What is borehole logging and water well logging?

Boreholes yield direct information about the structure of the ground below the surface. Drill cores show the lithology at the drill site, and physical parameter values and petrographic data can be obtained from them in the laboratory. For technical and economic reasons, however, drill cores are not taken from every borehole. 

Borehole logging (also called well logging or downhole logging) provides in-situ information in addition to the direct geological information. Geophysical borehole logging is the process of measuring physical, chemical, and structural properties of penetrated geological formations using tools that are lowered into a borehole on a wireline cable. The measurements are electrically transmitted via cable to the surface where they are recorded in digital format as a function of depth. The physical properties of the rock strata and a lithological classification can be derived from the data. Measurements on the cores themselves are also sometimes made. Borehole logging has certain advantages with respect to core measurements. 

For example, in a hole where there is limited core recovery, the depth of the incomplete cores can be uncertain; logs provide a continuous depth record of formation properties. The in-situ nature of the downhole measurements is an important factor because measurements on recovered cores are affected by the fact that the material is no longer under the pressure and temperature conditions that exist at depth.