Wednesday, February 24, 2010
Happy down-hole loggers
This is Annick, one of the two down-hole loggers. Here she was getting ready to start her work. The down-hole loggers were very happy the last couple of days, because they finally had a chance to collect some data. Poor drillhole conditions and bad weather did not make it possible to log any previous boreholes. The down-hole logging is very important for our studies. It is the only way to collect an almost continous record of the formations in the subsurface. During drilling there are always core breaks or sections that are not recovered, e.g. because the beds in the subsurface have too many large stones in them, or because the material is too soft or too loose. The down-hole logging provides a means to collect data on these missed sections. The loggers use several sets of tool, or instruments that are lowered into the borehole. One of them is the formation microscanner (FMS), which provides an image of the borehole wall. You can even see rock clasts in the borehole wall with this. Another tool is the gamma-ray tool. This tool is able to detect potassium-bearing minerals by exciting the potassium atoms with a radioactive source. In many cases the potassium-bearing phases are clay minerals, so it can be used to pick up fine-grained beds that were missed when coring. Other tools measure the resistivity of beds by sending a current trough the formation. Sands usually have high resistivity, so it is very useful to pick up coarse-grained beds. You can see some of the tools in the box on the photo to the right. These tools are lowered into the borehole on a wire, after the hole has been prepared for logging.
Posted by Sandra Passchier at 6:54 AM