Dacite – Igneous volcanic rock (~65% SiO2) that is less mafic than andesite, and typically fine-grained and light in color.
Data – Measurements of a physical phenomenon such as a field, flux, current, or force, etc. They should be accompanied by an estimated uncertainty. To understand and work with data in inversion, it is imperative that details of the survey and instrumentation are known, via locations/orientations of transmitters and receivers, transmitter strength, receiver gains and any processing that has been done to the data.
Data Misfit – Useful model of the Earth recovered by inversion must be capable of causing the dataset that was observed. This is tested by comparing field measurements to a synthetic dataset generated by forward Modelling based upon this recovered model. We say the model “fits the data” if it is capable of generating data that match the field measurements to within a specified degree. Data misfit describes how close predicted data are to the field measurements. Often we plot the real and synthetic datasets to compare for similarity. Sometimes a plot of the difference between the two datasets is generated to emphasize that variations between the two are small. Data misfit is commonly quantified with RMS or L2 difference norms.
Datum Plane – Artificially established, well surveyed, horizontal plane against which elevations, depths, tides, etc. are measured, such as mean sea-level.
Daughter Element – Element that occurs in a rock as end product of the radioactive decay of another element. Also often referred to as a Daughter Product.
DC Resistivity Method – Observation of electric fields caused by current introduced into the ground as a means for studying earth resistivity. The term normally includes only those methods in which a very low frequency or direct current is used to measure the apparent resistivity.
DCIP3D – Program library that performs forward modelling and inversion of DC resistivity and IP data over a 3D distribution of electrical conductivity and chargeability. The programs in the library work with data that are acquired using general electrode configurations and at arbitrary observation locations either on the Earth’s surface or in a borehole. 3D surface topography is also fully incorporated in the modelling and inversion.
Dead Time – Amount of time, in nuclear logging, required for the system to be ready to count the next pulse. Pulses occurring during dead time are not counted.
Debris Avalanche – Fast downhill mass movement of soil and rock from an unstable slope.
Decay – Process, in nuclear physics, of disintegration of an unstable radioisotope by the spontaneous emission of aged particles or photons.
Decentralize – Forcing a logging probe against one side of the drill hole.
Declination – At any location on Earth, the angle between the magnetic and rotational poles.
Decline – Sloping underground opening for machine access from level to level or from surface; also called a ramp.
Decompression Melting – Melting that occurs without additional heat when a mineral deep within the Earth at high temperature and pressure migrates to a zone at lower pressure closer to the surface.
Deconvolution – Data processing technique applied to seismic reflection data to improve the detection and resolution of reflected events. The process reverses the effect of linear filtering processes, commonly known as convolution, that have been applied to the data by recording instruments or other processes.
Deflation – Removal of clay and dust from dry soil by strong winds.
Delta – Accumulation of sediment deposited where a river empties into a larger body of water.
Delta Kame – Deposit having the form of a steep, flat topped hill, left at the front of a retreating continental glacier.
Dendrites – Small branching patterns on rocks made of iron and manganese oxides that show the passage of fluids through the rock.
Dendritic Drainage – Stream system that branches irregularly and resembles a branching tree in plan.
Departure Curves – Graphs that show the correction that may be made to logs for some extraneous effects, such as drill hole diameter, bed thickness, temperature, and so on.
Density – Mass per unit volume of a substance, commonly expressed in grams per cubic centimeter.
Density Contrast – When different rock types have different mass densities, then there will be spatial variation of mass density between, and sometimes within, rock units. This variation, known as the density contrast and it affects both the local gravitational field that is measured in gravity surveys as well as the acoustic impedance contrast.
Density Current – Current in either a liquid or a gas that is kept in motion by the force of gravity acting on differences in density, and it stops moving when the two fluids mix.
Density Log – Gamma photons from a radioactive source in the sonde are backscattered to a detector; the backscattering is related to the bulk density of the material around the sonde. Sometimes referred to as a gamma log.
Dense-Non-Aqueous-Phase Liquids – DNAPLs are organic liquids that are denser than water. They often coalesce in an immiscible layer at the bottom of a saturated geologic unit.
Depleted Uranium Tails – Uranium where the U-235 content is below the naturally occurring 0.71% as the result of the enrichment process.
Deposit – Mineralized body which has been physically delineated by sufficient drilling, trenching, and/or underground work, and found to contain a sufficient average grade of metal or metals to warrant further exploration and/or development expenditures; a deposit does not qualify as a commercially mineable ore body or as containing mineral reserves until certain legal, technical and economic factors have been resolved.
Deposition – Accumulation of sediments by mechanical settling from water or air or by chemical sedimentation or evaporation.
Deposition Remnant Magnetization – Weak magnetization created in sedimentary rocks by the rotation of magnetic crystals into line with the ambient field during settling.
Depth of Invasion – Radial distance from the wall of the hole to which mud filtrate has invaded.
Depth Reference or Datum – Zero reference for logs of a well in the oil and gas industry where a Kelly-bushing may be used if the rig is still on the well. A ground level or top of casing is frequently used.
Depth Section – Cross section to which a velocity function has been applied thus converting arrival times of reflections to depths.
Desert Pavement – Residual deposit produced by continued deflation, which removes the fine grains of a soil and leaves a surface covered with closely packed cobbles.
Detector – Any kind of a sensor used to detect a form of energy, but usually refers to nuclear detectors, such as scintillation crystals.
Detrital Sediment – Sediment deposit formed from solid particles of pre-existing rocks or organic debris.
Development – Preparation of a known commercially mineable deposit for mining.
Deviation – Departure in degrees between the drill hole or probe axis and vertical.
Diabase – Dark-coloured intrusive rock, found as dykes or sills, composed of mainly feldspar, pyroxene and/or olivine; it is the shallow intrusive equivalent of basalt and gabbro.
Diagenesis – Physical and chemical changes undergone by a sediment during lithification and compaction, excluding erosion and metamorphism.
Diamond Drill – Rotary type drill using small diamond chips embedded on the head of the drill bit that cuts out a solid, long cylindrical core of rock as the drill turns. The cutting is done by abrasion rather than by percussion. This in-situ record of the rock mass intersected in the form of solid cylinders of rock, are referred to as drill core.
Diamond Drilling – Drilling method whereby rock is drilled with a diamond impregnated, hollow drilling bit which produces a continuous, in-situ record of the rock mass intersected in the form of solid cylinders of rock, which are referred to as core.
Diastrophism – Process of deformation by which the major features of the earth’s crust, including continents, mountains, ocean beds, folds, and faults, are formed.
Diatom – Common type of phytoplankton which may be unicellular or exist as colonies. Diatoms usually have siliceous skeletons.
Diatomite – Diatomite, or diatomaceous earth, is a siliceous sedimentary rock formed from the accumulations of diatoms or other nanoplankton.
Diatom Ooze – Fine muddy sediment consisting of the hard parts of diatoms.
Differentiated Planet – One that is chemically zoned because heavy materials have sunk to the center and light materials have accumulated in a crust.
Diatreme – Volcanic vent filled with breccia by the explosive escape of gases.
Dielectric Constant – Measure of the ability of a material to store when an electric field is applied.
Dielectric Permittivity – The ge separation or polarization in a medium.
Differential Log – Log which records the rate of change of some logged value as a function of depth; the differential log is sensitive to very small changes in absolute value.
Digital Log – Recorded as a series of discrete numerical values.
Dilution – Reduction in ore grade that occurs during mining as a result of the unavoidable inclusion of waste rock together with the ore. Dilution varies with, among other things, mining method, ground conditions and style of mineralization.
Dilutions – Effect of waste or low-grade ore being included unavoidably in the mine ore, lowering the recovered grade.
Dimension Stone – Natural building stone that may be cut to specific size requirements. It includes granite, gabbro, anorthosite, marble, limestone, slate and sandstone. It can be rough or finished stone.
Diorite – Grey intrusive igneous rock composed of mostly feldspar and biotite, pyroxene or amphibole; it forms dykes and small stocks; it is the intrusive equivalent of andesite.
Dip – Angle between the horizontal and a sloping surface such as a bedding plane or fault when the dip is measured perpendicular to strike direction; usually from the horizontal.
Dipmeter – Multielectrode, contact-resistivity probe that provides data from which the strike and dip of bedding can be determined.
Dipole – Pair of equal ges or poles of opposite signs.
Directional Survey – Log that provides data on the azimuth and deviation of a borehole from the vertical.
Discretization – Although the Earth has a continuous distribution of physical properties we simplify this with a discretization that describes the earth as a model containing a number of cells each having a constant physical property. This model is defined on a 1D, 2D, or 3D grid or mesh. The size of the cells should reflect the resolving power of the survey. If the cells are too large important, geologic features may not be adequately modeled. If they are too small, it shouldn’t adversely affect the inversion outcome but it may slow the process down due to an increase in the size of the system of equations to be solved.
Dispersion – Property of seismic surface waves in which their velocity (as well as their penetration into the subsurface) is dependent on their frequency. The basis of methods such as SASW in which seismic wave velocity is analyzed as a function of wave frequency.
Displacement Currents – Movement of ge within a material by polarization, as opposed to the flow of free ions or electrons. Related to the applied electric field by the electric permittivity or dielectric constant.
Dissemination – Scattered distribution of generally fine grained minerals within a rock.
Divide – Ridge of high ground separating two drainage basins emptied by different streams.
Dolomite – A carbonate sedimentary rock composed of more than 50 percent of the mineral calcium-magnesium carbonate (CaMg(CO3)2). It is the principal component of dolostone.
Dolostone – Sedimentary rock formed by precipitation from seawater and is composed mainly of dolomite.
Dome – Round or elliptical upwarp of strata resembling a short anticline.
Dore – Impure and unrefined mixture of metallic gold and silver produced through the smelting of gold and silver concentrate, sand or precipitate. Typical impuritics include base metals. Doré is further refined to almost pure gold by a smelter or refinery.
Drainage Basin – Region of land surrounded by divides and crossed by streams that eventually converge to one river or lake.
Drift – Glacial drift is a collective term for all the rock, sand, and clay that is transported and deposited by a glacier either as till or as outwash.
Drill Core – The cylindrical piece of rock, usually between one and three inches in diameter brought to surface as a result of diamond drilling.
Drilling Core – Drilling method that uses a rotating barrel and an annular shaped diamond impregnated rock-cutting bit to produce cylindrical rock cores and lift such cores to the surface, where they may be collected, examined and assayed. Reverse circulation: a drilling method that uses a rotating cutting bit within a double-walled drill pipe and produces rock chips rather than core. Air or water is circulated down to the bit between the inner and outer wall of the drill pipe. The chips are forced to the surface through the centre of the drill pipe and are collected, examined and assayed. Conventional rotary: a drilling method that produces rock chips similar to reverse circulation except that the sample is collected using a single-walled drill pipe. Air or water circulates down through the centre of the drill pipe and returns chips to the surface around the outside of the pipe. In-fill: the collection of additional samples between existing samples, used to provide greater geological detail and to provide more closely-spaced assay data. Exploration Prospecting, sampling, mapping, diamond drilling and other work involved in searching for ore. deposit a mineralized body that has been physically delineated by sufficient drilling, trenching and/or underground work, and found to contain a sufficient average grade of metal or metals to warrant further exploration and/or development expenditures; such a deposit does not quantify as a commercially mineable ore body or as containing mineral reserves, until final legal, technical and economic factors have been resolved.
Drumlin – Elongated mound of glacial sediment deposited parallel to ice flow.
Dual Laterolog – Focused resistivity log with both shallow and deep investigation wherein usually gamma, SP, and microfocused logs are run simultaneously.
Dry Wash – Intermittent streambed in an arroyo or canyon that carries water only briefly after a rain.
Dune – Elongated mound of sand formed by wind or water.
Dyke – Sheet of igneous intrusive rock that cuts across the surrounding host rock at a high degree of angle. Dykes can range from a few centimetres thick to tens of metres thick and run for numerous miles.