Deformation and failure of rock samples probed by T1 and T2 relaxation.
The objective of the study was to pinpoint the effect of stress induced rock matrix alterations on NMR-wireline-log measurements by means of laboratory T1 and T2 relaxation time measurements. The research activities were subdivided into two major parts: NMR relaxation measurements on a brine saturated outcrop sandstone (Red Wildmoor Sandstone) during uniaxial compressional tests and NMR relaxation measurements on artificial sandstone samples prepared with defined crack patterns. T1-measurements performed on Red Wildmoor samples during compaction showed a decrease in the mean relaxation rate 1/exp<ln (T1) up to a point assumed to characterize the yield strength of the rock. The rate decreased most probably as a result of the compaction of micropores. This trend was reversed when the rock failed. The relaxation rate increased again, most probably through the generation of fresh mineral surfaces at broken cementations or grain contacts. T2-relaxation was measured on artificial sandstone samples using CPMG pulse sequences with echo times between 175 micros and 1400 micros. Samples with larger crack volumes and crack orientation orthogonal to the B0-field showed consistently smaller exp<ln (T2) than the samples without any cracks. Further calculations indicate that internal magnetic field gradients were largest for samples having a larger number of cracks and crack orientations orthogonal to the B0-field.
Novel Multidimensional Inverse Laplace Nuclear Magnetic Resonance Spectroscopy Techniques
This thesis presents the new development and application of multidimensional inverse Laplace nuclear magnetic resonance spectroscopy techniques. We present a new NMR technique which relates the longitudinal relaxation rate of the NMR signal to the internal gradients in the sample. We perform the experiment on a large range of magnet strengths to provide experimental evidence for the theory of how internal gradient intensity scales with pore size as a function of field strength. We make the first attempt of quantisation of two dimensional inverse Laplace experiments. We perform a transverse relaxation exchange experiment on several samples for a range of mixing times. We then integrate the peaks in the resulting spectra and plot them as a function of mixing time. By fitting the experimental results to theory, we can estimate the molecular exchange between pores of differing sizes. We then modify the transverse relaxation experiment to include diffusion attenuation so that we can see the separate signals for oil and water. We use this to look at the effect wettability has on the movement of the different fluids between pores. We then present the first experiment to combine two inverse Laplace dimensions with a Fourier dimension. We add a propagator dimension to the transverse relaxation exchange experiment to measure how far the molecules move during the mixing time. Quantisation of the results allows us to estimate the exchange rate between pores of similar sizes in addition the exchange rate between pores of different sizes. We are also able to estimate pore radii, inter-pore spacing and tortuosity. Lastly, we attempt a three dimensional inverse Laplace experiment by correlating transverse relaxation, diffusion, and internal gradients. While the three dimensional inversion techniques require more development, the results show resemblance to those seen from two dimensional experiments.
Archaeological Survey of Dangoyama Kofun by 3DGPR
We presented a three-dimensional ground penetrating radar (3DGPR) system for the archeological investigation in a harsh site, where we have to deal with many trees and/or great topographic variations. This system consists of a commercial GPR system with a laser positioning system. With this system, we can move the GPR antenna freely, on which a positioning detector can record accurate position for each GPR data trace. We applied our 3DGPR system at Dangoyama Kofun, a burial mound, in Fukushima prefecture, Japan. A lot of trees and bushes covered the survey area and cannot be cut. We acquired six datasets in two days in this area. We successfully imaged the boundary between the original surface of the burial mound and the covering soil clearly at about 0.3 m depth around the mound. Furthermore, we found some strong reflections appeared at 0.8 m depth in the top area of the mound, which can be from the artificial parts of the burial mound. The results would greatly aid the following archaeological excavation. In addition, we compared the non-interpolated depth slice with interpolated depth slice at same position and demonstrated that the interpolation process on threedimensional GPR data may introduce some artifacts which might mislead our interpretation.