The Costa Rica hydrogeology operation (301T), which took place during the transit between Expeditions 301 and 303, was designed to replace CORK-II downhole instrument strings in holes drilled at Ocean Drilling Program Leg 205 Sites 1253 and 1255 offshore Costa Rica. The CORKs are instrumented in a fractured horizon in the oceanic section of the incoming plate (Site 1253) and in the décollement zone (Site 1255) to investigate fluid geochemistry and flow across the margin and their implications for the seismogenic zone and subduction factory. This operation was necessary because during Atlantis cruise 11-8 (27 February–7 March 2004) the Alvin was unable to recover and thus redeploy the downhole OsmoSamplers and miniaturized temperature loggers. The OsmoSamplers installed during Leg 205 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 associated temperature loggers had been programmed to stop recovering data after 6 October 2004.
The primary operational objective during Expedition 301T was to successfully recover the OsmoSamplers installed during Leg 205 at Sites 1253 and 1255 and to install replacement OsmoSamplers.
The science goals of Expedition 301T remain those of ODP Leg 205—to investigate active fluid flow across the Costa Rica margin and its implications for the seismogenic zone and subduction factory. The specific opportunity this operation affords is the combination of pressure data recovered during the Atlantis cruise with downhole temperature variations through time and the time series chemical data to investigate both the transient pressure events recorded at Site 1253 and the overpressured décollement at Site 1255.