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Active prospection methods.  Geophysical techniques that produce an artificial electrical, electromagnetic, or acoustic signal that is transmitted into the ground. Variations in the subsurface interact with and modify these signals according to their specific physical properties, producing an altered return signal that is measured by instrumentation at the surface.

Anomaly.  A term used in geophysics referring to measurements that stand out as being different from typical background measurements, indicating the presence of a local geological, biological, or archaeological feature with different physical properties.

Archaeogeophysics.  The application of geophysical methods, principles, and theory to archaeological contexts.

Attenuation. The loss of power in a transmitted signal that occurs with all active geophysical methods, particularly in reference to GPR.

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Clipping.  An image processing technique that removes a percentage (usually 5 - 10%) of the most extreme values in a data set to allow focus on the more characteristic central measurements, usually resulting in a contrast enhancement.

Concatenation.  An image processing technique used in geophysics to merge data from multiple survey grids into a single data set and image.

Contrast enhancement.  An image processing technique that accentuates image detail by redistributing the range of color or gray display values, often by assignment to a narrower range of prevalent data values (see Clipping).

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Data logger.  A computer memory device used to automatically store large volumes of information.

Despiking.  An image processing technique for the removal of isolated extreme measurements in a scene.  These measurements are often regarded as noise that may result from modern litter like metallic debris in geophysical surveys.

Diurnal variation.  Daily variation, usually in reference to changes in natural properties of the Earth like magnetic field strength or temperature.

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Edge matching.  An image processing technique for the normalization of multiple grids of geophysical survey data to a common level.  Differences in instrument calibration or soil conditions between surveys can cause arbitrary changes in mean measured values.  Mean values of one edge of a matrix of measurements are commonly matched with the mean value of an adjacent edge in another matrix through the addition or subtraction of a constant.

Electrical resistance.  Resistance to the flow of an electrical current as determined by Ohm's Law, measured in ohms.  In electrical resistivity surveys the measurement of soil resistance is partially dependent on probe configuration.  Resistance is a parameter employed in the estimation of resistivity, an intrinsic soil property.

Electrical resistivity.  1. An intrinsic soil property measured in ohm-meters (see Ohm-meter). 2. An active geophysical survey technique that injects a current into the earth to assess soil resistivity.

Electromagnetic conductivity.  An active geophysical survey technique that transmits radio frequency energy into the earth, which induces a current and secondary magnetic field proportional to soil conductivity sensed by a receiver.  Measured in milliSiemens/meter (see milliSiemens/meter).

EM.  1. An abbreviation of "electromagnetic." 2. A term often used to refer to the electromagnetic conductivity method.

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Gamma. See nanotesla.

Geophysics. A technique that remotely investigates and defines an object or material in the subsurface without direct contact.

GPR. See ground penetrating radar.

GPR antenna. A GPR antenna for transmitting radar energy into the earth, usually coupled with a receiving antenna for sensing signals reflected from subsurface discontinuities.

GPR transducer.  See GPR antenna.

Gradiometer. See magnetic gradiometer.

Ground penetrating radar.  An active geophysical remote sensing device that transmits microwave (radar) energy into the earth which is reflected by discontinuities caused by changes in soil dielectric properties.

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Hertz (Hz).  Cycles per second, used to express frequency, typically in thousands (KHz; 10³ Hz), millions (MHz; 10⁶ Hz), or billions (GHz; 10⁹ Hz) of cycles per second.

Horizontal dipole mode.  See vertical and horizontal dipole mode.

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Image processing.  The manipulation of digital image data or other similarly structured matrices by computers.

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KHz.  See Hertz.

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Magnetic dipole.  Magnetic poles of opposite sign located in close proximity and resulting from a single, highly magnetic source that mimics a bar magnet.

Magnetic gradiometer.  A magnetometer with twin, vertically separated sensors, that simultaneously measure the difference, or gradient, in the magnetic field, eliminating the effects of diurnal variation.

Magnetic susceptibility.  The ability of a substance, usually a soil, to become magnetized.

Mho. See Millisiemens/meter

MHz.  See Hertz.

Millisiemens/meter (mS/m).  The unit of measurement of soil conductivity, an intrinsic bulk property of a soil that depends on type, particle size, compaction, dissolved ion content, moisture, and other factors.  0.001 Siemens/meter. Siemens/meter = (ohm-meter)^-1. (Formerly the millimhos/m where 1 millimho/m [MKS system of units] = 1 mS/m [SI system of units].)

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Nanosecond (nS).  10^-9 second.

Nanotesla (nT).  10^-9 Tesla, a measure of magnetic flux density per unit area (formerly the gamma).

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Ohm's Law.  Allows determination of earth resistance as the ratio of voltage to current: R = V / I, where R is resistance, V is volts, and I is current.

Ohm-meter.  The unit of resistivity measurement, an intrinsic property of a soil that depends on type, particle size, compaction, dissolved ion content, moisture, and other factors.  One approximation gives: ohm-m = 2aR, where a is the interprobe spacing (in a Wenner array) and R is the measured resistance in ohms.

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Passive prospection methods.  Geophysical techniques that detect variations within the natural fields of the Earth, like magnetism or gravity over the Earth's surface.

Pixel.  A picture element in a computer display, usually representing a single measured value in the underlying data.

Principal components analysis.  A multivariate statistical method that extracts the principal dimensions of variation, referred to as "components," in a set of correlated variables. One characteristic of principal components is that they are uncorrelated. Used in multidimensional image analysis.

Proton precession magnetometer.  A magnetic sensor containing a coil in a proton-rich liquid.  Application of a current polarizes the protons, which then precess to the magnetic field of the earth when the current ceases.  The frequency of the precession is proportional to the strength of the field.

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Relative dielectric permitivity.  The ability of a substance to propagate radar energy, calculated as the ratio of its electrical permitivity to that of a vacuum.

Resolution.  In geophysics, the ability to discriminate small objects or features, which depends on such factors as sampling density, wavelength, instrument sensitivity, depth, and soil conditions.  In image processing, the on-the-ground area represented by a pixel.

Resistivity.  See electrical resistivity.

Resistivity tomography.  Tomographic methods applied to resistiivity data acquired at multiple probe separations.

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Siemen. See Millisiemen.

Signal-to-noise ratio. In a geophysical data set, the ratio of the "signal" (anomalies that represent the targets or the object of the survey) to "noise" (all other variation in a data set). The goal is a high signal-to-noiose ratio.

Soil velocity analysis. Methods employed in GPR studies for the determination of the velocity of electromagnetic energy of specific wavelengths within various materials.

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Thermoremanent magnetism.  A permanent magnetism derived from heating a material beyond its Curie point.  The orientation of the magnetism reflects its relative position in the earth's magnetic field at the time of cooling.

Tomography. 1. A method by which a three-dimensional image of the internal structure of a solid (e.g., an archaeological site) is generated by recording its effects on electrical, electromagnetic, or acoustic energy impinging on those structures. 2. A three-dimensional image of a subsurface structure constructed by the computer through a series of planes or cross-section images made along various axes.

Total field magnetic survey.  A survey over an area that systematically records variations in the total magnetic field stemming cumulatively from the Earth's core and all other sources, including underlying geology and soil properties.  Measured in nanoteslas (nT; see Nanotesla).

Twin electrode array.  A modification of the Wenner array where the probes are separated into two pairs, each consisting of  current and voltage probes.  Only one pair is moved during a survey, facilitating data gathering and generating information with some advantages for interpretation.  The other pair remains fixed at a remote location.

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Vertical and horizontal dipole modes.  Two modes of use for electromagnetic conductivity instruments that align transmitting and receiving coils perpendicular to or parallel with the ground surface, respectively, affecting depth of prospection.

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Wenner array.  A linear, equally spaced, four-probe configuration for measuring soil resistivity, where the probe interval width is approximately equal to the depth of prospection.  The outer probes inject a current while the inner probes measure voltage, allowing computation of resistance in ohms and resistivity in ohm-meters.

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