Geophysical Techniques - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 209

function of two processes: relaxation in the bulk fluid and relaxation on the
solid pore surfaces. Surface relaxation is the faster of the two processes,
dominating the response, and leading to a relationship between relaxation time
and the ratio of the pore's surface area to volume, which is related to grain size.
An empirical correlation between rock type and decay time was used by Shirov
et al. (1991) to estimate grain size. Minimization of the misfit between the
calculated and measured responses is via inversion is used to extract both the
total amplitude and the relaxation-time constants.
Paramagnetic species (such as Fe 3+ ) can cause dramatic changes in T 2 so
that the direct nuclear magnetic resonance link to the ratio of the surface area to
volume breaks down. These effects, which make it much more difficult to
obtain estimates of permeability, were examined by Bryar et al. (2000) and
Knight et al. (1999). For example, two sands whose pore size and distribution
and grain size are identical could appear to have different permeabilities if one
had a high Fe 3+ content and the other did not. The variation in the content of Fe 3
+ and other paramagnetic species could complicate or negate permeability
estimates based on NMR data for near-surface applications. T 2 would be
affected, but to a different extent, depending on the specific location of the Fe 3+
(i. e., in pore water, adsorbed to the solid phase, or in a solid mineral grain).
BOREHOLE GEOPHYSICS
The vast majority of surface geophysical techniques can be modified for
application in borehole environments. This includes resistivity, electromagnetic,
gravity, radar, radiometric, and seismic methods (e.g., Daniels and Keys, 1990;
Howard, 1990).
In some cases, borehole geophysical measurements are made to help tie the
borehole samples to surface geophysical data. In other cases, logs are used to
help interpret the samples themselves. For example, in the petroleum industry,
to calculate the hydrocarbon concentration, the- resistivity log is used to infer
the percentage of saturation with hydrocarbons, where the salinity of the
formation water is known.
Sometimes a particular log will be diagnostic in a particular environment,
and other times the geology will defy rational analysis by even the most
sophisticated suites of logs. On balance, however, it is remarkable how much
geologic information can be derived from simple suites of logs, given the gross
physical assumptions that are made in logging.
Properties that can often be directly or indirectly determined from borehole
geophysics include, but are not limited to, the following: