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D
DÕA: Digital-to-Analog q.v. . D & A: Dry and Abandoned; refers to an abandoned well. daisy chain: A method of propagating signals along a bus, often used in applications in which devices not requesting a signal respond by simply passing the signal on. The first device requesting the signal breaks the daisychain continuity. A daisy-chain scheme assigns priorities based on the electrical positionof a device along the bus. damped error multipliers: The ratio of damping factors to the spectral value, indicating the change of parameters during successive iterations. See Raiche et al. 1985 . damping: 1. A slowing down or opposition to oscillation caused by dissipation of the oscillation energy. „a… Critical damping, c , is the minimum damping that will prevent oscillation from taking place.„b… The damping factor is the ratio of the system friction to that necessary for critical damping, or the quotient of the logarithm of the ratio of two successive oscillations if the system is underdamped. The damping factor is one for critical damping, less than one for an underdamped system which will oscillate , and greater than one for an overdamped system. See Figure D-1. „c… Most geophones areslightly underdamped, often having optimum damping that is 0.66 c , which provides the most rapid convergence. 2. Site damping is concerned with natural resonances and the response to standing-waves. Site damping can be thought of as either the rate of amplitude decrease after cessation of excitation or as the decrease in amplitude response as the excitation frequency differs from the resonantfrequency. Measuring damping and Q generally requires the use of controlled vibrators. Daniell window: dan’ y l See Figure W-12. ¯ darcy: dar’ se A unit of permeability; the permeability that will allow a fluid flow of one milliliter per second of one centipoise viscosity fluid through one square centimeter under a pressure gradient of one atmosphere per centimeter. The unit commonly used is themillidarcy. Darcy’s law: A relationship for 1D fluid flow rate q:

q

kA/

p/ x,

where k permeability, A cross-sectional area, viscosity, and p pressure differential across the thickness x. For radial flow into a cylindrical borehole,

q

k/

2 rh

/ r ,

where r radial distance and h height of the cylinder. dar Zarrouk: dar zar ruk’ The name given by Maillet to resistivity parameters orcurves that deal with layered anisotropic materials. The dar Zarrouk variable is the transversal unit resistance ( T ), the depth integral of the transverse resistivity perpendicular to the strata; the dar Zarrouk function is the longitudinal unit conductance (1/ L ), the depth integral of the conductivity parallel to the strata; and the dar Zarrouk curve is a plot of the mean resistivity ( T )of the formation down to the depth z, plotted versus the anisotropy coefficient times z( T L ) 1/2 :
T iz i

and 1/

L

z i/ i,

FIG. D-1. Damping. (a) Effect of damping on an impulsive input. (b) Effect of damping on the amplitude of a periodic input. 78

where z i are layer thicknesses. See Maillet 1947 . DAS: Deconvolution After Stack. data bank: A database q.v. . database: 1.Computerized record-keeping system with the purpose of maintaining information and making it available on demand. 2. A collection of data organized and managed by a central facility, not necessarily digital or computer based. A database management system „DBMS… is a centralized computer facility to manage and provide consistent and secure access to a database. data compression: A scheme for packing datainto a smaller space. data dimensionality: The smallest number of variables that could be used to represent the data in a set. data fusion: Merging data, often at different scales. data harvesting: 1. Retrieving data that has been stored temporarily at a geophone or ocean-bottom seismom-

data influence matrix

79

decay curve

eter or other type of station. 2. Collection of data from a...
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