The Costa Rica hydrogeology operation proposes to replace CORK-II downhole instrument strings in holes drilled at Ocean Drilling Program (ODP) Leg 205 Sites 1253 and 1255 (SeptemberNovember 2002) off Costa Rica. The CORKs are instrumented in a fractured horizon in the oceanic section of the incoming plate and in the décollement zone to investigate fluid flow across the margin and its implications for the seismogenic zone and subduction factory. During Atlantis cruise 11-8 (27 February7 March 2004) the Alvin was unable to recover the downhole osmotic fluid samplers and miniaturized temperature loggers and was thus unable to deploy the new instrument strings. Apparently, soft debris in the borehole buried the latch, and the limited bottom time, lighter-weight running tools necessary for the Alvin, and limited capacity of the wellhead winch frustrated recovery efforts. These impediments present no significant obstacles to a drillship, as discussed below. Pressure data downloaded from the samplers at Sites 1253 and 1255 record two transient events; evaluation of temperature and fluid chemistry data from the downhole instruments at the two sites is critical to investigating strain or hydrological origins of the pressure events. The osmotic samplers currently installed were designed to collect a time series of samples for fluid and gas analyses over a 2 y period; after 2 y, samples and the information they contain are progressively lost. The temperature loggers will stop recovering data after 6 October 2004. The drillship work proposed here will be done during the transit following Integrated Ocean Drilling Program (IODP) Expedition 301 after the scheduled port call at Astoria, Oregon, en route to the Panama Canal.
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