Chikyu Report
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LWD paradise!April 28, 2012

Data from the record-breaking JFAST hole turned from a trickle into an avalanche last night. The logging while drilling (LWD) tool measures a variety of physical properties of the rock penetrated by the drill bit. During the drilling operation, measurements are sent back up the drill string to the ship via a pressure-pulse in the drilling mud. We were receiving these measurements throughout the drilling, watching the monitor for a new point every 2 minutes. The communication system transfers data at a much slower rate than data are collected, so the majority of the measurements is stored locally on memory inside the tool itself. After the decision was taken to stop penetrating further into the subsurface, the tool was withdrawn and arrived on deck yesterday.

The new data were immediately downloaded from the memory inside the tool and the logging staff scientists and contractors got to work processing to make them available to the science team. We now have huge print outs of the entire section penetrated by the LWD hole, as well as the digital versions of the data. It’s like a treasure trove of information suddenly arrived on deck! Everyone in the science team is fascinated to see what the results show: the resistivity measurements and images and the gamma ray measurements give information on the different rock types, various deformation features, and the stability of the hole itself. But the big question on everyone’s mind is ‘where is the earthquake fault?’ The logging while drilling scientists are in overdrive getting the data processed, and interpretations developed. Several scenarios are possible based on the LWD data so we might need the long-term observatory data and the core observations to confirm the location of the fault. The LWD tool has given us the first glimpse into the subsurface and the plate boundary interface.

The science team devours the 5m long print out showing the newly available LWD data.

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