oa MRS Tests in Eastern Canada - Coping with Magnetite Dissemination

From late 1990s to early 2000s several MRS (Magnetic Resonance Sounding) missions in Europe and Southern Africa have shown the appropriateness of the MRS technique for groundwater studies. However MRS tests in Eastern Canada in 2003, did not show significant NMR responses over shallow, saturated, clean sand targets. Upon examination, it was found that disseminated magnetite was likely the cause of high magnetic gradients at the scale of water containing pores. This was later confirmed at the Rice U. NMR lab (Hirasaki, personal communication 2004).<br>the 2003 tests were documented in a paper with a recommendation to focus on T2 rather than T2*. Thanks to IRD, a spin echo mode of MRS survey was implemented on a light version MRS tool: NUMISLITE. This implementation was field tested in August 2008 over most of the sites tested in 2003. Field NMR spin echo responses were observed over these sites. All the tested sites represented shallow aquifers.<br>in the mean time numerical modeling of the field-scale spin echo mode was developed at IRD, Grenoble, France.<br>in a subsequent MRS mission - summer 2010 - a full power MRS tool (NUMISPLUS) was used to demonstrate that no spurious surface effects were observed during the 2008 tests. Sites in the same geological environment, with magnetite dissemination, were systematically investigated with aquifers at least 10 m below surface. Also a key site made in 2008 was repeated to insure instrumental integrity of the procedure. Clear NMR spin echo responses were observed at all the tested sites. <br>As a side issue, field data acquisition was also modified to allow acquiring MRS data at locations where the Larmor frequency is an harmonic of the local power line frequency.<br>All in all, the MRS technique is gradually improving toward a maturity status.