Joint inversion of Surface Nuclear Magnetic Resonance and Vertical Electrical Sounding

During the excavation of underground tunnels and ore mining, accidents related to sudden water inflows often occur. As the only technique used for the direct detection of groundwater, nucl...

First Evidence of the Detection of an Underground Nuclear Magnetic Resonance Signal in a Tunnel

The water content of underground structures can be imaged using surface nuclear magnetic resonance (NMR) in one dimension, two dimensions, or recently, three dimensions. The maximum efficiency can be obtained with a coincident loop configuration in a large-scale area, although the resolution is limited. We have developed a quasi-2D block inversion to characterize a horizontal continuous underground aquifer using a laterally constrained model to improve the resolution of thin aquifers and deep aquifers. Through the simulation of flat surface and irregular surface models, a horizontally layered aquifer with continuously variable water content and T2* can be imaged through quasi-2D block inversion, which is more optimized than the traditional 1D block or 1D or 2D smooth inversion methods. Finally, using a series of profile data obtained from surface NMR field measurements in Mongolia, we determine that quasi-2D block inversion based on a laterally constrained model can significantly improve the disco...

Quasi-2D block inversion of large-scale surface nuclear magnetic resonance profile data using a laterally constrained model

Surface nuclear magnetic resonance (surface NMR) has up to now rarely been applied to 3D subsurface modeling. Inversion approaches currently in use are smooth inversion techniques that are not useful for identifying sharp geologic boundaries. Although they are already computationally expensive, the resulting models are restricted to imaging the subsurface water content distribution and do not deliver relaxation times T2* based on the QT inversion scheme established elsewhere. We have developed a method of 3D block QT inversion that uses horizontal smoothness constraints to resolve sharp boundaries in the vertical direction and the distributions of the water content and relaxation time T2*. We have improved the computational efficiency, i.e., the ability to perform the inversion using a common desktop computer, by gating the surface NMR data, reducing the model space to monoexponential decays within the subsurface bodies, and inverting based on blocklike structures instead of smooth distributions. ...

3D block QT inversion of surface nuclear magnetic resonance data

A combined geophysical survey has been conducted at the test site Nauen near Berlin, prospecting a shallow aquifer in fluvial sediments bordered by glacial till. Some of the geophysical results have been reported earlier [1]. Comprehensive investigations of geoelectric sections and depth-soundings as well as georadar profiles were interpreted to describe subsurface conditions. The structure of the dipping glacial till is well recognisable up to a depth of 13 m. The sandy sediments above represent the aquifer with the water table at a depth of about 2 m. The radar velocities and resistivities from Block-Inversion of geoelectric sections have been used to derive hydrological parameters. The aquifer is accordingly determined to have a porosity around 24 % and a water content of 5 % in the vadose zone. At the main profile, five SNMR soundings have been carried out in order to compare the results with those from conventional geophysical methods. Inversion results show mobile water contents of less than 20 % in the first few meters of the vadose zone, increasing rapidly to 30 %. This means an aquifer with an effective porosity of at least 30 % plus adhesive water content. Decreasing water contents below 15-18 m relate to the glacial till which has low permeability and less mobile water. Increasing water contents in larger depth again to values above 30 % indicates another aquifer which is not verified with other methods yet. The decay times within the aquifer are generally about 150-250 ms and correspond in average to medium sands [2]. However, in detail they are somewhat ambiguous.

M. Hertrich

Papers

Examination of surface-NMR within an integrated geophysical survey in Nauen, Berlin