Use of Geophysics for Transportation Projects (2006)

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29 Accelerometer—device that converts the effects of mechan- ical motion into an electrical signal that is proportional to the acceleration value of the motion. Alluvium—general term for unconsolidated material (e.g., clay, silt, sand, or gravel) deposited from running water. Often a sorted or semi-sorted sediment in the bed of a stream or on its flood plain or delta. Deposit may be in the form of an alluvial fan. Analytic signal—automated function that enables one to de- termine analytic signal depth solutions from gravity and magnetic profiles. Anomaly—deviation from uniformity in a physical property. Apparent resistivity/conductivity—resistivity of a homoge- neous isotropic ground that would give the same voltage/ current or secondary/primary field ratios as observed in the field with resistivity or electromagnetic methods. The apparent conductivity is the reciprocal of the apparent resistivity. Aquifer—rocks or unconsolidated sediments that are capable of yielding a significant amount of water to a well or a spring. Aquitard—geologic formation(s) of low hydraulic conduc- tivity, typically saturated, but yielding a limited amount of water to wells. Also referred to as a confining unit. Bedrock—general term referring to rock that underlies un- consolidated material. Bulk modulus—gives the change in volume of a solid sub- stance as the pressure on it is changed. The bulk modulus for a solid substance is its resistance to change volume un- der pressure. Common Mid-Point (CMP) survey—seismic reflection tech- nique for detecting geologic boundaries. Complex resistivity (CR)—geophysical effect, also the basis of the CR method, in which polarization within the medium results in the voltage and applied current being out of phase—that is, their ratio is complex. Also known as spectral induced polarization (IP). IP is one form of complex resistivity. Conductance—product of conductivity and thickness (Siemens). Conductivity (electrical)—ability of a material to conduct electrical current. In isotropic material it is the reciprocal of resistivity. Units are Siemens/m. Crosshole—geophysical methods carried out between bore- holes (see also tomography). Crosshole seismic—seismic method between boreholes us- ing a source in one borehole and a receiver in two or more boreholes to measure the P- and S-wave velocities of the strata within the borehole. Detector—can be any kind of a sensor used to detect a form of energy, but usually refers to nuclear detectors, such as scintillation crystals. Dielectric constant—measure of the ability of a material to store charge when an electric field is applied. Dipole—pair of equal charges or poles of opposite signs. Elastic properties—elastic properties specify the stress–strain properties of isotropic materials in which stress is propor- tional to strain. They include bulk and shear moduli. Electrode—piece of metallic material that acts as an electric contact with a nonmetal. In chemistry, it refers to an in- strument designed to measure an electrical response that is proportional to the condition being assessed (e.g., pH, resistivity). Electromagnetic (EM) method—method that measures mag- netic and/or electric fields associated with subsurface cur- rents. Electromagnetic wave—electric field associated with sub- surface currents. Field—space in which an effect, such as gravity or magne- tism, is measurable. Frequency domain—in geophysics, refers to measurements analyzed according to their constituent frequencies. The usual alternative is time–domain measurement. Gamma—common unit of magnetic field intensity, equal to one nanoTesla (a Tesla is the SI unit). The Earth’s mag- netic field strength is about 50,000 gammas () in mid- latitudes. Geophones—in seismic geophysical methods, receivers used to record the seismic energy arriving from a source. Gravity—lateral density changes in the subsurface cause a change in the force of gravity at the surface. The intensity of the force of gravity owing to a buried mass difference (concentration or void) is superimposed on the larger force of gravity owing to the total mass of the earth. Thus, two components of gravity forces are measured at the Earth’s surface, total field, and second, a component of much smaller size that varies as a result of lateral density changes (the gravity anomaly). Ground penetrating radar (GPR)—geophysical method in which bursts of electromagnetic energy are transmitted downwards from the surface, to be reflected and refracted by velocity contrasts within the subsurface. Also known as ground probing radar. Induced polarization (IP)—geophysical effect whereby elec- trical charge is momentarily polarized within a material, usually a disseminated ore or a clay. This effect is the ba- sis for the IP method, in which a decaying voltage owing to this polarization is measured following the turn-off of the activating current in time–domain surveying. See also complex resistivity. Induction (EM), induce—process, described by Faraday’s Law, whereby a variable magnetic field generates an elec- tric field (voltage) that, in the presence of a conductor, will produce electric currents. GLOSSARY

30 Karst—topographic area that has been created by the disso- lution of carbonate rock terrain. It is characterized by cav- erns, sinkholes, and the absence of surface streams. Magnetics—the Earth possesses a magnetic field caused pri- marily by sources in the core. The form of the field is roughly the same as would be caused by a dipole or bar magnet lo- cated near the Earth’s center and aligned subparallel to the geographic axis. Many rocks or minerals are weakly mag- netic or are magnetized by induction in the Earth’s field, and cause spatial perturbations or “anomalies” in the Earth’s main field. Man-made objects containing iron or steel are of- ten highly magnetized and locally can cause large anomalies. Magnetics is the geophysical method used to measure anom- alies in the subsurface owing to a high ferrous composition. Magnetic permeability—characteristic of a material, it is proportional to the magnetism induced in that material di- vided by the strength of the magnetic field used. Magnetic susceptibility—measure of the extent to which a substance may be magnetized, it represents the ratio of magnetization to magnetic field strength. Magnetization—magnetic moment per unit volume; a vector quantity. Mapping—locating geological, chemical, or geophysical in- formation in space (as opposed to time, which is monitor- ing). Results are usually summarized as maps. Mechanical caliper—borehole tool used to measure the di- ameter of a borehole. The shape of the borehole is a result of the subsurface lithology and the drilling technique. Monitoring—observing the change in a geophysical, hydro- geological, or geochemical measurement with time. Multi-Channel Analysis of Surface Waves (MASW)—in situ method that analyzes dispersion of surface waves and inverts it in terms of mechanical properties of the soil. Nondestructive Testing (NDT)—uses geophysical methods to test engineered structures for integrity. Permittivity—property that enables a three-dimensional ma- terial to store electrical charge; that is, its capacitivity. Poisson’s Ratio—ratio of lateral strain and axial strain. Ratio of transverse contraction strain to longitudinal extension strain in the direction of stretching force. Tensile deforma- tion is considered positive and compression deformation is considered negative. Profiling—in geophysics, an investigation method whereby an array of sensors is moved along the Earth’s surface without change in its configuration to detect lateral changes in the properties of the subsurface (faults, buried channels, etc.). The alternative is usually a sounding. Pseudosection—cross section showing the distribution of a geophysical property, such as seismic travel time, from which the distribution of the geological property of inter- est (e.g., depth to bedrock) can be interpreted. Radioactivity—energy emitted as particles or rays during the decay of an unstable isotope to a stable isotope. Rayleigh Wave—waves that travel along the free surface of a solid material. Particle motion is always in a vertical plane, elliptical and retrograde to the direction of propagation. Raypath—direction a seismic generated source travels in the subsurface. Receiver—part of an acquisition system that senses the in- formation signal. Refractor—portion of the raypath that travels along the in- terface of two solid materials that have two different ve- locities. Lower material has a higher velocity than the overlying material V1  V2. Resistivity (electrical)—electrical resistance to the passage of a current, expressed in ohm-meters; the reciprocal of conductivity. Rippability—ease with which soil or rock can be mechani- cally excavated. Seismic—see seismic reflection and seismic refraction. Seismic reflection—surface geophysical method recording seismic waves reflected from geologic strata, giving an es- timate of their depth and thickness. Seismic refraction—surface geophysical method recording seismic waves refracted by geological strata. Seismic velocity—if the ground is stressed by a hammer blow or explosion, three types of waves propagate into the sub- surface; P-Primary waves, S-Secondary waves, and Surface waves. The rate at which these waves travel is the seismic velocity measured in meters per second or feet per second. Shear modulus—stress–strain ratio for simple shear in isotropic materials that obey Hooke’s law. Ratio of shear stress to engineering shear strain on the loading plane. Shear wave—acoustic wave with direction of propagation at right angles to the direction of particle vibration (S-wave). Shear zone—subsurface area in the lithology that causes an acoustic wave to propagate at right angles to the direction of particle vibration. Soil resistivity—mineral grains composed of soil and rocks are essentially nonconductive. The resistivity of soils and rocks is governed by the amount of pore water, its resis- tivity, and the arrangement of pores Sounding—in geophysics, an investigation method whereby the geometry and/or frequency of an array of sensors are varied so as to measure the physical properties of the earth as a function of depth beneath the configuration. The al- ternative is usually profiling. Spectral Analysis of Surface Waves (SASW)—in situ seis- mic method that analyzes dispersion of surface waves and inverts it in terms of mechanical properties of the soil. Spontaneous-potential log—log of the difference in DC volt- age between an electrode in a well and one at the surface. Most of the voltage results from electrochemical poten- tials that develop between dissimilar borehole and forma- tion fluids. Statics—time shift corrections to individual traces to com- pensate for the effects of variations in elevation, surface layer thickness or velocity, or datum references. Surface wave—wave that travels along, or near to, the surface; its motion dropping off rapidly with distance from it. A dis- tinct seismic mode from the body waves (P- and S-waves).

31 S-wave—a body wave in which particles move perpendicu- lar to the direction of propagation. Also known as sec- ondary or shear wave. Terrain conductivity—geophysical method in which EM methods measure directly the average electrical conduc- tivity of the ground. Operates at low induction number. Time domain—in geophysics it refers to measurements ana- lyzed according to their behavior in time. The usual alter- native is frequency domain measurements. Tomography—method for determining the distribution of physical properties within the earth by inverting the results of a large number of measurements made in three dimen- sions (e.g., seismic, radar, resistivity, and EM) between different source and receiver locations. Transducer—any device that converts an input signal to an output signal of a different form; it can be a transmitter or receiver in a logging probe. Unexploded ordnance (UXO)—any munition that has not functioned properly during its firing, where the munition is dangerous and potentially capable of exploding. Variable-density log (VDL)—a log of the acoustic wave train that is recorded photographically, so that variations in darkness are related to the relative amplitude of the waves. Also called a three-dimensional log. Vibroseis—mechanical device used as a seismic source in- stead of a hammer or explosives. Well logging—geophysical method used in boreholes to pro- vide waveforms that are interpreted into geologic units. Televiewer logging provides actual pictures of the bore- hole or casing surface. Young’s modulus—ratio of normal stress to strain in the loading plane. It is the ratio of equilibrium length over the change in length times the force applied over the area.