TY - ABST

T1 - Full-waveform Inversion of Crosshole GPR Data Collected in Strongly Heterogeneous Chalk

T2 - EGU General Assembly 2015

AU - Keskinen, Johanna

AU - Zibar, Majken Caroline Looms

AU - Nielsen, Lars

AU - Klotzsche, Anja

AU - van der Kruk, Jan

AU - Moreau, Julien

AU - Stemmerik, Lars

AU - Holliger, Klaus

PY - 2015

Y1 - 2015

N2 - Chalk is an important reservoir rock for hydrocarbons and for groundwater resources for many major cities. Therefore, this rock type has been extensively investigated using both geological and geophysical methods. Many applications of crosshole GPR tomography rely on the ray approximation and corresponding inversions of first break traveltimes and/or maximum first-cycle amplitudes. Due to the inherent limitations associated withsuch approaches, the resulting models tend to be overly smooth and cannot adequately capture the small-scale heterogeneities. In contrast, the full-waveform inversion uses all the information contained in the data and is able to provide significantly improved images. Here, we apply full-waveform inversion to crosshole GPR data to image strong heterogeneity of the chalk related to changes in lithology and porosity.We have collected a crosshole tomography dataset in an old chalk quarry in Eastern Denmark. Based on core data (including plug samples and televiewer logging data) collected in our four ~15-m-deep boreholes andresults from previous related studies, it is apparent that the studied chalk is strongly heterogeneous. The upper ~7 m consist of variable coarse-grained chalk layers with numerous flint nodules. The lower half of the studied sectionappears to be finer-grained and contains less flint. However, still significant porosity variations are also detected in the lower half. In general, the water-saturated (watertable depth ~2 m) chalk is characterized by high porosities,and thus low velocities and high attenuation, while the flint is essentially non-porous and has correspondingly high velocities and low attenuation. Together these characteristics form a strongly heterogeneous medium, whichis challenging for the full-waveform inversion to recover. Here, we address the importance of (i) adequate starting models, both in terms of the dielectric permittivity and the electrical conductivity, (ii) the estimation of the sourcewavelet, (iii) and the effects of data sampling density when imaging this rock type. Moreover, we discuss the resolution of the bedding recovered by the full-waveform approach. Our results show that with proper estimatesof the above-mentioned prior parameters, crosshole GPR full-waveform tomography provides high-resolution images capturing a high degree of variability that standard methods cannot resolve in chalk. This in turn makescrosshole full-waveform inversion a promising tool to support time-lapse flow modelling.

AB - Chalk is an important reservoir rock for hydrocarbons and for groundwater resources for many major cities. Therefore, this rock type has been extensively investigated using both geological and geophysical methods. Many applications of crosshole GPR tomography rely on the ray approximation and corresponding inversions of first break traveltimes and/or maximum first-cycle amplitudes. Due to the inherent limitations associated withsuch approaches, the resulting models tend to be overly smooth and cannot adequately capture the small-scale heterogeneities. In contrast, the full-waveform inversion uses all the information contained in the data and is able to provide significantly improved images. Here, we apply full-waveform inversion to crosshole GPR data to image strong heterogeneity of the chalk related to changes in lithology and porosity.We have collected a crosshole tomography dataset in an old chalk quarry in Eastern Denmark. Based on core data (including plug samples and televiewer logging data) collected in our four ~15-m-deep boreholes andresults from previous related studies, it is apparent that the studied chalk is strongly heterogeneous. The upper ~7 m consist of variable coarse-grained chalk layers with numerous flint nodules. The lower half of the studied sectionappears to be finer-grained and contains less flint. However, still significant porosity variations are also detected in the lower half. In general, the water-saturated (watertable depth ~2 m) chalk is characterized by high porosities,and thus low velocities and high attenuation, while the flint is essentially non-porous and has correspondingly high velocities and low attenuation. Together these characteristics form a strongly heterogeneous medium, whichis challenging for the full-waveform inversion to recover. Here, we address the importance of (i) adequate starting models, both in terms of the dielectric permittivity and the electrical conductivity, (ii) the estimation of the sourcewavelet, (iii) and the effects of data sampling density when imaging this rock type. Moreover, we discuss the resolution of the bedding recovered by the full-waveform approach. Our results show that with proper estimatesof the above-mentioned prior parameters, crosshole GPR full-waveform tomography provides high-resolution images capturing a high degree of variability that standard methods cannot resolve in chalk. This in turn makescrosshole full-waveform inversion a promising tool to support time-lapse flow modelling.

M3 - Conference abstract in journal

VL - 17

JO - Geophysical Research Abstracts

JF - Geophysical Research Abstracts

SN - 1607-7962

M1 - EGU2015-14581

Y2 - 12 April 2015 through 17 April 2015

ER -