There are many ways of collecting data from boreholes, and this can be done while drilling the hole, or once drilling has stopped. These data can give valuable information about the borehole, including a wide variety of geological information collected by direct measurements, or by carefully monitoring the drilling mud. One of the most important uses for these data is to provide information about the borehole condition.
Logging (Geophysical well logging)
Geophysical data are measured using various tools and sensors inside the borehole. Some of these data are: formation natural gamma ray, electrical resistivity, density, porosity (ratio of void space within the rock formation or sediment), acoustic velocity, seismic velocity, borehole image, natural gamma ray spectral, borehole diameter, and temperature. Rock strength tests, sampling cores from borehole wall, and analysis and sampling of formation fluid are carried out occasionally. These tools are often used to transmit live data back to the ship using mud pulses, while more comprehensive data are stored in memory inside each tool.
Collecting measurements using a special armored wireline cable uses sensors suspended inside the borehole via the cable from the ship. Here, sensors and tools are lowered into the borehole (either inside casing or in the open borehole) and log geophysical measurements. They are powered from the ship, and send their data back through the cable to the ship. Various and high-performance tools are available.
LWD (Logging While Drilling)
Logging while drilling (LWD) measures formation properties during drilling with the tools sensors mounted just behind the drilling bit. In unstable formations, LWD is less operationally risky than wireline logging, because the LWD tools are connected to the strong drilling pipe. The LWD tools are powered by a downhole electric dynamo using the pumped drilling mud circulation and an onboard battery. A limited real time data stream from the downhole LWD tools is pulsed back to the ship, while high resolution data are recorded in memory. The mud pulse system uses mud pressure control as a data communication system.
Logging data from Chikyu expeditions are available on this website.
In riser drilling, cuttings and gases trapped in the sediments and rocks are returned and collected on the ship transported along with the drilling mud. Solid dugs (cuttings), gas (muddy gas) and drilling mud are separated, and the drilling mud is recycled and reconditioned as drilling mud again. The cuttings and muddy gases are sampled and analyzed. They provide valuable information on the geology under the seabed and are used as a safety evaluation during drilling and investigation of sediments and rocks.
Cuttings are sorted into size fractions with sieves. The properties of these cuttings fractions are analyzed and examined with optical and scanning electron microscopes