th  - 1988 - 51st ENC Conference

THE VISUALIZATION OF PROBE ELECTRIC FIELDS
by
190 j D.I. Hoult* and C-N. Chen
Biomedical Engineering and Instrumentation Branch, Division of Research Services,
Bldg. 13, Rm. 3W13, National Institutes of Health, Bethesda, MD 20892.
Conservative electric fields in the sample volume of NMR probes are responsible for twin
evils - the detuning of the probe upon insertion of a sample with high dielectric constant (e.g.
tissue or water, E = 80), and losses which result in sample heating and reduced signal-to-noise
ratio. Several probe designs have adopted the strategy of distributing the tuning capacitance
about the probe coil in order to reduce the electric fields and extend the usable frequency range,
but it is always difficult to know whether or not the adopted strategy has been successful, the
principal difficulty's being in distinguishing" dielectric" loss from "magnetic" (induction) loss.
Differentiation of these two mechanisms usually requires a tedious plot of loss versus
frequency, the onset of dominant dielectric loss being the point at which losses increase far more
rapidly than the usual = (02 dependency.
Working with a simple method first described by Chute and Vermeulen t, we have been able
to produce cheaply and quickly a color picture of the electric fields in a probe - an inestimable
aid in deciding whether a design is viable. The required components are a sheet of resistive paper
glued to mylar film containing temperature-sensitive liquid crystals. The combination is then
mounted as necessary to simulate the sample, and when power is applied to the probe, that
portion of the sheet that is in a sizable electric field is warmed, whereupon the crystals change
color. Clearly, contours of constant color deliniate regions of constant electric field, and
undesirable "hot spots" are quickly noticed. Thus the poster will give fabrication details, and
results from several common coil configurations will be displayed.
IF. S. Chute and F. E. Vermeulen, AJP 42, 1075-1077, 1974.
191 ] NMR ANALYSIS AND IMAGING OF OIL CORES: W. A. Edelstein .1,
H. J. Vinegar 2, P. B. Roemer 1, P. N. Tutunjian 2, and O. M. Mueller 1. (1) GE Corporate Research and
Development Center, Schenectady, NY 12345. (2) Shell Development Company, Houston, TX 77025.
Oil cores in the form of cylinders up to 6" diameter are routinely taken during oil exploration and
production. It is important to measure several petrophysical properties of the oil cores, such as oil and
water saturation, porosity and permeability. Traditional methods of obtaining such information involve hot
solvent extraction and take several days or longer. NMR can, in many cases, make the required
measurements in minutes. Imaging allows variations of the rock and fluid properties to be visualized on
the scale of mm and allows discrimination against artefacts such as fractures and invasion of the core by
drilling mud. There are a number of technical difficulties involving the NMR. In certain rock formations,
such as clean sandstones and carbonates, the NMR linewidth is narrow and the oil and water saturation
can be easily separated by chemical shift spectroscopy or imaging. In other cases, such as shaly
formations, the water and oil linewidths may be inhomogeneously broadened. NMR imaging under these
circumstances requires fast and strong magnetic field gradients. We have shown that the NMR signals in
some of the shaly sandstones can be refocussed and should be imageable with very fast data acquisition.
We are presently examining the relaxation mechanism in sandstones by comparative proton/deuterium
studies, and are investigating the suitability of other nuclei (i.e. carbon and sodium) for
measurementftmaging of core properties.
194