Hydraulic properties at the North Sea island of Borkum derived from joint inversion of magnetic resonance and electrical resistivity soundings

Abstract

Abstract. For reliably predicting the impact of climate changes on salt/freshwater systems below barrier islands, a long-term hydraulic modelling is inevitable. As input we need the parameters porosity, salinity and hydraulic conductivity at the catchment scale, preferably non-invasively acquired with geophysical methods. We present a methodology to retrieve the searched parameters and a lithological interpretation by the joint analysis of magnetic resonance soundings (MRS) and vertical electric soundings (VES). Both data sets are jointly inverted for resistivity, water content and decay time using a joint inversion scheme. Coupling is accomplished by common layer thicknesses.We show the results of three soundings measured on the eastern part of the North Sea island of Borkum. Pumping test data is used to calibrate the petrophysical relationship for the local conditions in order to estimate permeability from nuclear magnetic resonance (NMR) data. Salinity is retrieved from water content and resistivity using a modified Archie equation calibrated by local samples. As a result we are able to predict porosity, salinity and hydraulic conductivities of the aquifers, including their uncertainties.The joint inversion significantly improves the reliability of the results. Verification is given by comparison with a borehole. A sounding in the flooding area demonstrates that only the combined inversion provides a correct subsurface model. Thanks to the joint application, we are able to distinguish fluid conductivity from lithology and provide reliable hydraulic parameters as shown by uncertainty analysis.These findings can finally be used to build groundwater flow models for simulating climate changes. This includes the improved geometry and lithological attribution, and also the parameters and their uncertainties.

Numerical modelling of climate change impacts on freshwater lenses on the North Sea Island of Borkum using hydrological and geophysical methods

“…Both are related to the hydraulic conductivity of the aquifer and can, after calibration, provide non-invasive estimates of the hydraulic parameters (Günther and Müller-Petke, 2012). In a preliminary study, four soundings have been carried out, one of them at the CLIWAT II borehole and one at an existing well.…”

“…The measurements were jointly inverted with vertical electrical soundings. As a result, lithology can be differentiated from salinity and total dissolved solids (TDS) concentrations as well as sediment porosities can be predicted (Günther and Müller-Petke, 2012). However, due to the extremely limited data, the results were solely used for corroboration of the model results.…”

On determining uncertainties of magnetic resonance sounding estimated transmissivities for groundwater modeling

“…Plata and Rubio (2008) provide a detailed analysis of the importance of the uncertainty of transmissivities estimated using hydrological methods, but they do not give a detailed description on how to take this uncertainty into account. One of the few studies documenting the methodology used for estimating MRS-based transmissivity uncertainties is the study of Günther and Müller-Petke (2012). However, they do not take into account the potential MRS parameter correlation between, e.g., layer thickness and water content, which may or may not be profound.…”

“…References suggesting how uncertainty of transmissivity (or hydraulic conductivity) can be estimated from MRS are numerous (e.g., Legchenko et al, 2004;Chalikakis et al, 2008;Plata and Rubio, 2008;Boucher et al, 2009;Günther and Müller-Petke, 2012;Vilhelmsen et al, 2014) but the methods or their documentation are often incomplete. In some references, it is just stated that the uncertainty given for the MRS-estimated transmissivities takes into account the uncertainty of the MRS data, MRS parameters, and transmissivity estimates (Legchenko et al, 2004;Boucher et al, 2009), but the method is not documented or explained.…”

Quantifying aquifer properties and freshwater resource in coastal barriers: a hydrogeophysical approach applied at Sasihithlu (Karnataka state, India)

“…Vouillamoz et al (2007) has proposed a more comprehensive approach to estimate the 1-D properties of coastal aquifers but it was applied in confined aquifer conditions, thus concerning elastic storage and not specific yield. More recently, Günther and Müller-Petke (2012) presented an improved methodology to estimate hydraulic properties of an island unconfined aquifer. This paper presents a ready-to-use methodology for estimating not only hydraulic properties of coastal aquifer but also for mapping fresh groundwater reserve and estimating recharge in unconfined conditions.…”