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Downhole measurements focus on characterizing in situ formation conditions and properties, as well establishing the link between the cores, logs, and seismic data. Since LWD data were not collected during Expedition 314, downhole measurements are essential to the success not only of the expedition but also of the whole NanTroSEIZE project.
The primary downhole measurements conducted during coring operations will consist of formation temperature and pressure measurements with the SET-P tool. We will stabilize the hole with weighted mud when making the pressure measurements. IODP Expedition 308 used this technique successfully to extend formation measurement time without circulation (Long et al., 2008). The degree of induration will also increase with depth, however, and we have concerns about the reliability of pressure data if insertion of the tool tip causes the mudstone to fracture or within formations characterized by low hydraulic diffusivity that require unreasonably long times (>2 h) to obtain any meaningful pressure data. Thus, our deeper tests will be attempted within intervals of uncemented sand in the lower Shikoku Basin turbidite facies at proposed Site NT1-07A.
Once coring is completed in the first hole, we will conduct a series of downhole logging runs. Given the history of difficult formation conditions in convergent margins and its impact on the successes of downhole logging, weighted mud will be used to stabilize the hole to increase chance of obtaining downhole logs. Our logging plan consists of three primary tool strings (Fig. F11):
These tool strings are described in detail at www.jamstec.go.jp/chikyu/eng/Expedition/logging.html.
The first run measures basic and important properties: natural gamma ray, resistivity, density, PEF, and porosity. These data help characterize finer scale stratigraphy that can be associated with cyclicity within turbidite intervals. The caliper log provided by the HLDS will also allow us to assess the hole conditions and chances of success of subsequent wireline runs. Since these measurements coincide with those in the LWD holes in Expedition 314 (Kinoshita et al., 2008), the data will be integrated.
The second run will include EDTC, HNGS, DSI, and FMS. The high-resolution FMS images will provide fine-scale stratigraphy and the best information about the extent of deformation and brecciation within the upper basement. The DSI will provide the first measurements of in situ formation sonic velocity that will allow generation of synthetic seismograms for detailed seismic log correlations and help characterize the petrophysical properties of the sediments.
The final run will be a VSP whose primary goal will be to tie precisely the well data to the 3-D seismic survey. This will be recorded with the VSI (containing a three-axis geophone). As the difficulty of getting proper mechanical coupling between the tool and the formation increases with the number of shuttles, the actual tool configuration will ultimately depend on the results of the caliper log and sea conditions.