Aquifer characterization using elastic full-waveform inversion

This study exploits the benefits of elastic full-waveform inversion (FWI) as an effort to improve aquifer characterization for better understanding of their properties and delineating their structure. Conventional methods (e.g. boreholes, pumping tests) have usually one-dimensional nature and cannot provide information concerning the lateral heterogeneities of such complex subsurface environments. In the past, many studies were conducted using ground-penetrating radar (GPR), electrical resistivity tomography and seismics which result in two-dimensional tomographic images and provide spatially highly resolved mapping of aquifer heterogeneities. In this research, we focus in the seismic method and in particular we apply FWI to seismic data acquired at the Krauthausen test site. We compare our findings with borehole data and GPR FWI from previous studies. We conclude that combining the results of FWI with additional geophysical techniques can provide more reliable subsurface models and reduce uncertainties on reconstructing the aquifer architecture with higher spatial resolution. INTRODUCTION A detailed characterization of aquifers strives to improve the prediction of groundwater flow and to evaluate possible contaminant hazards. Conventional methods (Döring, 2002) either have limited spatial sampling volume but high resolution (e.g. boreholes), or they record an average response over a large volume (e.g. pumping tests). The inherently one-dimensional nature of these methods cannot provide information concerning the lateral heterogeneities of such subsurface environments. Doetsch et al. (2010) and Klotzsche et al. (2015) studied the use of ground penetrating radar (GPR) and electrical resistivity tomography (ERT) methods to obtain spatially highly resolved mapping of aquifer heterogeneities, taking advantage of the two-dimensionality of the tomographic images provided by these methods. Along with GPR and ERT, shallow seismic applications are also important for hydrogeological site characterization. In particular fullwaveform inversion (FWI; Tarantola, 1984) allows us to derive information of the elastic parameters and reconstruct accurate subsurface models that describe our measured data. Several studies revealed the applicability of FWI in the case of near-surface applications (Köhn et al., 2012, Athanasopoulos and Bohlen, 2017b, Groos et al., 2017 andWittkamp et al., 2019). In this study, we demonstrate the value of using elastic FWI of seismic data acquired at the Krauthausen test site for improved aquifer characterization. We conclude that combining the results of FWI with additional geophysical techniques can provide more reliable results and reduce uncertainties on reconstructing the aquifer architecture with higher spatial resolution. STUDY AREA The location of the survey is the Krauthausen test site, situated in the North-West Germany between the cities of Jülich and Düren (figure 1a). A detailed description of the test site, which was set up in 1993 by the research center Jülich is given by Döring (2002). According to Döring (2002) the subsurface on top of the aquifer can be divided into three layers (figure 2, right column): (a) a bottom layer composed of sandy 88 Annual WIT report 2018 to gravelly grain size (from 6 to 11.5 m depth), (b) a well sorted sand layer (from 4 to 6 m depth), and (c) a poorly sorted gravel layer (from 1 to 4 m depth). The base of the aquifer is formed by thin layers of clay and sand, at approximately 12 m depth. The groundwater level shows variations from 1 to 3 m depth depending on the annual season. The aquifer material is composed of alluvial terrace sediments, deposited by the river Rur. Numerous studies were performed in the area (Döring, 2002; Klotzsche et al., 2015; Gueting et al., 2015) with the goal to study the spatial distribution of aquifer parameters. The true aquifer architecture exhibits lateral variations in layer thickness and properties and additional information from various geophysical methods is necessary for the adequate characterization of these spatial distributions. Klotzsche et al. (2015) studied the area extensively by applying GPR full-waveform inversion and revealed the heterogenous nature of the subsurface (figure 1c). The focus of our study lies on characterizing the aquifer by applying elastic FWI (seismic) and reconstruct the P-wave velocity (vp), S-wave velocity (vs) and density (ρ) models of the subsurface. The seismic line was choosen in the center of the profile crossing several borehole and GPR transects (figure 1b), in order to allow a further qualitative comparison between the various methods. (Döring, 1997) and by a mean total porosity of 26% (Vereecken et al., 2000). The mean hydraulic conductivity based on the results of a large scale pumping test is 3.8 ! 10" 3 m/s (Englert, 2003). It is important to note that the generalized aquifer cross-section shown in Fig. 2a represents a simplified conceptual model. Treating the aquifer as a perfectly stratified medium is an oversimplification of the aquifer’s true structure (at Krauthausen and elsewhere). For the true aquifer architecture we expect significant lateral variations in layer thickness and properties, as well as the existence of discrete non-layered structures. Characterization thereof is what the present study focuses on. 2.2. Crosshole ground penetrating radar tomography 2.2. 1. Data acquisition GPR data were acquired along a transect in the central part of the test site, where six closely spaced boreholes were available (Fig. 1). In total, the transect covers a lateral distance of 22.7 m and consists of five adjacent crosshole tomographic planes with individual borehole separations ranging from 2.6 to 6.2 m. We used the Sensors and Software pulseEKKO Pro system with 200 MHz antennas and a semi-reciprocal acquisition setup, with transmitter and receiver spacing of 0.5 m and 0.1 m, respectively (Fig. 2b). This setup was recently found to represent a compromise between sufficient ray coverage for high resolution inversion results, on the one hand, and affordable acquisition time and computational costs, on the other hand (Klotzsche et al., 2010; Oberröhrmann et al., 2013). In order to avoid refracted/reflected radar waves associated to the groundwater table (Klotzsche et al., 2010, 2012), GPR data were acquired only within the saturated zone. Measurements were conducted from 3 m depth, which was around 1 m below the groundwater table during the measurements, down to approximately 8–10 m depth, depending on the depth of the boreholes. Note that the transmitter and receiver positions within the 10 15 20 25 30 35 92 94 96 98 100 102