Een modelonderzoek naar het mechanisme van de natuurlijke verdichting van een zandgrond

In het kader van het onderzoek aan bodemverdichting na een losmakende bewerking door de afdeling bodemtechniek, wil men nagaan welke de mechanismen van natuurlijke bodemverdichting zijn

Surface nuclear magnetic resonance imaging of water content distribution in the subsurface. 1998 annual progress report

'The objective of the project is to evaluate Surface Nuclear Magnetic Resonance Imaging ( NMRI) for determining water content distribution in the subsurface. In NMRI the interaction of the magnetic moment of hydrogen ( protons) nuclei with external applied electromagnetic ( EM ) fields is measured. In surface NMRI the Earth''s magnetic field causes alignment of the spinning protons. An alternating EM field is generated by a loop of wire laid on the Earth surface. The alternating current driven through the loop at the Lamor frequency of protons in liquid water. The component of the EM field perpendicular to the Earth''s field causes a precession of protons from thier equilibrium position. Water content distribution in the subsurface is derived from measurements on the EM field caused by the return of the precessing protons to equilibrium after the current in the transmitter loop is terminated. The scientific goals of the R and D are: to verify and validate the theoretical concepts and experimental results of Russian scientists, who first introduced this method; to evaluate the range of applications and limitations of this technology for practical field measurements. NMRI has the potential of providing a remote, direct, unique method for subsurface water measurements. All present methods are either intrusive or indirect ( e.g. electrical resitivity measurements). In the past year progress has been made along two separate paths. These are: (1) Field Measurements. Surface NMRI equipment manufactured by IRIS Instruments of France was tested over a number of sites with good hydrogeologic control. The results of these measurements can be summarized as follows: The NMRI measurement directly and uniquely determines water distribution in coarse grained aquifers; geologic formation from which water can be readily withdrawn. Water content can not be determined by this technique in fine grained sediments. The signal to be measured is very small and EM interference''s from power lines makes NMRI a difficult measurement in an urban setting. The presence of minerals with a high magnetic susceptibility interfere with reliable measurements. (2) Theoretical Computations. The inversion of the experimental measurements requires the computation of the EM field within the Earth. The authors have extended these computations with the design of fast algorithms for computing the EM field for Earth stratified in electrical resitivity.

Nuclear magnetic resonance imaging of subsurface water content. 1997 annual progress report

'During the period from October 1997 to January 1998 the author has further developed the understanding of NMR physics, improved software for forward and inverse modeling of the NMR signal, and conducted field tests on sites in Colorado and New Mexico. One important result from the forward modeling was that the field strength of the signals is concentrated under the loop. This indicates that little lateral dissipation occurs. The author received the NUMIS/NMR system (manufactured by IRIS Instruments, France) in late July, 1997. In July and early August, 1997, potential test sites were visited, and several test sites were selected and permitted. The first NMR test measurements were made in mid-August, 1997. The instrument malfunctioned during mid-September, 1997, and was returned to IRIS for repairs. Time lost due to malfunction, repairs, and shipping was about one month. Many NMR measurements have been made at sites in Colorado and New Mexico. Parks often have been selected as test sites due to ease of permitting, the relatively large open space, and general lack of powerlines. Noise from power lines severely degrades the NMR data quality. The NMR data acquired at the first three sites in Colorado (Bear Creek, Clear Creek, and Prospect) was either severely distorted by powerline noise or did not indicate significant groundwater occurrences. The NMR data taken at Cherry Creek were of good quality and also indicated significant groundwater. The NMR data acquired at three sites with relatively shallow ground water levels around Socorro, New Mexico, did not detect any ground water due to severe signal distortion by magnetite, a magnetic mineral. Measurements in a compact sand stone near Santa Rosa and in a limestone near Artesia, New Mexico, gave excellent results. Overall, the NMR technique proves capable of detecting subsurface ground water under the right conditions: little noise from power lines and absence of magnetite.