Caliper Logs - measure the diameter of an uncased borehole versus depth. Borehole enlarged by caving or dissolution of minerals. Hole diameter may be less than drill-bit because of mud cake or squeezing out of a soft clay by the weight of overlying sediment. When the caliper tool is rotated as it is lowered it is possible to see deformation (elongation) of the borehole, which can be used to infer the stress field

Temperature logs - can be used to reveal different thermal zones due to changes in thermal conductivity, variations in basal heat flow, temporal variations in surface temperature (i.e., climate change) or fluid flow in permeable layers. Drilling and mud circulation profoundly changes the temperature field. Logging must be done after the borehole has had time to equilibrate.

Single-Point Resistance - tool measures resistance between 2 current electrodes. If borehole fluid is homogeneous, variations in resistance are caused by lithology. Clay and shale have low resistance and sand and gravel have high resistance. Increasing pore water salinity will cause a decrease in resistance.

Resistivity - tool measures resistivity (as opposed to resistance) with 2 current electrodes and 2 measurement electrodes. 3 different configurations: short-normal, long-normal, and lateral. Short normal has the smallest distance between 2 adjacent electrodes (16 inches). It is the most sensitive to thin layers but is also influenced by the drilling mud. Lateral log has the longest distance between two adjacent electrodes 18 feet 8 inches). It samples resistivity over a large section of sediment/rock away from the borehole. Lateral log may miss thin beds.

Spontaneous Potential - natural processes produce an electrical current.

Occurrence of Self-Potentials

Liquid-junction - when formaton water and mud filtrate have different salinities, ions migrate from the high salinity fluid to the low salinity fluid. Chloride anions migrate more readily than cations, which produces an electrical current.

Membrane - Ions also migrate through the shale. Cations migrate more readily than chloride anions, which produces an electrical current.
 

Measurement of self-potentials - Voltage is measured between a ground electrode at the surface and an electrode lowered down the well. When formation water is saltier than the drilling mud, shale has a positive SP response and sand a negative SP response.

Interpretation and Application - SP logs are used to distinguish shale from sand and to estimate percent sand. In addition, it can be used to estimate pore water salinity in clean sands. The SP log is also used in correlation work especially in conjunction with resistivity logs.

Gamma Ray - Natural decay of potassium 40. Feldspars and micas are rich in potassium. High levels of radiation next to clay layers. Log can be run in a cased well.

Neutron - tool contains a source of neutrons and a detector. Neutrons are slowed and scattered by collisions with hydrogen atoms primarily found in water or hydrocarbons filling pore spaces. High porosity, saturated rocks have a low neutron count. Some hydrated minerals such as gypsum also have a low neutron count.

Gamma-Gamma - A source of gamma radiation, such as cobalt, is lowered down with a detector. Absorption of gamma radiation is proportional to bulk density. Bulk density decreases with increasing porosity.