Well logging is an evaluation method in which a logging crew lowers a special tool, a sonde, into the well and then pulls it back up. As the sonde passes the formations on its way up the wellbore, it senses and measures electrical, radioactive, and acoustic (sound) properties of the rocks. Measuring and recording these properties provides a good deal of information about the rocks. The sonde transmits its measurements via conducting cable to the surface, where computers record them. Experts analyze the resulting recording. Analysis involves careful study of the curves on the log.