Electric resistivity and seismic refraction tomography: a challenging joint underwater survey at Äspö Hard Rock Laboratory

Tunnelling below water passages is a challenging task in terms of planning, pre-investigation and construction. Fracture zones in the underlying bedrock lead to low rock quality and thus reduced stability. For natural reasons, they tend to be more frequent at water passages. Ground investigations that provide information on the subsurface are necessary prior to the construction phase, but these can be logistically difficult. Geophysics can help close the gaps between local point information by producing subsurface images. An approach that combines seismic refraction tomography and electrical resistivity tomography has been tested at the Äspö Hard Rock Laboratory (HRL). The aim was to detect fracture zones in a well-known but logistically challenging area from a measuring perspective. The presented surveys cover a water passage along part of a tunnel that connects surface facilities with an underground test laboratory. The tunnel is approximately 100 m below and 20 m east of the survey line and gives evidence for one major and several minor fracture zones. The geological and general test site conditions, e.g. with strong power line noise from the nearby nuclear power plant, are challenging for geophysical measurements. Co-located positions for seismic and ERT sensors and source positions are used on the 450 m underwater section of the 700 m profile. Because of a large transition zone that appeared in the ERT result and the missing coverage of the seismic data, fracture zones at the southern and northern parts of the underwater passage cannot be detected by separated inversion. Synthetic studies show that significant three-dimensional (3-D) artefacts occur in the ERT model that even exceed the positioning errors of underwater electrodes. The model coverage is closely connected to the resolution and can be used to display the model uncertainty by introducing thresholds to fade-out regions of medium and low resolution. A structural coupling cooperative inversion approach is able to image the northern fracture zone successfully. In addition, previously unknown sedimentary deposits with a significantly large thickness are detected in the otherwise unusually well-documented geological environment. The results significantly improve the imaging of some geologic features, which would have been undetected or misinterpreted otherwise, and combines the images by means of cluster analysis into a conceptual subsurface model

DCIP tomografi för kartläggning av jorddjup och strukturer i berg

There are plans to build an energy storage in the rock at Önneslöv near Dalby in Skåne. The bedrock is of a part of Romeleåsen and mainly comprises gneiss, with elements of amphibolite and dolerite intrusions. The area is located immediately south of the Sydsten’s large rock quarry, which shows that the rock is mostly heavily fractured with clay weathered zones of different sizes. In connection with underground construction fractured zones and weathering constitutes a risk for problems with water inflow and stability. Furthermore, variations in depth to the upper surface of the rock can lead to stability problems in the upper portions of a planned construction.Electric resistivity tomography (ERT) is now an established pre-investigation method for tunnel projects, and it has been used on a large scale in connection with for example the Hallandsås Tunnel. The method provides continuous models of variations in the electrical properties of the rock in two (2D) and three dimensions (3D) that can be linked to variations in the rock mechanical and hydraulic properties. ERT measured by a combination of DC resistivity and induced polarization can be called DCIP tomography, which can provide additional information about the variation in the material properties of the rock. With the help of new and improved methods of data acquisition, processing and interpretation it is possible to collect large amounts of good quality IP data in a time and cost effective way, paving the way for better and more nuanced models of the rock and variations in its properties. The newly developed technology has been tested in full scale at Önneslöv. Three parallel DCIP sections about 1 km long, with a maximum survey depth of about 170 m were measured, plus a 800 m long cross-section. Furthermore, DCIP logging was carried out in two percussion drill holes down to 200 m depth in which it is furthermore made logging of diameter, natural gamma radiation, seismic velocity and flow rate during pumping.Geological interpretation of the DCIP results agree well with what one can expect from the documentation of soil depth, and variation in degree of fracturing and weathering as documented from drilling. A major advantage is continuous models that can be linked to variation in soil depth, structures in the rock and hydrogeological conditions. The combined surveys with surface-based measurements and borehole measurements are complementary and provide a more reliable overall picture of the variations in rock conditions

Detektion und Monitoring von Salzwasserintrusionen mittels stahlverrohrter Bohrlöcher als Langelektroden

Contaminations of freshwater aquifers by saltwater are a well-known problem in coastal regions, as well as on inland sites. They can be detected by electrical resistivity tomography (ERT) as recent widespread investigations show. The three-dimensional application is limited to small scales due to the enormous effort required by field surveys. This thesis presents an approach that uses the metal casing of boreholes as long electrodes for ERT surveys (LE-ERT) to detect and monitor saltwater intrusions. Three different approaches for modelling long electrodes exist: The complete electrode model (CEM) and the conductive cell model (CCM) lead to comparable results but are numerically expensive, while the shunt electrode model (SEM) has the lowest numerical effort for the same accuracy. Simulations reveal that the low vertical model resolution due to long electrodes can be enhanced by using electrodes of different length or additional surface electrodes. Three-dimensional inversion of synthetic data supports this assumption. They also show that varying contact impedances along metal casings are of secondary importance. Laboratory 2D and 3D experiments were conducted to monitor different scenarios of saltwater intrusions. Optimised data sets showed that dipole combinations with a high geometric factor and low voltage often possess a high information content. Due to their low signal strength, these combinations are difficult to measure and prone to noise. This can be considered by excluding or down-weighting data prior to the optimisation process. Field measurements were conducted on medium- and large-scaled test sites. Reduced data sets were generated to control the effort for the surveys. Inversion results of the medium-scaled site showed good agreement with geology, ERT profiles and in-situ fluid conductivity measurements. LE-ERT monitoring over a period of two years could sufficiently resolve small resistivity changes in the upper aquifer, validated by fluid conductivities. The feasibility of LE-ERT as a large-scale application was proved on a 7 x 7 km test site. The general vertical resistivity trend is in good agreement with electro-logs of three different boreholes. LE-ERT is an applicable cost-efficient method covering large areas.Salzwasserintrusionen in Frischwasser führende Aquifere sind sowohl in Küstenregionen, als auch im Landesinneren ein bekanntes Problem. Zur Detektierung können elektrische Verfahren wie ERT (electrical resistivity tomography) eingesetzt werden. Da die 3D Anwendung dieser Methode selbst auf kleinen Skalen enormen Aufwand erfordert, wurde in dieser Arbeit der Ansatz untersucht, stahlverrohrte Bohrlöcher als lange Elektroden für Geoelektrikmessungen (LE-ERT) zu verwenden. Um lange Elektroden zu diskretisieren, existieren drei Ansätze. Das Complete Electrode Model (CEM) und das Conductive Cell Model (CCM) sind zwei ähnliche Methoden, die vergleichbare Ergebnisse erzielen, jedoch nummerisch aufwändig sind. Das Shunt Electrode Modell (SEM) bietet eine Alternative mit geringem nummerischen Aufwand. Modellierungen zeigen, dass die vertikale Auflösung erhöht werden kann, indem verschieden lange Elektroden oder zusätzliche Punktelektroden verwendet werden, was durch Auflösungsanalysen unterstützt wird. Simulationen zeigten, dass der Einfluss von Kontaktimpedanzen eine untergeordnete Rolle spielt. Verschiedene Salzwasserszenarien wurden in 2D/3D Laborexperimenten simuliert. Während der Generierung optimierter Datensätze wurde herausgefunden, dass Kombinationen mit geringer Messspannung und hohem Geometriefaktor einen hohen Informationsgehalt besitzen. Diese müssen aufgrund der schweren messtechnischen Erfassung entweder vor dem Optimierungsprozess ausgeschlossen oder mit einem Fehlermodell gewichtet werden. Messungen auf mittel- und großskaligen Testfeldern zeigten, dass nur reduzierte Datensätze realisierbar sind. Inversionsergebnisse eines mittelskaligen Testfeldes stimmten mit 2D ERT Profilen, der Geologie und in situ Fluidleitfähigkeiten überein. Ein zweijähriges Monitoring konnte selbst kleine Widerstandsänderungen hinreichend auflösen. Die Machbarkeit von LE-ERT als großskalige Anwendung konnte auf dem zweiten 7 x 7 km Testfeld gezeigt werden. Der generelle Widerstandstiefenverlauf stimmt gut mit Elektrologs aus 3 Bohrlöchern überein. LE-ERT ist damit eine Methode, welche auf großen Flächen, wie Brunnenfeldern eingesetzt werden kann

Geophysical pre-investigation for a Stockholm tunnel project : Joint inversion and interpretation of geoelectric and seismic refraction data in an urban environment

Underground constructions for public traffic purposes are becoming increasingly important for urban areas in order to use the limited space more efficiently. Several electric resistivity tomography and seismic refraction tomography measurements were performed crossing a water passage near Stockholm during the pre-investigation phase of a tunnel building project. The objective was to determine the bedrock interface and qualitatively assess the rock quality. The scope of this study is to present a field case in an urban environment and show improvements of geophysical results due to additional model constraints by a joint inversion. Results of individual inversions show a large transition zone below the seabed from electric resistivity tomography. Some parts of the seismic refraction tomography have a low model resolution, due to gas-bearing sediments with a low velocity together with a high noise level, which leads to insufficient investigation depth that makes it difficult to determine the bedrock interface. However, the bedrock interface could be reconstructed in the resistivity model by performing a joint inversion, using the seismic velocity model to constrain the electric resistivity tomography model and vice versa. Adjacent geotechnical soundings support the joint inversion results. A vertical low resistive zone could be identified as a dominant fracture zone in the southern part of the investigated area. In general, the joint inversion approach significantly improved the electric resistivity tomography results and provided a more reliable bedrock estimation

Monitoring freshwater–saltwater interfaces with SAMOS – installation effects on data and inversion

A major problem for the freshwater supply of coastal regions is the intrusion of saltwater into aquifers. Due to extensive extraction of freshwater to suffice increasing drinking water demands and/or in periods of reduced groundwater recharge, the equilibrium state may be disturbed. The result is an upconing or movement of the fresh–saline groundwater interface, which reduces the local drinking water resources at coastal regions or islands. The saltwater monitoring system (SAMOS) is a vertical electrode chain installed in a backfilled borehole. It provides a solution to observe the transition zone in detail, both temporally and spatially. We present monitoring data of the first year from three locations - with different geological conditions that show disturbances in the resistivity distribution that result from the drilling processes. A clayey backfilling, for example, can lead to beam-like artefacts, and a mixed fluid within the backfilling changes its bulk resistivity, both leading to misinterpretations. We performed data inversion under cylindrically symmetrical conditions in full-space in order to separate these resistivity artefacts from the undisturbed background. Data inversion reveals that it is possible to separate drilling effects on the resistivity distribution from the undisturbed background. Thus, an interpretation of the natural transition zones can be made immediately after the installation. © 2020 The Authors. Near Surface Geophysics published by John Wiley & Sons Ltd on behalf of European Association of Geoscientists and Engineers

Monitoring freshwater–saltwater interfaces with SAMOS – installation effects on data and inversion

A major problem for the freshwater supply of coastal regions is the intrusion of saltwater into aquifers. Due to extensive extraction of freshwater to suffice increasing drinking water demands and/or in periods of reduced groundwater recharge, the equilibrium state may be disturbed. The result is an upconing or movement of the fresh–saline groundwater interface, which reduces the local drinking water resources at coastal regions or islands. The saltwater monitoring system (SAMOS) is a vertical electrode chain installed in a backfilled borehole. It provides a solution to observe the transition zone in detail, both temporally and spatially. We present monitoring data of the first year from three locations - with different geological conditions that show disturbances in the resistivity distribution that result from the drilling processes. A clayey backfilling, for example, can lead to beam-like artefacts, and a mixed fluid within the backfilling changes its bulk resistivity, both leading to misinterpretations. We performed data inversion under cylindrically symmetrical conditions in full-space in order to separate these resistivity artefacts from the undisturbed background. Data inversion reveals that it is possible to separate drilling effects on the resistivity distribution from the undisturbed background. Thus, an interpretation of the natural transition zones can be made immediately after the installation

Electric resistivity and seismic refraction tomography : A challenging joint underwater survey at Äspö Hard Rock Laboratory

Tunnelling below water passages is a challenging task in terms of planning, pre-investigation and construction. Fracture zones in the underlying bedrock lead to low rock quality and thus reduced stability. For natural reasons, they tend to be more frequent at water passages. Ground investigations that provide information on the subsurface are necessary prior to the construction phase, but these can be logistically difficult. Geophysics can help close the gaps between local point information by producing subsurface images. An approach that combines seismic refraction tomography and electrical resistivity tomography has been tested at the Äspö Hard Rock Laboratory (HRL). The aim was to detect fracture zones in a well-known but logistically challenging area from a measuring perspective. The presented surveys cover a water passage along part of a tunnel that connects surface facilities with an underground test laboratory. The tunnel is approximately 100ĝ€m below and 20ĝ€m east of the survey line and gives evidence for one major and several minor fracture zones. The geological and general test site conditions, e.g. with strong power line noise from the nearby nuclear power plant, are challenging for geophysical measurements. Co-located positions for seismic and ERT sensors and source positions are used on the 450ĝ€m underwater section of the 700ĝ€m profile. Because of a large transition zone that appeared in the ERT result and the missing coverage of the seismic data, fracture zones at the southern and northern parts of the underwater passage cannot be detected by separated inversion. Synthetic studies show that significant three-dimensional (3-D) artefacts occur in the ERT model that even exceed the positioning errors of underwater electrodes. The model coverage is closely connected to the resolution and can be used to display the model uncertainty by introducing thresholds to fade-out regions of medium and low resolution. A structural coupling cooperative inversion approach is able to image the northern fracture zone successfully. In addition, previously unknown sedimentary deposits with a significantly large thickness are detected in the otherwise unusually well-documented geological environment. The results significantly improve the imaging of some geologic features, which would have been undetected or misinterpreted otherwise, and combines the images by means of cluster analysis into a conceptual subsurface model

Saltwater intrusion under climate change in North-Western Germany - mapping, modelling and management approaches in the projects TOPSOIL and go-CAM

Climate change will result in rising sea level and, at least for the North Sea region, in rising groundwater table. This leads to a new balance at the fresh–saline groundwater boundary and a new distribution of saltwater intrusions with strong regional differentiations. These effects are investigated in several research projects funded by the European Union and the German Federal Ministry of Education and Research (BMBF). Objectives and some results from the projects TOPSOIL and go-CAM are presented in this poster

Structurally coupled inversion of ERT and refraction seismic data combined with cluster-based model integration

Electrical resistivity tomography (ERT) and refraction seismics are among the most frequently used geophysical methods for site-investigations and the combined results can be very helpful to fill in the gaps between the point measurements made by traditional geotechnical methods such as Cone Penetration Test (CPT), core-drilling and geophysical borehole logging. The interpretation of the results from a geophysical investigation constituting a single method often yields ambiguous results. Hence, an approach utilizing multiple techniques is often necessary. To facilitate interpretation of such a combined dataset, we propose a more controlled and objective approach and present a method for a structurally coupled inversion of 2D electrical resistivity and refraction seismic data using unstructured meshes. Mean shift clustering is used to combine the two images and to compare the separate and coupled inversion methodologies. Two synthetic examples are used to demonstrate the method, and a field-data example is included as a proof of concept. In all cases a significant improvement by the coupling is visible. The methodology can be used as a tool for improved data interpretation and for obtaining a more comprehensive and complete picture of the subsurface by combining geophysical methods