GEOPHYSICS 1 Flashcards
The splitting of an incident wave into two waves of different velocities and orthogonal polarizations. Birefringence occurs in optical mineralogy (see petrography) when plane-polarized light passes through an anisotropic mineral and emerges as two rays traveling at different speeds, the difference between which is characteristic of a mineral. In seismology, incident S-waves can exhibit birefringence as they split into a quasi-shear and a pure-shear wave. Although birefringence was first described by Danish physician Erasmus Bartholin (1625 to 1698) in crystals in 1669, the phenomenon was not fully understood until French physicist Etienne-Louis Malus (1775 to 1812) described polarized light in 1808.
birefringence / double refraction
A small, electrically activated explosive charge that explodes a larger charge. Detonators, also called caps,seismiccaps or blasting caps, are used for seismicacquisitionwith an explosivesourceto achieve consistent timing of detonation.
blasting cap
A device containing primary high-explosive material that is used to initiate an explosive sequence. The two common types of detonators are electrical detonators (also known as blasting caps) and percussion detonators. Electrical detonators have a fuse material that burns when high voltage is applied to initiate the primaryhigh explosive. Percussion detonators contain abrasive grit and primary high explosive in a sealed container that is activated by a firing pin. The impact force of the firing pin is sufficient to initiate the ballistic sequence that is then transmitted to the detonating cord. Several safety systems are used in conjunction with detonators to avoid accidental firing duringrig-up or rig-down. Safety systems also are used to disarm the gun or ballistic assembly if downhole conditions are unsafe for firing.
blasting cap
A shadow zone, or a zone through which waves do not pass, or cannot be recorded, or in which reflections do not occur.
blind zone
A layer or body of rock that cannot be detected by seismic refraction, typically because its velocity is lower than that of the overlying rocks; also known as a hidden layer.
blind zone
A permanently fixed marker cited in surveying, such as a concrete block or steel plate, with an inscription of location and elevation.
BM / benchmark
A wave that propagates through a medium rather than along an interface. P-waves and S-waves are examples of body waves.
body wave
Seismic data measured with receivers, sources or both in a well, such as a check-shot survey, vertical seismic profile (VSP), crosswell seismic data or single-well imaging. By directly measuring the acoustic velocity of each formation encountered in a well, the well logs and borehole seismic data can be correlated to surface seismic data more easily. Borehole seismic data, including both S- and P-waves, can be gathered in a cased or openhole. This term is commonly used to distinguish between borehole sonic data (with frequencies typically greater than 1000 Hz) and borehole seismic data (with frequencies typically less than 1000 Hz).
borehole seismic data
The remaining value of gravitational attraction after accounting for the theoretical gravitational attraction at the point of measurement, latitude, elevation, the Bouguer correction and the free-air correction (which compensates for height above sea level assuming there is only air between the measurement station and sea level). This anomaly is named for Pierre Bouguer, a French mathematician (1698 to 1758) who demonstrated that gravitational attraction decreases with altitude.
Bouguer anomaly
The adjustment to a measurement of gravitational acceleration to account for elevation and the density of rock between the measurement station and a reference level. It can be expressed mathematically as the product of the density of the rock, the height relative to sea level or another reference, and a constant, in units of mGal: δgB = 2 π G ρ h = 0.4193 ρ h, whereδgB = Bouguer correctionρ = rock density in kg/m3h = height difference between two locations in mG = gravitational constant = 6.67384 × 10−11 m3 kg−1 s−2. Strictly interpreted, the Bouguer correction is added to the known value of gravity at the reference station to predict the value of gravity at the measurement level. The difference between the actual value and the predicted value is the gravity anomaly, which results from differences in density between the actual Earth and reference model anywhere below the measurement station.
Bouguer correction
A concave-upward event in seismic data produced by a buried focus and corrected by proper migration of seismic data. The focusing of the seismic wave produces three reflection points on the event per surface location. The name was coined for the appearance of the event in unmigrated seismic data. Synclines, or sags, commonly generate bow ties.
bow tie
Anarrivalof energy propagated from the energysourceat the surface to thegeophonein the wellbore in verticalseismicprofiles and check-shot surveys, or an indication of seismic energy on a trace.
break
A seismic amplitude anomaly or high amplitude that can indicate the presence of hydrocarbons. Bright spots result from large changes in acoustic impedance and tuning effect, such as when a gas sand underlies a shale, but can also be caused by phenomena other than the presence of hydrocarbons, such as a change in lithology. The term is often used synonymously with hydrocarbon indicator.
bright spot
A processed seismic record that contains traces from a common midpoint that have been added together but has undergone only cursory velocity analysis, so the normal-moveout correction is a first attempt. Typically, no static corrections are made before the brute stack.
brute stack
Bubble pulses or bubble noise that affect data quality. In marine seismic acquisition, the gas bubble produced by an air gun oscillates and generates subsequent pulses that cause source-generated noise. Careful use of multiple air guns can cause destructive interference of bubble pulses and alleviate the bubble effect. A cage, or a steel enclosure surrounding a seismic source, can be used to dissipate energy and reduce the bubble effect.
bubble effect
A vibrator truck equipped with wide tires to allow access to rugged or soggy terrain while causing less damage to the environment.
buggy vibro
The ratio of stress to strain, abbreviated as K. The bulk modulus is an elastic constant equal to the change in applied pressure (∂P) divided by the ratio of the change in volume to the original volume of a body (∂V/V). K = −V (∂P / ∂V), whereK = bulk modulusV = volume∂P = partial derivative of pressure∂V = partial derivative of volume.
bulk modulus/ modulus of compression
The cable on which wireline logging tools are lowered into the well and through which signals from the measurements are passed. The cable consists of a central section with conductors surrounded by a metal, load-bearing armor.
cable
A bundle of electrical wires that connects geophones, or the entire carrier system for marine hydrophones, which includes the hydrophones, the electrical wires, the stress member, spacers, the outer skin of the cable, and the streamer filler, which is typically kerosene or a buoyant plastic. The cable relays data to the seismic recording truck or seismic vessel.
cable
The process of adjusting a measurement to a standard, so that copies of the same type of logging tool or laboratory instrument will read the same. The tool or instrument is placed in the presence of a calibrator or calibrating environment, for example, a source of gamma rays for a gamma ray tool, or the air, far from the ground, for an induction tool. Calibration coefficients, typically a gain and an offset, are calculated so that the tool or instrument reads correctly in the calibrator. The coefficients are then applied during subsequent measurements. The term master calibration is used for the regular, as for example quarterly, calibration of a logging tool in the workshop. For most wireline tools, a secondary calibrator is adjusted during the master calibration and taken to the wellsite so that a wellsite calibration can be done just prior to the logging job. Some tools, such as the gamma ray, are calibrated only at the wellsite. For most measurements-while-drilling tools, the environment requires that the calibration be performed at the workshop and only a verification made at the wellsite. For some measurements, there is a primary worldwide standard against which calibrators are calibrated, as for example, the radioactive formations at the University of Houston used to define gamma ray API units.
calibration
A method of adjusting a data set against a control that has properties to which the data set should conform.
calibration
A small, electrically activated explosive charge that detonates a larger charge. Caps, also called seismic caps or blasting caps, are used for seismic acquisition with an explosive source to achieve consistent timing of detonation.
cap/ blasting cap
A type of positive gravity anomaly that results from the presence of a dense cap rock overlying a relatively low-density salt dome.
caprock effect
An implosion produced by locally low pressure, such as the collapse of a gas bubble in liquid (the energy of which is used as the source of seismic energy from air guns).
cavitation