A’a’ Lava – There are two main types of lava, pahoehoe and `a`a which differ in certain properties, such as crystal/gas bubble content, and temperature. After they cool, pahoehoe and `a`a are easily identifiable by their distinct textures. `A`a has a rough and jagged surface. Pahoehoe has a smooth, billowy, and even ropy surface. If one were traversing `a`a type lava they would want to be wearing very sturdy, protective type of boots. In an eruption where a lot of lava is being discharged at once, a`a flows tend to form.

A Priori Information – Used to refer to knowledge about the earth model that is not classified as data in terms of an inversion. Additional information of the Earth such as a known background or reference model, an expected structure or geometry, and known specific physical properties that can be assigned to cells and used to constrain and guide the inversion.

Abyssal Plain – Offshore portion of the ocean floor which is often flat and on a slight slope.

Accrete – The adding of terranes such as small land masses or pieces of crust to another, usually larger, land mass.

Accuracy – Closeness of a measurement or calculation to the true value.

Acoustic Impedance – Seismic velocity multiplied by density.

Acoustic Impedance – Reflects the ability of a boundary to reflect seismic energy. Contrasts of density multiplied by velocity across the boundary.

Acoustic Log – Record of changes in the actions of sound waves as they are transmitted through liquid-filled rock; a record of the transit time (t) is the most common; amplitude and the full acoustic-wave form are also recorded.

Acoustic Televiewer LogRecord of the amplitude of high-frequency acoustic pulses reflected by the borehole wall. It provides location and orientation of bedding, fractures, and cavities.

Acoustic WaveSound wave transmitted through material by elastic deformation.

Activation LogRecord of radiation from radionuclides that are produced in the vicinity of a well by irradiation with neutrons; the short half-life radioisotopes usually are identified by the energy of their gamma radiation or decay time.

Adit – Near horizontal tunnel excavated to access a mine from the surface.

A’ electrodeOne of the current-emitting electrodes of a resistivity-logging system. The current return electrode is labeled, and sometimes referred to as C1 and C2.

Aeolian Process – Geological modification, such as sand dunes or erosion, caused by wind activity.

Ag – Chemical symbol for silver.

Age – Unit of geological time that is shorter than an epoch, commonly lasting several million years.

A-Horizon – Uppermost layer of a soil.

Algae – Group of varied plants that generally live in the sea or in fresh water.

Algal Mat – Layered communal growth of algae observed in fossils and in present day tidal zones associated with carbonate sedimentation.

Alkali Metal – Basic metal like potassium or sodium.

Alkaline – Basic, as opposed to acidic, substance such as hydroxide or potassium.

Allochthonous – Something formed elsewhere other than its current location.

Alluvial Fan – Triangular-shaped deposit of sediment at the point where a river emerges from a mountainous area which generally flows onto a more gently sloping plain.

Alluvium – General term for unconsolidated material (e.g. clay, silt, sand, and gravel) deposited by flowing water in the bed of a stream or on its floodplain or delta. The deposit may be in the form of an alluvial fan.

Amber – Fossilized resin from a coniferous tree that sometimes contains entrapped and preserved insects.

Amphibole – Group of important silicate minerals containing iron, magnesium and calcium.

AmplitudeMaximum departure of a wave from the average value.

Analog recording – Data are represented as a continuous record of physical variables instead of discrete values, as in digital recording.

Andesite – Dark-colored, fine-grained, volcanic rock composed of feldspar and biotite, hornblende or pyroxene; it is the volcanic equivalent of diorite.

Angle of Repose – Steepest slope angle at which particular sediment will remain in place without cascading down.

Angstrom – A length of 10-10 meter or one hundred millionth of a centimeter.

Angular Unconformity – Unconformity in which the bedding planes of the rocks above and below are not parallel.

Anisotropic – Contains a physical property which varies with direction.

Annulus – Space between the drill pipe or casing and the wall of the drill hole. In rocks saturated with hydrocarbons the annulus is the transition interval between the invaded zone and the uncontaminated zone.

Anomaly – Simply, anything that is “not normal”. A departure from the norm which may indicate the presence of mineralization in the underlying bedrock. Geochemical anomalies and geophysical anomalies are two of the most common. In the context of the Earth’s subsurface an anomaly is a feature that can be detected or characterized which could be referred to as an anomaly or an anomalous zone.

Anorthosite – Igneous rock predominantly composed of plagioclase feldspar (90–100%), and a minimal mafic component (0–10%). Consisting mainly of calcium-rich feldspar, known as labradorite.

Anthozoa – Produces a skeleton of calcium carbonate.

Anthracite – Most highly metamorphosed form of coal containing 92 to 98 percent of fixed carbon. Usually black, hard, and glassy.

Anticline – Fold of rock layers with a convex shape pointing upwards. It’s the opposite of syncline.

Antifor – Fold in which the limbs dip away from the axis of the fold. Should the oldest rocks be in the central core, it is referred to as an anticline.

Aphanitic – Grain size components in igneous rock in which the crystals are not distinguishable to the naked eye.

API Unit – The American Petroleum Institute (API) has established test pits for calibrating neutron and gamma logs. The API neutron unit is defined as 1/1,000 of the difference between electrical zero and the logged value opposite the Indiana limestone in the calibration pit with an average porosity of 19 percent. The API gamma unit is defined as 1/200 of the deflection between intervals of high and low radioactivity in the calibration pit.

Apparent Resistivity/Conductivity – Resistivity of a homogeneous isotropic ground that gives the same voltage/current or secondary/primary field ratios as observed in the field with resistivity or EM methods. Note that apparent conductivity is the reciprocal of the apparent resistivity.

Aquifer – Permeable formation of rocks or unconsolidated sediments that stores and transmits groundwater in sufficient quantity to supply wells.

Aquitard – Geologic formation(s) of significantly low hydraulic conductivity, typically saturated, but yielding a limited amount of water to wells. Sometimes called a confining unit.

Archipelago – Group of scattered islands.

Archean – Geologic time period from about 3800 to 2500 million years ago. The Archean had four distinct eras: Eoarchean, Paleoarchean, Mesoarchean, and Neoarchean.

Archeocyathid – Simple, reef-building, marine organism that lived during the Cambrian period of geological time about 500-600 million years ago.

Archie’s Law – an empirical relationship linking formation resistivity (rt), formation water resistivity (r w) and porosity.

Arkose – Kind of sandstone containing abundant feldspar and quartz that is often of angular, poorly sorted grains.

Arroyo – Steep-sided and flat-bottomed gully in an arid region that is occupied by a stream which contains water occasionally after rainfalls.

Arsenopyrite – Most common arsenic mineral and principal ore of arsenic. It occurs in many sulfide ore deposits, particularly those containing lead, silver and gold.

Artesian Well – Well that penetrates an impermeable layer of rock to reach an aquifer containing water under pressure, thus the water in the well rises above the normal surrounding water table.

Asbestos – A white to green, fibrous form of the mineral serpentine which is used widely in fire-retardant and heat- resistant applications.

Asphalt – Dark bituminous substance found in natural beds. It is a residue of petroleum distillation.

Assay – Chemical analysis of an ore, mineral, or concentrate of metal to-determine the amount of valuable elemental content. Content of precious metals are usually provided in ounces per short ton or grams per metric tonne, while base metals are given as a percentage.

Assemblage – Group of related rock types that together define a specific depositional environment.

Asthenosphere – Highly viscous, mechanically weak region of the upper mantle of the Earth that lies below the lithosphere at depths between 100 and 200 km below the surface.

Astrobleme – Circular erosional surface feature that has been attributed to the impact of a meteorite or comet.

Atmosphere – Layer of gases surrounding a planet or other material body of sufficient mass that is held in place by the gravity of the surrounded object.

Atmosphere ‘Unit’ – Unit of pressure equal to 101,325 newtons per square meter, or about 14.7 pounds per square inch.

Atoll – Continuous or broken circle of coral reef and low coral islands surrounding a central lagoon.

Atomic Number – Number of protons in the nucleus of an atom equal to the number of electrons in a neutral atom.

Atomic Weight – Total number of protons and neutrons in the nucleus of an atom.

Attenuation or Attenuate – A reduction in energy or amplitude caused by the physical characteristics of a transmitting system.

Au – Chemical symbol for gold.

Augen – Eye-shaped mineral grains in a mylonite schist or gneiss.

Autochthonous – Something that was formed in its present location.

Automatic Gain Control – AGC is a process for increasing signal amplitude of a signal through time, thus making all events on the trace appear to be of approximately the same amplitude.

Azurite – Soft, deep blue copper mineral produced by weathering of copper ore deposits.