Gamma ray attenuation

To determine the source intensity in practice and to correct for the attenuation in the liner walls first no core and then an empty core liner are placed between the gamma ray source and detector to measure the intensities (1 ) and (1, ). The difference (1 - 1, ) replaces the source 

Porosities are derived by rearranging equation 2.3 and assuming a grain density (p ). 

The specific Compton mass attenuation coefficient (p) is a material constant. It depends on the energy of the gamma rays and on the ratio (Z/A) of the number of electrons (Z) to the atomic mass (A) of the material (Ellis 1987). For most sediment and rock forming minerals this ratio is about 0.5, and for a Cs source the corresponding mass attenuation coefficient (p ) for sediment grains is 0.0774 cm g. However, for the hydro- 

The precision of wet bulk densities can be slightly improved, if the iterative scheme for the mass attenuation coefficient is applied (Fig. 2.5a). For core PS 1725-2 wet bulk densities computed with a constant mass attenuation coefficient are compared with those derived from the iterative procedure. Below 1.9 g cm the iteration produces slightly smaller densities than are determined with a constant mass attenuation coefficient and are 

If both data sets are plotted versus the wet bulk densities of the discrete samples the optimization essentially becomes obvious for high densities ( 2.0 g cm Fig. 2.5b). After iteration densities are slightly closer to the dotted 1 1 line. 

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Other Methods, Gamma-ray attenuation measures the density of the sample, which is related to changing oil or water content. Gamma-ray density meters are quite common in process monitoring, but they are useful for emulsion characterization only in cases where the solids content is known to be zero or completely constant (33). Otherwise the density information obtained cannot be reliably related to oil or water content. 

Charlton, B. Newton, J.M. Application of gamma-ray attenuation to the determination of density distributions within compacted powders. Powder Technol. 1985, 41, 123-134. 

In this demonstration a host of assumptions and simplifications have been made, which in practice can be corrected for accurately, shown to be unimportant, or made to cancel in the design of the measurements. These include neutron and gamma-ray attenuation in the sample, spatial gradients and temporal changes in neutron flux, differences in counting efficiency between sample and standard, multistep nuclear reactions, and instrumental nonlinearities at high counting rates. 

For the fluidized bed system illustrated in Fig. 9.1, the materials that cause gamma ray attenuation are the bed particles and the fluidization column wall. Thus, Eq. 9.2 can be expressed as... 

A monotonic relationship between intensity (i.e., count rate) and distance between the tracer and each detector was established by calibration. The density dependence of gamma ray attenuation through the bed made it necessary to calibrate in situ because of the inhomogeneity of the bubbling fluidized bed. The procedure involved positioning the tracer in a large number of distributed locations within the bed and then measuring the count rates of all detectors at each tracer location. 

A comparison of wet bulk densities derived from gamma ray attenuation with those measured on discrete samples is shown in Figure 2.4a for two gravity cores from the Arctic (PS 1725-2) and Antarctic Ocean (PS 1821-6). Wet bulk densities,... 

A detailed comparison of both data sets is shown in Figure 2.4b for two segments of core PS 1725-2. Wet bulk densities measured on discrete samples agree very well with the density log derived from gamma ray attenuation. Additionally,... 

Fig. 2.4 Comparison of wet bulk densities determined on discrete samples by weight and volume measurements and calculated from gamma ray attenuation, (a) Cross plot of wet bulk densities of gravity cores PS 1821-6 from the Antarctic and PS1725-2 from the Arctic Ocean. The dashed lines indicate a difference of 5% between both data sets, (b) Wet bulk density logs derived from gamma ray attenuation for two 1 m long core sections of gravity core PS1725-2. Superimposed are density values measured on discrete samples. Modified after Gerland and Villinger (1995).

Fig. 2.5 Influence of an iterative mass attenuation coefficient determination on the precision of wet bulk densities. The gamma ray attenuation log of gravity core PS 1725-2 was used as test data set. (a) Wet bulk densities calculated with a constant mass attenuation coefficient ( processing porosity =50%) are displayed versus the data resulting from die iteration. A pore fluid density of 1.024 g cm and a constant grain density of 2.7 g cm were used, and the iteration was stopped if densities of two successive steps differed by less than 0.1%o (b) Cross plot of wet bulk densities measured on discrete samples versus wet bulk densities calculated from gamma ray attenuation with a constant mass attenuation coefficient (O) and with the iterative scheme (+). (c) Influence of grain density on iteration. Three grain densities of 2.65, 2.75 and 2.1 g cm were used to calculate wet bulk densities. Modified after Gerland (1993).

The ratio of the resistivity (R ) in sediment to the resistivity (R. ) in pore water defines the formation (resistivity) factor (F). (a) and (m) are constants which characterize the sediment composition. As Archie (1942) assumed that (m) indicates the consolidation of the sediment it is also called cementation exponent (cf. Sect. 3.2.2). Several authors derived different values for (a) and (m). For an overview please refer to Schon (1996). In marine sediments often Boyce s (1968) values (a = 1.3, m = 1.45), determined by studies on diatomaceous, silty to sandy arctic sediments, are applied. Nevertheless, these values can only be rough estimates. For absolutely correct porosities both constants must be calibrated by an additional porosity measurement, either on discrete samples or by gamma ray attenuation. Such calibrations are strictly only valid for that specific data set but, with little loss of accuracy, can be transferred to regional environments with similar sediment compositions. Wet bulk densities can then be calculated using equation 2.3 and assuming a grain density (cf. also section 3.2.2). 

Though resistivities are only measured half-automatically including a manual insertion and removal of the probe, increments of 1 - 2 cm can be realized within an acceptable time so that more fine-scale structures can be resolved than by analysis of discrete samples. However, the real advantage compared to an automated gamma ray attenuation logging is that the resistivity measurement system can easily be transported, e.g. 

Fig. 2.9 Model based computation of a wet bulk density log from resistivity measurements on ODP core 690C. (a) Porosity log derived from formation factors having used Boyce s (1968) values for (a) and (m) in Archie s law. (b) Carbonate content (O Conell 1990). (c) Wet bulk density log analyzed from gamma ray attenuation measurements onboard of JOIDES Resolution (gray curve). Superimposed is the wet bulk density log computed from electrical resistivity measurements on archive halves of the core (black curve) having used the grain density model shown in (d). Unpublished data from B. Laser and V. SpieB, University Bremen, Germany.

Bodwadkar S.V., Reis J.C., 1994. Porosity measurements of core samples using gamma-ray attenuation. Nuclear Geophysics 8 61-78... 

Boyce R.E., 1976. Definitions and laboratory techniques of compressional sound velocity parameters and wet-water content, wet-bulk density, and porosity parameters by gravity and gamma ray attenuation techniques. In Schlanger S.O., Jackson E.D. et al (eds) Initial Reports of the Deep Sea Drilling Project 33, Washington, US Government Printing Office, pp 931-958... 

Gerland S., Villinger H., 1995. Nondestructive density determination on marine sediment cores from gamma-ray attenuation measurements. Geo-Marine Letters 15 111-118... 

Whitmarsh R.B., 1971. Precise sediment density determination by gamma-ray attenuation alone. Journal of Sedimentary Petrology 41 882-883... 

A direct method is the analysis by weight and volume, indirect methods are gamma ray attenuation and electrical resistivity measurements. 

Analysis by weight and volume primarily determines the wet bulk density, gamma ray attenuation the wet bulk density (by assumption of a processing porosity), and electrical resitivity the porosity. Porosity values can be converted to density values (and vice versa) by using equation 2.3. 

The third correction factor, which is the ratio of the adsorbed dose buildup factors in the sample and the dosimeter, is usually ignored, but is shown in this paper to be very important. The absorbed dose buildup factor is defined in this paper analogous to the dose buildup factor, a notation used when the unit roentgen was still the unit of radiation dose. This paper shows the magnitude of this third correction factor, which is caused by differences in gamma-ray attenuation coefficients and softening of the gamma-ray spectrum. As an illustrative example, the dose in different dosimeters is calculated as a function of the distance from a point isotropic cobalt-60 source in water. 

Still other problems can arise when a radionuclide has not been equilibrated with its carrier, when a radiochemical separation is carried out imperfectly, or when the counting efficiency differs for sample and standard (different shapes or sizes, different gamma ray attenuation). 

On-line moisture monitors, which are based on infra-red absorption, nuclear magnetic resonance, capacitance and microwave attenuation are used in numerous process industries. The applicability of these techniques to the coal preparation industry has been reviewed (13,14). The CONAC and "Elemental Analyzer" units use a microwave attenuation method for the moisture measurement. This information is used to correct the PNAA hydrogen assay data in order to calculate the hydrogen content of the coal. Kay-Ray, Inc., (Arlington Heights, Illinois) has developed an instrument that measures the moisture content of a coke stream and uses a combination of microwave and gamma ray attenuation measurements. 

Gamma ray attenuation Absorption of gamma rays passing through the sediment is caused by Compton attenuation (scattering), pair production, and by photoelectric absorption. The amount of gamma ray attenuation is a measure of bulk density, from... 

Gamma ray attenuation porosity evaluator (GRAPE) Instrument that measures gamma ray attenuation through unsplit cores and provides an estimate for bulk density, water content, and porosity. 

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