The carbonate gamma ray log is a gamma ray log from which the uranium contribution has been subtracted. Self/spontaneous potential The Spontaneous Potential (SP) log measures the natural or spontaneous potential difference between the borehole and the surface, without any applied current. It was one of the first wireline logs to be developed, found when a single potential electrode was lowered into a well and a potential was measured relative to a fixed reference electrode at the surface. The most useful component of this potential difference is the electrochemical potential because it can cause a significant deflection in the SP response opposite permeable beds. The magnitude of this deflection depends mainly on the salinity contrast between the drilling mud and the formation water, and the clay content of the permeable bed. Therefore, the SP log is commonly used to detect permeable beds and to estimate clay content and formation water salinity. The SP log can be used to distinguish between impermeable shale and permeable shale and porous sands. Miscellaneous logs Caliper A tool that measures the diameter of the borehole mechanically, using either 2 or 4 arms, or through high-frequency acoustic signals. Because most logs are dependent on borehole regularity to record accurately, the caliper log can indicate where logs are potentially compromised due to the borehole being either over-gauged (due to washout) or under-gauged (like mudcake buildup). Nuclear magnetic resonance Nuclear magnetic resonance (NMR) logging uses the NMR response of a formation to directly determine its porosity and permeability, providing a continuous record along the length of the borehole. The chief application of the NMR tool is to determine moveable fluid volume (BVM) of a rock. This is the pore space excluding clay bound water (CBW) and irreducible water (BVI). Neither of these are moveable in the NMR sense, so these volumes are not easily observed on older logs. On modern tools, both CBW and BVI can often be seen in the signal response after transforming the relaxation curve to the porosity domain. Note that some of the moveable fluids (BVM) in the NMR sense are not actually moveable in the oilfield sense of the word. Residual oil and gas, heavy oil, and bitumen may appear moveable to the NMR precession measurement, but these will not necessarily flow into a well bore. Spectral acoustic logging Spectral acoustic logging is a acoustic measurement technique used in oil and gas wells