRO) decided to conduct field testing to determine the density state of the sediment. The water surface within the reservoir was at elevation 244m, or the depth of water was over 71m. Due to the excessive depth of water, conventional soil boring and sampling methods were not applicable. The research team combined the techniques of flat dilatometer (DMT) and time domain reflectometry (TDR) to perform DMT/TDR tests from a barge. The DMT/TDR probe was extended to a maximum of 80m below water surface. The density of the sediment was inferred by combined DMT tests and sediment conductivity according to TDR signals.