Mike Müller-Petke

Comparison and optimal parameter settings of referencebased harmonic noise cancellation in time and frequency domains for surface-NMR

The technique of surface nuclear magnetic resonance (surface-NMR) provides information on porosity and hydraulic conductivity that is highly valuable in a hydrogeological context. However, the applicability of surface-NMR is often limited due to a bad signal-to-noise ratio. In this paper we provide a detailed insight into the technique of harmonic noise cancellation based on remote references to improve the signal-to-noise ratio. We give numerous synthetic examples to study the influence of various parameters such as optimal filter length for time- domain approaches or the necessary record length for frequency-domain approaches, all of which evaluated for different types of noise conditions. We show that the frequency-domain approach is superior to time-domain approaches. We demonstrate that the parameter settings in the frequency domain and the decision whether or not to use separated noise measurement depend on the actual noise properties, i.e., frequency content or stability with time. We underline our results using two field examples.

Two-dimensional distribution of relaxation time and water content from surface nuclear magnetic resonance

Development in instrumentation and data analysis of surface nuclear magnetic resonance has recently moved on from one-dimensional (1D) soundings to two-dimensional (2D) surveys, opening the method to a larger field of hydrological applications. Current analysis of 2D data sets, however, does not incorporate relaxation times and is therefore restricted to the water content distribution in the subsurface. We present a robust 2D inversion scheme, based on the qt approach, which jointly inverts for water content and relaxation time by taking the complete data set into account. The spatial distribution of relaxation time yields structural information of the subsurface and allows for additional petrophysical characterization. The presented scheme handles separated loop configurations for increased lateral resolution. Assuming a mono-exponential relaxation in each model cell, using irregular meshes, and gate-integrating the signal, the size of the inverse problem is significantly reduced and can be handled on a standard personal computer. A synthetic study shows that contrasts in both the quantities – water content and relaxation time – can be imaged. Inversion of a field data set outlines a buried glacial valley and allows the distinguishing of two aquifers with different grain sizes, which can be concisely interpreted together with a resistivity profile. The impact of the anisotropic weighting factor and subsurface resistivity on the inversion result are shown and discussed. A comparison of the results obtained by the previously used initial value and time-step inversion approaches illustrates the improved stability and resolution capabilities of the 2D qt inversion scheme.

Surface nuclear magnetic resonance—A unique tool for hydrogeophysics

Surface nuclear magnetic resonance (surface NMR) is the only geophysical exploration method that nondestructively provides direct information on subsurface aquifer properties (i.e., geometry, water content, hydraulic conductivity, and, partially, resistivity). The method combines the information accessible via nuclear magnetic resonance (NMR) measurements with a nondestructive surface acquisition approach to derive subsurface water information. These characteristics have made surface NMR a useful tool for hydrogeophysics during the last decade.

Reliability and limitations of surface NMR assessed by comparison to borehole NMR

The measurements of nuclear magnetic resonance (NMR) parameters to investigate petrophysical properties related to fluid (e.g., water) storage and transport processes provide unique insights compared to other geophysical methods and have become a very useful tool for geophysicists during the last decades at the laboratory scale and as a borehole tool. We investigated, at a groundwater test site in the desert of Abu Dhabi, the reliability and limitation of surface NMR, a new but establishing technique that measures the NMR parameter from the surface by comparing its results to borehole NMR logs. Surface NMR or magnetic resonance sounding measurements (MRS) were conducted along a profile, close to several boreholes. The available borehole NMR logs were used to i) evaluate the potential of surface NMR derived results comparing them with borehole NMR measurements and to ii) extend the hydrogeological knowledge of the groundwater site. Firstly, we show how to carefully handle short relaxation signals of surface NMR data. The most significant steps during this process are: i) broad-band filtering to preserve the short decaying NMR signals, ii) correction for relaxation during pulse effects and iii) QT-inversion to extract reliable subsurface parameter distribution. By comparing surface NMR results with borehole NMR logs we found the following limitations: i) surface NMR is not able to detect borehole NMR measured water content related to T2 decay times lower than ≈80 ms T2 decay time. This reduced detectable water content of surface NMR is due to an instrumental dead time of 40 ms, measured T2* relaxation times and a lower Larmor frequency of 2 kHz and ii) borehole NMR has significantly higher vertical resolution. Taking this into account, surface NMR is in good agreement with borehole NMR. Secondly, on a profile of 1.3 km length 11 MRS measurements were carried out to map the lateral aquifer structure. The obtained results show that surface NMR provides unique lateral information of demanded aquifer properties complementary to e.g., transient electromagnetic.