Glossary

vapor dominated	A geothermal system in which pressures are controlled by vapor rather than by liquids.

vaporization	The process by which a liquid is converted into vapor.

vapor pressure	The pressure at which a liquid and its vapor are in equilibrium at a given temperature.

variable density	Variable intensity. Intensity modulated-time presentation of the acoustic wave train in which the amplitude of the wave form produces a variable photographic density which is displayed versus time. The variations in darkness or density represent relative amplitude. See intensity modulated-time and acoustic log. Compare amplitude-time. See illustration at wave train display.

variable density log	An acoustic log in which the acoustic wave train is recorded in the variable photographic density or intensity modulated-time mode. See illustration at wave train display.

variable intensity	Variable density. An intensity modulated-time mode of acoustic wave train display .

vector plot	Arrowplot. See tadpole plot.

velocimeter	A device which measures fluid flow; a flowmeter.

vertical	(1) an imaginary line perpendicular (at an angle of 90°) to the plane of the horizon. (2) Said of a borehole which is straight. Not deviated.

vertical resolution	The capability to resolve thin beds. Often expressed as the minimum thickness of formation that can be distinguished by a tool under operating conditions.

vertical seismic profile	VSP. A collection of seismic traces made from one-, two-, or three-dimensional geophones mounted in the same downhole tool which is anchored to the borehole wall. These traces are taken at sequential depths in a well and record the energy over a period of several seconds received from one or more sources located at the surface. The resulting profile displays the direct, reflected, refracted, and diffracted waves in both compressional and shear modes from all interfaces in the sedimentary column. It is the most accurate correlation tool for relating the well logs and lithologic logs to the field seismograms.

viscometer	A device for measuring viscosity Also called viscosimeter.

viscosity	Resistance of a fluid to flow. Internal friction caused by molecular cohesion in fluids. The internal properties of a fluid that offers resistance to flow. Viscosity of drilling muds may be reported in different ways: (1) Marsh funnel seconds. The time it takes for 1000 cm³ of drilling mud to flow through the funnel. The longer the time in seconds, the more viscous is the mud. (2) Yield point and plastic viscosity in centipoises. Using the combination of plastic viscosity and yield point, the plastic viscosity indicates the flow characteristics of the mud when it is moving rapidly, and the yield point indicates the flow characteristics when it is moving very slowly or at rest. In both cases, higher values indicate a more viscous mud.

volcanic	Of, pertaining to, like, or characteristic of a volcano; characterized by or composed of volcanoes; produced, influenced, or changed by a volcano or by volcanic agencies; made of materials derived from volcanoes.

voltmeter	An instrument used to measure, in volts, the potential difference in an electrical circuit.

vug	Solution cavity. See vugular porosity.

vugular porosity	A form of secondary porosity resulting from the dissolution of more soluble portions of rock or solution enlargement of pores or fractures. Common in carbonates. See also porosity.