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SM5.2

EDI
Geophysical imaging of near-surface structures and processes

Geophysical imaging techniques are widely used to characterize structures and processes in the shallow subsurface. Methods include active imaging using seismic, (complex) electrical resistivity, electromagnetic, and ground-penetrating radar methods, as well as passive monitoring based on ambient noise or electrical self-potentials. Advances in experimental design, instrumentation, data acquisition, data processing, numerical modelling, and inversion constantly push the limits of spatial and temporal resolution. Despite these advances, the interpretation of geophysical images often remains ambiguous. Persistent challenges addressed in this session include optimal data acquisition strategies, (automated) data processing and error quantification, appropriate spatial and temporal regularization of model parameters, integration of non-geophysical measurements and geological realism into the imaging process, joint inversion, as well as the quantitative interpretation of tomograms through suitable petro-physical relations.

In light of these topics, we invite submissions concerning a broad spectrum of near-surface geophysical imaging methods and applications at different spatial and temporal scales. Novel developments in the combination of complementary measurement methods, machine learning, and process-monitoring applications are particularly welcome.

Convener: Florian WagnerECSECS | Co-conveners: Ellen Van De VijverECSECS, James Irving, Adam Booth, Frédéric Nguyen
Presentations
| Wed, 25 May, 14:05–18:30 (CEST)
 
Room D2

Wed, 25 May, 13:20–14:50

Chairpersons: Ellen Van De Vijver, Frédéric Nguyen

14:05–14:10
Introduction

14:10–14:20
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EGU22-9493
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ECS
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solicited
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On-site presentation
Matthias Steiner et al.

The hydraulic conductivity (K) is a relevant parameter in soil sciences as it is critical, e.g., for hydrological modeling or the characterization of groundwater flow and atmosphere-soil interactions. Field investigations allow for the direct assessment of K, yet the number and distribution of the sampling points limit the spatial resolution. To assess the spatial variability of K at the field scale, pedotransfer functions (PTFs) have been developed, which estimate K from soil parameters such as soil texture or bulk density. On the other hand, geophysical methods have also revealed their potential to quantify K in laboratory investigations, yet investigations at the field scale are still rare. In this study, we investigate the estimation of K through the simultaneous inversion of seismic and electrical data in an imaging framework. We use an open-source joint inversion framework, and adapt the underlying petrophysical model to take into account the (electrical) surface conductivity during parameter estimation. In particular, we invert resistivity data collected at a low and a high frequency, considering that the associated difference accounts for the (electrical) surface conductivity based on the frequency dependence of the observed electrical response. Accordingly, our joint inversion scheme solves, among other parameters, for water content, cation exchange capacity, polarization and porosity, which we use to quantify K through different models developed from laboratory investigations. Moreover, we investigate the possibility to enhance the quantification of K by adapting the joint inversion scheme to solve directly for the parameters required by the employed models. We illustrate our approach based on data collected at the Hydrological Open Air Laboratory (HOAL), a thoroughly investigated and monitored catchment located in Petzenkirchen (Lower Austria). We use available information about soil properties to calibrate our joint inversion scheme and evaluate the resolved K models based on K values obtained through direct investigations or the use of PTFs, respectively. Our approach contributes to the field-scale estimation of process-relevant subsurface properties at high spatial resolution by means of non-invasive geophysical imaging techniques – a key objective of the hydrogeophysical discipline.

How to cite: Steiner, M., Pavlin, L., Katona, T., Wagner, F. M., Roser, N., Strauss, P., Blöschl, G., and Flores Orozco, A.: Imaging of hydraulic conductivity from seismic and electrical data in a joint inversion framework, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9493, https://doi.org/10.5194/egusphere-egu22-9493, 2022.

14:20–14:26
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EGU22-5788
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ECS
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Highlight
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Virtual presentation
Adrian White et al.

Flood embankments (levees or dykes) are used worldwide to protect homes, industry and farmland from flooding caused by extreme weather events and tidal surges. Their role is becoming increasingly important for two key reasons: climate change is causing larger and more frequent floods, and the number of people living on floodplains is increasing globally. Both these factors necessitate that flood defences are well maintained to minimise failure during flood events, and reduce disruption, damage, and even loss of life.

 

There are more than 10,000 km of flood embankments in UK alone, so condition monitoring must be rapid. Current monitoring relies on qualitative walkover surveys every 6-12 months, but this can only detect the surface features that form in response to subsurface processes or characteristics. If we could detect subsurface properties and deterioration features directly it would enable us to identify areas at risk significantly earlier, minimising both risk and mitigation costs. Two complementary geophysical methods stand out: Electrical Resistivity Tomography (ERT) and Multi Channel Analysis of Surface Waves (MASW). These are sensitive to different hydro-mechanical properties of the materials that make up flood embankments and their foundations. ERT is sensitive to moisture content, clay content and porosity, whereas MASW is sensitive to elastic properties controlled by material strength, density, porosity and saturation.

 

In this work we combine co-located ERT and MASW surveys with time-lapse airborne lidar on three contrasting embankments on the River Thames, River South Tyne, and the Humber Estuary. Each site was selected based on known anomalies or the availability of existing geotechnical information to ground truth the geophysical measurements. The three embankments represent a range of different soil types, ages and varying foundation materials, making an ideal suite of targets to test the different geophysical methods.

 

In total c. 1 km of embankment was surveyed. Preliminary analysis shows good spatial agreement between units imaged by the ERT and those identified in the borehole data for each site. Areas of greatest settlement identified using time-lapse lidar also correlate with low resistivity anomalies indicating areas of soft clay and peat. Further data analysis will incorporate the MASW results and use clustering to quantitatively divide the subsurface into units with similar electrical and seismic properties. Geotechnical properties will then be attributed to each of the clusters, allowing more accurate fragility analysis of the embankment during flood conditions to be conducted.

How to cite: White, A., Chambers, J., Wilkinson, P., Wookey, J., Kendall, J. M., Dashwood, B., Whiteley, J., Boyd, J., Watlet, A., Morgan, D., Ball, J., and Binley, A.: Seeing inside flood embankments: combining electrical and seismic imaging., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5788, https://doi.org/10.5194/egusphere-egu22-5788, 2022.

14:26–14:32
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EGU22-12546
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Highlight
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On-site presentation
Maike Offer et al.

Degradation of mountain permafrost poses an increasing hazard to the stability of high-alpine infrastructures, which are predominantly located in low porosity bedrocks. Considering the dramatic climate change-induced temperature increase and the recent tourism expansion in these regions, safe long-lasting constructions and maintenance of infrastructures at high altitudes requires a complete process understanding of these permafrost systems. 

Non-invasive, geophysical measurements such as Electrical Resistivity Tomography (ERT) and Seismic Refraction Tomography (SRT) are the state of the art today in permafrost research due to their capability to distinguish between frozen and unfrozen medium. Thanks to their complementary sensitive records, it is common to combine electrical and seismic data sets by using petrophysical relations. Several multimethod approaches were already successfully applied in ice-rich conditions, however quantitative studies in ice-poor bedrock characterized by different physical properties are rarely investigated.  

In this study, we present a quantitative multimethod approach for long-term monitoring of low porosity permafrost bedrock. ERT and SRT data sets were recorded between 2010 and 2021 at the Zugspitze crest (Germany, 2.885 m asl) and in the Hanna-Stollen at the Kitzsteinhorn (Austria, 3.029 m asl). Both locations are visited every day by thousands of tourists, present infrastructure founded in bedrock with porosity of 0.2 to 5.0 % and are affected by degrading permafrost, although showing different lithologies. A combined analysis of resistivities and p-wave velocities, supported by their laboratory temperature calibrations with water-saturated samples from the field, allowed us to quantitatively estimate site-specific permafrost changes. The preliminary results show a clear warming of the permafrost core and a thickening of the active layer, well in agreement with other long-term permafrost observation at the Zugspitze summit and at further alpine sites (e.g. Scandroglio et. al, 2021).

In summary, our quantitative multimethod analysis for ice-poor bedrock provides fundamental contributions for planning and maintenance of permafrost-founded infrastructure under the influence of climate change. In the future, we aim at developing a new benchmark approach for hazard potential assessment of high-alpine infrastructures with foundations and anchoring in thawing permafrost.

 

Scandroglio, R., Draebing, D., Offer, M., & Krautblatter, M. (2021). 4D quantification of alpine permafrost degradation in steep rock walls using a laboratory-calibrated electrical resistivity approach. Near Surface Geophysics, 19, 2:241-260, doi: 10.1002/nsg.12149.

How to cite: Offer, M., Scandroglio, R., Mamot, P., Keuschnig, M., and Krautblatter, M.: Using combined quantitative geophysical methods to delimit physical properties of low porosity permafrost bedrocks , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12546, https://doi.org/10.5194/egusphere-egu22-12546, 2022.

14:32–14:38
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EGU22-2292
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Highlight
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On-site presentation
Mariusz Majdański et al.

The rapid climatic changes and their impact in the regions where unique environmental balance is not polluted by human existence are strongly visible. One of those places, exposed to Arctic amplification, is the newly deglaciated areas of Southern Spitsbergen. One of the most important scientific aspects is understanding their response to climate and environmental changes. To do that, an extensive geophysical approach is required integrating results from multiple imaging techniques. To derive spatial information from complex geomorphological terrain, joint interpretation of three non-intrusive geophysical methods were applied: Electrical Resistivity Tomography, Ground Penetrating Radar, and time-lapse Seismic Tomography. These were used to identify subsurface structures in the forefield of the retreating Hansbreen glacier in SW Spitsbergen, Svalbard. As a result, the authors distinguish three main zones, with different responses to the freezing-thawing effect: outwash plain, terminal moraine at the last glacial maximum, and glacial forefield proximal to the glacier front. The obtained data allowed for differentiation between geological and periglacial structures, and seasonally changing effects. The estimation of the impact of the freezing-thawing effect based on the VP velocity change reveals, that changes are deeper than previously believed reaching down to 30 metres of flat terrain and even deeper to 40 metres at the slope area with the strong subsurface flow. The study provides a unique snapshot of the current situation on the forefield of retreating Hansbreen concerning the current climate state.

This research was funded by National Science Centre, Poland (NCN) Grant UMO-2016/21/B/ST10/02509 and, Poland (NCN) Grant 2020/37/N/ST10/01486.

How to cite: Majdański, M., Dobiński, W., Marciniak, A., Osuch, M., Wawrzyniak, T., Owoc, B., and Glazer, M.: The effect of subsurface freezing-thawing in the SW Svalbard on the newly deglaciated areas, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2292, https://doi.org/10.5194/egusphere-egu22-2292, 2022.

14:38–14:44
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EGU22-6851
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ECS
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Highlight
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Virtual presentation
Jakob Gallistl et al.

Engineering-geophysical tasks in urban environments pose significant challenges for both the collection and interpretation of geophysical data. Typical problems that arise consist of background noise affecting seismic methods (i.e., moving cars or pedestrians), electromagnetic fields due to power lines and other infrastructure distorting electrical and electromagnetic methods and even traffic itself, requiring a thorough planning of fieldwork in order to minimize interruptions. To mitigate such limitations, commonly a combination of several geophysical methods is applied to counterbalance the caused distortions by a careful analysis of the data and combined modelling of the available information. Following this notion, we present a case study conducted in an urban setting in the first district of Vienna, a busy place in terms of both traffic and number of pedestrians. The objective was to delineate possible infiltration pathways of surface water or shallow subsurface water, infiltrating into an ancient cellar complex with a delay of two days after rainfall. The geophysical imaging included seismic refraction (RST) and multichannel analysis of surface waves (MASW), complex conductivity imaging (CCI) and ground-penetrating radar (GPR) measurements to characterize the subsurface architecture below the street (i.e., above the cellar) and CCI and GPR from within the cellar along the outer wall (i.e., below the street). Based on a combined analysis of the datasets from the street and the cellar itself, and incorporating 3D information from LiDAR within the cellar, we propose a model of infiltration pathways, as well as zones in the cellar wall possibly already strongly weakened by continuous high soil moisture.

 

 

How to cite: Gallistl, J., Bano, G., and Schlögel, I.: Detecting infiltration pathways by means of multi-method geophysical interpretation: an urban case study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6851, https://doi.org/10.5194/egusphere-egu22-6851, 2022.

14:44–14:50
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EGU22-5554
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ECS
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Highlight
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Virtual presentation
Anna Hettegger et al.

The main objective of this study is to understand salt dynamics in the soda lakes of the Austrian National Park Neusiedlersee Seewinkel and the influence of climate change in these dynamics. To achieve this, we investigated the application of electrical and electromagnetical methods to quantify spatial and temporal variations in the salt and water content. Built on the link between electrical conductivity and salt content, we mainly present results obtained with low induction number electromagnetic (EMI). EMI data were collected with the CMD Explorer and CMD Mini Explorer (from GF Instruments). Our investigations present, on the one hand, mapping of four lakes, each related to a different degree of degradation, with the aim of understanding the general patterns of electrical conductivity and their link to vegetation and surface cover. On the other hand, we conducted measurements at different time-lapses to monitor changes in the electrical conductivity along three profiles. The monitoring datasets combined EMI and time-domain induced polarization (TDIP) imaging and were repeated every two weeks between April and October 2021 near the Wörthenlacken. Monitoring measurements were conducted with both horizontal and vertical coplanar configurations with 3 separate receiver coils, for a total of 12 measurements for each point with a maximal nominal depth up to 6.7 m. Mapping measurements were collected only with horizontal configurations with both instruments, for six measurements with the same nominal depth of investigation. The initial analysis of the raw data demonstrates changes in the electrical conductivity related to changes in vegetation. In a second step, we inverted the EMI data using the open-source application EMagPy to resolve vertical variations of electrical conductivity along the monitoring profiles. Based on these results we evaluated the variation in electrical conductivity and potential salinity accompanying seasonal variations. Our interpretation incorporates non-geophysical data (temperature, water level, and precipitation) collected in observation wells near the study area. Moreover, we compared EMI monitoring results with those obtained from the inversion of TDIP datasets.

How to cite: Hettegger, A., Aigner, L., Armas, M. A., Moser, C., Cimadom, A., and Flores-Orozco, A.: Application of electrical and electromagnetic methods to delineate changes in salt content of soda lakes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5554, https://doi.org/10.5194/egusphere-egu22-5554, 2022.

Wed, 25 May, 15:10–16:40

Chairpersons: Ellen Van De Vijver, Frédéric Nguyen

15:10–15:16
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EGU22-10826
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ECS
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Virtual presentation
Huieun Yu et al.

For the detection of contaminated zones based on geophysical surveys, we make numerical experiments since it is not easy to make or maintain contaminated test-beds for field surveys. In the numerical experiments, we numerically simulate and analyze the responses of geophysical surveys including electrical resistivity tomography (ERT), induced polarization (IP) and ground penetrating radar (GPR), for numerical models, each of which was constructed based on the results of field geophysical survey obtained from contaminated site. ERT, which can image electrical resistivity of subsurface, is one of the most common geophysical tools, while IP survey can provide additional electrical information of the subsurface. Besides GPR can suggest the structure of geology.

For each model, whose geological structure was composed as similar to that of the corresponding field-survey site, we first numerically simulated corresponding field surveys performed in the site and compare with field data to verify the properness of the model. For the verified models we performed numerical simulation of geophysical surveys about various scenario of contaminations (e.g., oil pollution, leachate, heavy metal, etc.), and analyzed resulting responses in order to make strategies for the detection of contaminated zones bases on geophysical surveys. Further, we considered time-lapse geophysical surveys for changing contamination scenarios with respect to time. In the case of models including clay media, the responses of IP were remarkable useful in locating the clay media when compared ERT-only survey.

This work was supported by the Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20194010201920) and Korea Ministry of Environment as "The SEM projects; 2018002440005"

How to cite: Yu, H., Kim, B., Cho, A., Joung, I. S., Jeong, J., Jang, H., Jang, S., and Nam, M. J.: Comparative analyses on geophysical survey responses for various numerical models constructed based on field data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10826, https://doi.org/10.5194/egusphere-egu22-10826, 2022.

15:16–15:22
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EGU22-7503
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Virtual presentation
Ester Piegari et al.

The detection and imaging of landfills is a challenging task for geophysical methods because major pitfalls may arise, in such complex areas, from the speculative interpretation of geophysical anomalies as geological or antrophic features. In fact, when we face a multi-layered scenario, with numerous resistive to conductive transitions (that is the case of landfills), the actual shape and position of the anomalies (e.g. due to leachate accumulation) can be biased. The use of electrical resistivity tomography (ERT) in combination with the induced-polarization (IP) method, can help in this sense, even though may be not sufficient to completely remove ambiguities in interpretation of inverted models.

In this work, we present an application of an unsupervised machine learning k-means algorithm to ERT and IP data acquired in two urban waste disposal sites. The aim of the cluster analysis is to reduce the ambiguity on geophysical model interpretation and to improve the accuracy on detection of anomalous zones related to leachate accumulation. Experimental 2D field data were firstly inverted separately for resistivity and chargeability, using a Gauss-Newton algorithm. Then, joint 2D sections were obtained using k-means clustering of electrical resistivity, chargeability and normalized chargeability (chargeability divided by the resistivity) data. The retrieved model sections provide a quantitative integration of distinct geophysical data, which can offer new perspectives for the characterization of leachate distribution in landfills.

How to cite: Piegari, E., De Donno, G., and Paoletti, V.: k-Means Clustering of geophysical tomographic data for landfill characterization, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7503, https://doi.org/10.5194/egusphere-egu22-7503, 2022.

15:22–15:28
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EGU22-8876
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ECS
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Highlight
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Andrea Balza Morales et al.

Proper characterization of geologic structures that host geothermal systems is crucial for the efficiency and safety of their energy production. This includes estimating layer boundaries, complex geologic features, and lithology through means of inversion and its regularization. However, existing advanced regularization techniques (e.g., geostatistical regularization, minimum-gradient support, etc.) fail to capture the complexity of 3D geological models including fault networks, fault–surface interactions, unconformities, and dome structures. Förderer et al (2021) propose a solution by means of structure-based inversion, which implements implicit geological modeling and low-dimensional parametrization to produce sharp subsurface interfaces in 2D. This work aims to extend their approach to image realistic and complex geometries in 3D. We continue with the example of electrical resistivity tomography (ERT) and synthetic data; however, this approach is aimed towards independent and joint inversion of geophysical methods that are commonly used in geothermal exploration such as magnetotellurics, gravity, and seismic techniques.

The 3D geological model is created using GemPy, an open-source Python library, which constructs a structural geological model from interface points and orientations using an implicit approach based on co-kriging (de la Varga et al., 2019). Subsequently, the 3D model is discretized, and physical parameters are assigned using minimal pilot points that are then interpolated. We use pyGIMLi (Rücker et al., 2017), another open-source multi-method library for geophysical modelling and inversion, to perform a structure-based inversion, where we include the interface points in the primary model vector of the inversion to update these points iteratively to estimate a geological model in agreement with the geophysical observations.

In this work, special focus is placed on the sensitivity of each model parameter. To maintain low parametrization and account for the increase in computational power, the cumulative sensitivity is calculated and tested under criteria to optimize the model updates. This is relevant for geometries where the interface and pilot points are more influential in one dimension than others. The workflow has also been adapted to include more complex structures that can be defined in 3D, especially those that reflect geothermal systems. This work is part of the Innovative Training Network EASYGO (www.easygo-itn.eu), which aims to improve the efficiency and safety of geothermal operations but can be readily used in other applications.

 

References:

Förderer, A., Wellmann, F., and Wagner, F.: Geoelectrical imaging of subsurface discontinuities and heterogeneities using low-dimensional parameterizations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10012, https://doi.org/10.5194/egusphere-egu21-10012, 2021.

de la Varga, M., Schaaf, A., and Wellmann, F., 2019. GemPy 1.0: open-source stochastic geological modeling and inversion, Geosci. Model Dev., 12, 1–32, doi:10.5194/gmd-12-1-2019.

Rücker, C., Günther, T., Wagner, F.M., 2017. pyGIMLi: An open-source library for modelling and inversion in geophysics, Computers and Geosciences, 109, 106-123, doi: 10.1016/j.cageo.2017.07.011.

 

How to cite: Balza Morales, A., Maurer, H., Wellmann, F., and Wagner, F. M.: Towards structure-based joint geological-geophysical inversion for improved characterization of geothermal reservoirs , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8876, https://doi.org/10.5194/egusphere-egu22-8876, 2022.

15:28–15:34
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EGU22-10857
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ECS
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Highlight
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On-site presentation
Itzel Isunza Manrique et al.

There are two main drivers to integrate former metallurgical residues or mine waste into the circular economy through an efficient resource recovery. First, the economic driver, which considers land and resource recovery of high value elements such as critical raw materials. Secondly, the environmental and human health driver, as these types of residues might be a potential source of pollution. In both scenarios, there is a need to improve the characterization of past metallurgical sites and to locate and quantify materials of interest. To this aim, mostly geoelectric methods applied in the laboratory and/or in the field have been used and they are often complemented with geochemical or mineralogical studies. In this contribution, we present an approach that integrates a 3D Electrical Resistivity Tomography (ERT) and Induced Polarization (IP) acquisition in the field, lab measurements of ERT, IP and Spectral Induced Polarization (SIP) in several samples together with chemical analysis, to predict the metallic content in a slag heap from a former iron and steel factory located in Belgium. The samples were collected at locations targeting the observed geophysical anomalies. We first look for correlations between geophysical lab measurements and the chemical analysis to identify the variables which could potentially have a larger impact or control in the metallic content. Second, we use the geophysical lab measurements to improve the deterministic constrained inversion carried out for the 3D field data. Third, we use a supervised learning algorithm - Gaussian Process Classification (GPC)- to predict the metallic content of the slag heap from the 3D inverted resistivity/chargeability model. Overall, we found that variations in the chargeability are correlated with changes in iron, calcium and silicon content. Additionally, the GPC represents a suitable algorithm to integrate the uncertainty in the prediction results as well as the uncertainty that arises from the direct comparison of field and lab data. Finally, this methodology which integrates geophysical field data, targeted sampling, lab measurements and supervised learning can be applied in broader contexts where such data are available.

How to cite: Isunza Manrique, I., Caterina, D., Dumont, M., and Nguyen, F.: Integrated Approach to Identify Variables for the Prediction of Metallic Content in a Slag Heap using Time-Domain ERT and SIP Laboratory Measurements, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10857, https://doi.org/10.5194/egusphere-egu22-10857, 2022.

15:34–15:38
Seismic methods

15:38–15:44
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EGU22-9800
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Highlight
Lars Houpt et al.

Persistent microseismicity in the East Eifel Volcanic Field occurs along the Ochtendung Fault Zone (OFZ) just SE of Laacher See Volcano. In addition, deep-low-frequency earthquakes close by are a strong indication for active magmatic processes. No surface expression is known for the OFZ, therefore an active seismic study was conducted in the summer 2021 aiming to detect the near-surface structure of the fault.

The survey follows a line nearly perpendicular to the assumed fault orientation. The total length of the survey is 4,500 m with 5 m geophone distance and a maximum offset of 1000 m. Additional to these vertical component geophones, 3-component sensors were deployed at several sites along the profile in order to record far offsets. A drop-weight served as a seismic source.

1,022 shots lead to a total of more than 225,000 channels with maximum offsets of up to 1km, if including the 3-component sensors even up to 5km. Standard QC procedures and the stacking of the single shots at each shot point was done. The data set comprises 177 shot gathers with up to 221 receivers active at the same time. On these data the first onset P-wave arrivals were determined resulting in more than 35000 picks.

The refraction tomography uses an innovative inversion technique harnessing the power of a transdimensional, hierachical Markov chain Monte Carlo (McMC) algorithm without the need of a priori assumptions. The number of Voronoi cells describing the Earth structure model and the level of data noise is automatically determined during the inversion process. The forward modelling is performed by a fast, finite-difference based eikonal solver. Starting several hundred McMC-chains across multiple CPU-cores leads to the parallelism needed for efficient sampling of the model space, thus computing of a refraction tomography 2-D Earth structure model including its uncertainty.

We achieve a good resolution in depth down to about 200 m throughout our model. The thickness of the tephra layer covering the Rhenish shield is increasing from SW (few meters) to NE (80 m) along the profile. Further studies are still needed to illuminate the shallow structure of the OFZ.

How to cite: Houpt, L., Frietsch, M., Rietbrock, A., Ryberg, T., Haberland, C., Ritter, J., Schmidt, B., Reicherter, K., and Hertweck, T.: The Active Ochtendung Fault Zone Seismic Experiment – Shallow refraction tomography in the East Eifel Volcanic Field, Germany, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9800, https://doi.org/10.5194/egusphere-egu22-9800, 2022.

15:44–15:50
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EGU22-8695
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ECS
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Virtual presentation
Tzu-Chi Lin et al.

Fault zone head waves (FZHWs) generated by bimaterial interfaces are the first arriving seismic phases at near-fault stations on the slower side of the fault. Since FZHWs spend almost their entire path along the fault interface, imaging methods based on these phases can provide high-resolution information on fault structure at seismogenic depths. In Eastern Taiwan, many past catastrophic earthquakes highlight the need for an improved understanding of fault characteristics and mechanical properties in this tectonically active environment. Here we use FZHWs to examine the northern segment of the Longitudinal Valley (LV) fault system, a suture zone between the Eurasian Plate and the Philippine Sea plate. We focus on 44 stations within a 70 km by 28 km area located along the northern segment of the LV fault system and ~8800 small-to-moderate earthquake seismograms recorded between 2012 and 2018 by those stations. We apply a set of algorithms developed by Ross and Ben-Zion to automatically detect and pick direct P waves, S waves and potential head waves contained in the seismograms. We augment the detection using finite-difference simulations to study the effect of varying source mechanisms on the first motion polarity characteristics of the P wave and FZHW. The results indicate that head waves will be generated not only by a clean strike-slip fault but by a wider range of focal mechanisms. We discuss 414 robustly detected head waves excited by 204 events that are located within a thin volume along the west-dipping Central Range fault, which now suggests—for the first time—the existence of a consistent velocity contrast across that fault segment. We apply particle motion, polarization, and moveout analyses to confirm the robustness of our FZHW phase picks obtained with the automatic method. Most particle motion and polarization results show that the azimuths calculated from windows containing direct P waves do not consistently point to the epicentres. This variability in horizontal particle motion is likely associated with the complex structure beneath the receivers and changes in topography. To the first order, the moveout pattern of the P wave to FZHW time difference is constant. This indicates a shallow bimaterial interface along the fault that affects the wavefield near or below the stations. We fit the moveout and constant arrivals with two models to the differential P and head wave arrival times to explore possible local variations of the generally constant trend. The Akaike Information Criterion applied to the constant and the sloping moveout pattern suggests spatially complex significant results that together with the depth distribution of the involved events indicate a deeper-reaching bimaterial interface. For these configurations, the obtained average velocity contrast ranges from 0.75 to ~3 per cent.

How to cite: Lin, T.-C., Hillers, G., Lee, S.-J., and Hung, S.-H.: Seismic Velocity Contrast Along the Longitudinal Valley Fault System, Taiwan, from Analysis of Fault Zone Head Waves and Direct P Arrivals, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8695, https://doi.org/10.5194/egusphere-egu22-8695, 2022.

15:50–15:56
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EGU22-977
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Virtual presentation
Mahmoud Gomaa et al.

The use of microseismic technology to optimize fracture connectivity in  unconventional reservoirs

Mahmoud Mowafi, Ibrahim Alhilali, Ali AlEid, Mohammed Hamza, Sherif Allam and SanLinn Kaka

College of Petroleum Engineering and Geosciences

King Fahd University of Petroleum and Minerals

Dhahran 31261, Saudi Arabia

 

Unconventional reservoirs represent a challenging scenario to optimize the subsurface connectivity and production efficiencies. Heterogeneity is one of the main factors impacting well capability. Furthermore, the size and quality of the interconnected fracture network consequently enhance reservoir stimulation and permeability. We undertake this study to review the impact and efficiency of microseismic monitoring technology in  1) determining the growth of hydraulic fracture, 2) enhancing the understanding of fracture networks,  3) evaluating risks, and  4) estimating the production values of unconventional resources.  We subsequently develop a workflow to predict the possible range of stimulated reservoir volume using available data compiled from various literature. Our data were mainly from   North American and  China. The data were processed and analyzed considering the variations in rock properties and the hydraulic fracturing designs. Fracture heights and growth geometry were identified. No significant variations within the fracture heights were noticed among different plays. The workflow developed in this study enables us to predict stimulated reservoir volume in order to optimize the fracturing design which plays an important role in improving the recovery ratio of unconventional reservoirs.

How to cite: Gomaa, M., Alhilali, I., AlEid, A., Hamza, M., Allam, S., and Kaka, S.: The use of microseismic technology to optimize fracture connectivity in  unconventional reservoirs, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-977, https://doi.org/10.5194/egusphere-egu22-977, 2022.

15:56–16:02
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EGU22-5753
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ECS
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Virtual presentation
Yihe Xu et al.

Surface waves propagating from earthquakes, active sources or within the ambient noise wavefield are widely used to image Earth structure at various scales, from centimetres to hundreds of kilometres. The accuracy of surface-wave, phase-velocity measurements is essential for the accuracy of the Earth models they constrain. Here, we identify a finite-frequency phase shift in the phase travel time that causes systematic errors in time-domain, phase-velocity measurements. The phase shift arises from the approximation of monochromatic surface waves with narrow-band filtered surface waves. We derive an explicit formula of the finite-frequency phase shift and present a numerical method for its evaluation and for the correction of the measurements. Applications to high-frequency and long-period examples show that the phase shift is typically around π/60-π/16 for the common settings of ambient-noise imaging studies, which translates to 0.2-0.8% phase-velocity measurement errors. The finite-frequency phase shift depends on the (1) second derivative of the wavenumber with respect to frequency; (2) width of the narrow-band filter; (3) epicentral or interstation distance; (4) centre frequency of the filter. In conversion to phase velocity, the last two factors cancel out. Frequency-domain methods for phase-velocity measurements have the advantage of not producing the finite-frequency phase shift. Both time- and frequency-domain measurements, however, can be impacted by a break-down of the far-field approximation (near-field phase shift), which our calculations also show. Our method offers an effective means of improving the accuracy of the widely used time-domain, phase-velocity measurements via the evaluation of and corrections for the finite-frequency phase shift.

How to cite: Xu, Y., Lebedev, S., and Meier, T.: Improving the accuracy of time-domain surface-wave measurements: evaluation and correction of the finite-frequency phase shift, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5753, https://doi.org/10.5194/egusphere-egu22-5753, 2022.

16:02–16:08
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EGU22-154
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ECS
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Virtual presentation
anas charbaoui

Charbaoui A.(1*), Jaffal M.(1,2), Kchikach A.(1,2), Eljabbar B.(1), Bodinier J,L.(1,3), Rochdane S.(1), Khadiri O.(4), Jourani E.(4)

 

(1) Mohammed VI Polytechnic University, Geology and Sustainable Mining (GSM), Benguerir, Morocco

(2) Cadi Ayyad University, Georessources, Geoenvironment & Civil Engineering Laboratory, Marrakech, Morocco

(3) University of Montpellier & CNRS, Geosciences Montpellier, Montpellier, France.

(4) OCP Group, Casablanca, Morocco

*Email: anas.charbaoui@um6p.ma

 

Abstract

Seismic reflection is extensively used in petroleum exploration because it is recognized as an excellent tool of geological imaging, especially for sedimentary basins. In recent years, a variant of this method, namely the high-resolution seismic reflection (HRS) has experienced a rapid development due its implementation in many shallow geophysical investigations including studies in geotechnics, hydrogeology, structural geology, seismic hazard estimation, etc. This method can provide continuous coverage of the underground in two-dimensional 2-D, as well as in three-dimensional 3-D spaces. The HRS method has a lot of successful applications in shallow underground prospecting for various purposes (Tallini et al. 2020, Ahokangas et al. 2020).

The present project is concerned with the study of the Bahira basin, which hosts some of the most important phosphate deposits of Morocco. Its main objective is to provide a detailed seismic imaging of the phosphatic series, particularly in the area where it is hidden by a plio-quaternary cover. The Bahira phosphatic series is made of a Maastrichtian to Ypresian regular intercalation of phosphate beds and sterile layers.

The studies planned in order to reach this objective include first and foremost, the knowledge of the different terms of phosphatic series through (i) the analysis the available boreholes data, so as to gather information about the thickness of the layers, their lithological lateral change, etc., and (ii) the measurement of the rock properties necessary for the seismic modelling. The next step is to carry out numerical simulations that would help establish the expected seismic response of the phosphatic series. This also would aid to determine the appropriate parameters of the subsequent data acquisition. The third step is to perform HRS survey on the areas of interest. This will includes 2D surface seismic profiling and borehole vertical profiling (VSP). The projected studies also include performing measurements of Electrical Resistivity Tomography along the same survey profiles as the HRS. This would help realize a joint inversion of the two type of data and contribute to a better understanding the phosphatic series deep structure.

 

References:

[1] Tallini M., Spadi M., Cosentino D., Nocentini M., Cavuoto G., Di Fiore V., 2020. High-resolution seismic reflection exploration for evaluating the seismic hazard in a Plio-Quaternary intermontane basin (L'Aquila downtown, central Italy). Quaternary Iternational (In press)

[2] Ahokangas E., Mäkinen J., Artimo A., Pasanen A., Vanhala H., 2020. Reflection method with landstreamer in SW Finland, Journal of Applied Geophysics 177, 104014

Keywords: Phosphatic series, high-resolution seismic imaging, numerical simulations, Joint inversion, Bahira basin, Morocco.

How to cite: charbaoui, A.: High-resolution seismic investigation in phosphate mining, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-154, https://doi.org/10.5194/egusphere-egu22-154, 2022.

16:08–16:14
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EGU22-2718
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ECS
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On-site presentation
Marine Deheuvels et al.

In this work, we recover physical properties of a material with a focus on the attenuation, using a laboratory-scale sample. We develop a method to accurately invert the attenuation models, illustrating with 3D simulations the seismic wave propagation in the frequency domain considering different rheological viscoelastic models.

First, we consider a simplified numerical case where we avoid wave reflections from boundaries. Our analysis allows to characterize the wave behavior and the attenuation properties of the medium. Here, we use a complex wavenumber analysis, to recover a complex-valued mechanical modulus that accounts for the viscoelastic behavior.

Secondly, we consider numerically an experimental configuration, with free-surface conditions on the sample boundaries, and measurements restricted to the faces of the sample. In this case, the free-surface boundaries lead to multiple reflections and wave conversions that must be taken into account to analyze both the body waves and surface waves displacements to recover the representative viscoelastic properties. 

Finally, we carry out laboratory-scale experiments on various rock samples designed to find out their attenuation properties. For this purpose, we run an experimental setup using piezoelectric transmitters acting as a seismic source, and a laser-doppler vibrometer for non-contact time-domain measurements. Then, we have to recover the appropriate attenuation laws and their corresponding parameters, depending on the nature of samples. This eventually serves to build initial models to perform iterative reconstruction with Full Waveform Inversion.

How to cite: Deheuvels, M., Faucher, F., and Brito, D.: Quantitative analysis of seismic waves attenuation: numerical analysis and laboratory-scale experiments., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2718, https://doi.org/10.5194/egusphere-egu22-2718, 2022.

16:14–16:20
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EGU22-7080
Sigitas Radzevicius et al.

The sub-Cambrian peneplain is a well-known geological phenomenon in Scandinavia and it is found in sub-aerial outcrops in Finland, Sweden, and Norway. It is suggested that the peneplain formed within the Baltic Sea region in Cryogenian and Ediacaran / early Cambrian, when Baltica, a part of Rodinia at most of that time, experienced a tectonic stability, strong sheet-wash weathering and glaciations. While the peneplain outcrops at the surface in the Baltic Shield region, the remaining part of this peneplain is buried under the Phanerozoic sediments that comprise the Baltic Basin. This buried part of sub-Cambrian peneplain is known to have several inselbergs that occur as isolated ones or as sparse groups of inselbergs. From the results of interpretation of 2D and 3D onshore seismic data newly acquired in Western Lithuania, the continuation of sub-Cambrian peneplain was identified in Western Lithuania, a large array of inselbergs was mapped to the detail that the 3D seismic can permit, and the change in paleo-topography character of the Precambrian basement from flat to hilly and rough, was observed. The flat western side of our study area is interpreted as a continuation of the sub-Cambrian peneplain, which outcrops sub-aerially in Scandinavia while to the southeast it is buried under the strata of Baltic Basin. The southeastern part of our study area has many closely spaced hill-like features of various size, it is of considerable extent (at least 30 km) and does not comply with the peneplain’s definition on a local scale. It is interpreted as a part of large array of inselbergs. Though some of the largest palaeo-topography features in the study area were documented before, only the detailed mapping revealed that in Western Lithuania there is a large and dense array of inselbergs. This array of inselbergs is exceptional because it is the largest and densest of all known clusters of inselbergs in the Baltic Basin.

How to cite: Radzevicius, S., Grendaitė, M., and Michelevičius, D.: Large array of inselbergs and continuation of sub-Cambrian peneplain in the Baltic Basin based on interpretation of seismic data, Western Lithuania, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7080, https://doi.org/10.5194/egusphere-egu22-7080, 2022.

16:20–16:26
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EGU22-6031
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Virtual presentation
Ramon Carbonell et al.

Control and natural source seismic reflection records were acquired in the early fall in 2018 in the Sotiel-Elvira mining prospect, as part of the SIT4ME project (funded by EIT RawMaterials). Over 700 seismic digital instruments were deployed in a pseudo-3D grid to register these seismic signals in order to image and characterize the subsurface of the study area. The 2- and 3-component instruments used recorded wide azimuth and relatively long offset data. A 32 Tn Vibroseis truck was used in 900 vibration points to complete the controlled source component of the experiment. The array of receivers (deployed within a grid of 10x10m) recorded P- and S- waves and allowed to develop seismic velocity models derived from first arrival travel time tomography and multichannel analysis of surface waves (MASW). Results are further constrained by density measurements of rock samples and surface geology. The integrated information places structural constrains in the subsurface and allows us to depict areas where higher than average P and S wave velocities, characteristic of massive sulphides, might point out to the existence of new or better delimited deposits within the Iberian Pyrite Belt (SW- Iberia). The area is under assessment for potential future exploitation. This experiment further demonstrates the potential of non-invasive and relatively inexpensive seismic techniques to address high-resolution imaging of mineralized areas.

How to cite: Carbonell, R., Handoyo, H., Martinez, Y., Marti, D., Alcalde, J., Ruiz, M., Ayarza, P., and Tornos, F.: Multi-seismic imaging: Development of geological and geophysical models of the subsurface in the Iberian Pyrite Belt (SW-Iberia) , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6031, https://doi.org/10.5194/egusphere-egu22-6031, 2022.

16:26–16:32
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EGU22-12314
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ECS
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On-site presentation
Laura Frahm et al.

We present velocity images across a bottom-simulating reflector (BSR) recovered using 3D high-resolution full-waveform inversion (FWI) and discuss its use as a tool for understanding the nature of the BSR.

FWI is a seismic imaging technique which generates highly resolved physical property models of the subsurface. FWI uses the full recorded waveform for inversion which leads to a superior resolution compared to other imaging methods, but also makes it computationally more expensive. Relative to 2D inversions, 3D FWI leads to image improvements due to an increase in azimuthal coverage and ability to map out-of-plane arrivals to the correct location, which is particularly important in a complex geological setting. Therefore, next to the advantage of a fully resolved 3D structure, the model will also be more accurate.

Caused by gas hydrate in an upper layer and/or free gas in a lower layer, a BSR indicates the base of the gas hydrate stability zone. This significant change of the physical properties in the upper few hundred meters of the marine sediment produces a distinct reflection, i.e. the BSR, that can be seen in the seismic image.

We are imaging and investigating a BSR at Puke Ridge, a thrust ridge on the accretionary wedge of the northern Hikurangi subduction margin, offshore the North Island of New Zealand. We are using seismic multichannel streamer data, belonging to the NZ3D dataset collected in 2018, to invert for the P-wave velocity. The resolved velocity model displays the geometry and the structure of a BSR characterised by a velocity increase followed by a sudden decrease and provides us with accurate velocities which we can use for rock physics modelling and interpretation.

How to cite: Frahm, L., Davy, R., Bell, R., Morgan, J., Arai, R., Bangs, N., Henrys, S., and Barker, D.: Using 3D full-waveform inversion to investigate bottom-simulating reflectors, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12314, https://doi.org/10.5194/egusphere-egu22-12314, 2022.

16:32–16:38
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EGU22-10210
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On-site presentation
Antonio García-Jerez et al.

Preceding the seismic modelling of Campo de Dalías sedimentary basin by single-station ambient noise measurements, research focused on the reliability of the methodology employed, namely the microtremor horizontal-to-vertical spectral ratio (MHVSR), is conducted. It is known that MHVSR may present some dependence on weather and site-specific conditions as topographic effects, anthropogenic activities or the variability in the microtremor source distribution. In this context, the stationarity of MHVSR curves with their fundamental peaks below 1 Hz is studied after the installation of three long-term stations in rural sites and another one for a week in the urban area of El Ejido town.

The robustness of the MHVSR methodology is often assured by looking into the stationarity of mainly two peak parameters: frequency and amplitude. In this study, two new parameters are tested: the peak-width and the trough frequency. We have up to two years of microtremor and weather data that helped to track the variability of wind speed, atmospheric pressure, and temperature, as well as sea tide and aquifer levels compared to the peak shape of MHVSR curves.

Most weather variables only show short or punctual correlations with MHVSR parameters, which is the case of wind gusts above 10 m/s that totally blurred the MHVSR peak-shape for periods of a few days in the more poorly isolated station. The wind-speed time series collected in Campo de Dalías show high correlations with the total microtremor energy in the frequency band of secondary microseisms (0.3 - 1 Hz) with a clear seasonal behaviour. However, the MHVSR peaks studied in that band are uncorrelated with them. Our results show that the piezometric level maintains a moderate to high correlation with MHVSR peak-variability during a time span of 9 months. Campo de Dalías hosts a system of karst aquifers, which constitutes the main water supply for this semi-arid region. Modelling the groundwater flow in that kind of aquifers is a challenging field of research and monitoring it by means of investigation wells has a high cost. The results observed in this study widen the possibilities of MHVSR for being an aquifer-monitoring tool on time scales as short as a few days.

How to cite: García-Jerez, A., Seivane, H., Navarro, M., and Molina, L.: Capabilities for Aquifer Monitoring of long-term MHVSR Observations in Campo de Dalías (Almería, SE Spain), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10210, https://doi.org/10.5194/egusphere-egu22-10210, 2022.

Wed, 25 May, 17:00–18:30

Chairpersons: Ellen Van De Vijver, Frédéric Nguyen

17:00–17:06
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EGU22-905
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ECS
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Virtual presentation
Atikul Haque Farazi et al.

The densely populated Bangladesh occupies most of the part of the Bengal Basin; the basin is located just above the subduction margin extended with an N-S alignment between the Indian and the Burmese Plates. The subduction tectonics along this Indo-Burmese plate boundary has put this locality to high seismic risk, which is also supported by the historical earthquake records. Moreover, being in the foothill of the Himalayan Mountains and the Indo-Burmese Ranges, respectively, to the North and the East, this country has become extremely riverine to be filled by sediments. Soft sedimentary layers over geophysical bedrock, with shear-wave velocity, VS > 760 m/s, can significantly amplify earthquake ground motion to cause damage to infrastructure. In addition, bedrock depth significantly controls the phenomena of soil-infrastructure vibration resonance. That is why, for seismic risk evaluation, it is essential to have adequate information on soft sediment thickness or depth to the sediment-bedrock interface.

The continuously subsiding Sylhet Basin (SB, Zone 1), being a sub-basin within the northern limit of the Bengal Basin, is the flexural depocenter in the northeastern Bangladesh with possibly the thickest (~ 25 km) sedimentary successions (Bürgi et. al. 2021). The active Dauki Fault demarcates the northern limit of the Sylhet Basin as well as the Bengal basin, along which the Shillong Plateau has been uplifted.

In this work, we present VS velocity up to 3000 m beneath three seismic stations in the Sylhet Basin, namely JAML, SUST and JAFL, data of which are available in the Incorporated Research Institutes for Seismology (IRIS) website. Here, subsurface VS profile is estimated by inversion of single-station horizontal-to-vertical (H/V) spectral ratio curve within 0.2 to 10 Hz. The H/V curves at three stations are obtained from 15 days continuous recordings of seismic ambient noise data. We use HV-Inv software (García-Jerez et. al. 2016) for the inversion, in which the H/V ratio is interpreted under the diffuse filed assumption (Sánchez-Sesma et. al. 2011) for full H/V inversion considering contribution from the full noise wavefield. The inversion process is constrained using the existing general lithological information as well as unpublishable VP data from active seismic surveys of Bangladesh Petroleum Exploration Company Ltd. (BAPEX).

From this analysis, we find that geophysical bedrock depth is approximately at 180 m, 220 m and 160 m, respectively, below the stations JAML, SUST and JAFL. To the best of our knowledge, neither the current approach of VS estimation was applied nor such high-resolution VSinformation of sedimentary successions was reported in the study area previously. The presented velocity information could be crucial for engineering development, seismic hazard mitigation, and exploration purpose in the Sylhet Basin.