2 Physical Properties of Marine Sediments - Blogs Unpad

2.2 Porositiy and Wet Bulk Densitythe centre of the coil (Gerland et al. 1993). Thisinduced electric field contains information on themagnetic and electric properties of the sediment.Generally, the coil characteristic is defined bythe quality value(Q)L( ω)Q = ω ⋅(2.13)R ( ω)(L(ω)) is the inductance, (R(ω)) the resistance and(ω) the (angular) frequency of the alternatingcurrent flowing through the coil (Chelkowski1980). The inductance (L(ω)) depends on the numberof windings, the length and diameter of thecoil and the magnetic permeability of the coilmaterial. The resistance (R(ω)) is a superpositionof the resistance of the coil material and losses ofthe electric field induced in the core. It increaseswith decreasing resistivity in the sediment.Whether the inductance or the resistance is ofmajor importance depends on the frequency of thecurrent flowing through the coil. Changes in theinductance (L(ω)) can mainly be measured ifcurrents of some kilohertz frequency or less areused. They simultaneously indicate variations inthe magnetic susceptibility while the resistance(R(ω)) is insensitive to changes in the resistivityof the sediment. In contrast, operating withcurrents of several megahertz allows to measurethe resistivity of the sediment by changes of thecoil resistance (R(ω)) while variations in themagnetic susceptibility do not affect the inductance(L(ω)). The examples presented here weremeasured with a commercial system (Scintrex CTU 2)which produces an output voltage that is proportionalto the quality value (Q) of the coil at afrequency of 2.5 MHz, and after calibration isinversely proportional to the resistivity of thesediment.The induced electric field is not confined tothe coil position but extends over some sedimentvolume. Hence, measurements of the resistance(R(ω)) integrate over the resistivity distribution onboth sides of the coil and provide a smoothed,low-pass filtered resistivity record. The amount ofsediment volume affected by the inductionprocess increases with larger coil diameters. Theshape of the smoothing function can be measuredfrom the impulse response of a thin metal plateglued in an empty plastic core liner. For a coil ofabout 14 cm diameter this gaussian-shapedfunction has a half-width of 4 cm (Fig. 2.10), sothat the effect of an infinitely small resistivityanomaly is smeared over a depth range of 10 - 15 cm.This smoothing effect is equivalent to convolutionof a source wavelet with a reflectivity functionin seismic applications and can accordinglybe removed by deconvolution algorithms. However,only few applications from longcore paleomagneticstudies are known up to now (Constableand Parker 1991; Weeks et al. 1993).Fig. 2.10 Impulse response function of a thin metal plate measured by the inductive method with a coil of about 14 cmdiameter. Modified after Gerland et al. (1993).41