Natural gamma ray logging was carried out at the five wells in Izena Hole off-shore Okinawa by D/V Chikyu on November to December in 2016. This is one of the research cruise under an umbrella of Cross-ministerial Strategic Innovation Promotion Program (SIP), "Next-generation technology for ocean resources exploration (Zipangu in the Ocean program)". The purpose of this survey is to understand geological profile and distribution of the subsurface hydrothermal deposit. It is difficult to take good quality and recovery cores in hydrothermal brittle formation. On the other hand, the continuous natural gamma ray logging data allows us to describe continuous vertical formation profile. The memory-type logging tool was stuck out just below the drill bit at the bottom of borehole after cutting cores. The tool measures logging data during pulling out of the hole. Natural gamma ray, temperature and pressure were measured in the boreholes. The gamma ray data by logging tool has good correlation with that in the core data by laboratory measurement until ~30–80m below sea floor at the good core recovery intervals. The logging data are only continuous profile below the depth till maximum 180m below sea floor. The gamma ray data indicate classification of sediment layer, ore body, and alternate layer in this region. The temperature logging indicates potential of hydrothermal deposit activity. The pressure data shows static water column.


Surveys for sub seafloor hydrothermal fluid deposit off-shore Okinawa have started since 2014 for SIP. Analysis with cores, well log data, observatory allows us to understand characteristics and mechanism of hydrothermal deposit. It is very challenging to acquire full volume and good quality of cores at hydrothermal deposit in deep sea subsurface, because of its fragile formation. To compensate this disadvantage, LWD was used to measure continuous depth profile.

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