SM5.1

Reflection seismic is indispensable in evaluating depth, lateral extent, and heterogeneity of the shallow aquifers related to e.g., glacial sediments. Kurikka area in western Finland is an example of a complex groundwater system. It is being studied for its potential to supply water to the nearby city of Vaasa (with around ~150000 people and large industries in the area). The confined aquifer is topped with soft lake sediments and clays. Below the clays, a mixture of sands and gravels extends down to the bedrock, which can be as deep as 70-100 m. Felsic rocks (granites, granodiorites) constituting bedrock are weathered, fractured, and faulted. Because of the sparse outcrops, mapping of the bedrock fracture zones was based on gravity data and boreholes. Location, dimension, and connectivity of those fractures constitute a big uncertainty in the groundwater flow modeling.

We performed a multicomponent seismic survey in October 2021 west of the town of Kurikka along 3 seismic profiles in order to characterize the fractured and weathered bedrock, as well as the internal structure of the aquifer and its seal. The seismic profiles were acquired along gravel roads and were crossing the inferred fractures and a zone of a rapid change of bedrock elevation. Besides the geological objectives, we tested the performance of the lightweight Vibroseis source (SeismicMechatronics Lightning) and the operational aspects of the Finnish national pool of seismic instruments (Flex-EPOS) consisting of nodal 3C recorders and 3C geophones (https://wiki.helsinki.fi/display/FLEX/Large-N+Devices). Lightning is an electrically driven seismic vibrator (E-vib) based on the linear synchronous motor principle. It weights ca. 90 kg and can be used in both P- and S-wave mode offering 1.3 and 1.8kN force, respectively, with a full-force sweep frequency of 8-400 Hz (capable of sweeping between 1-1000 Hz at lower force). All profiles were shot in P-wave and SH-wave mode, resulting in a comprehensive and good quality 6C dataset. Lightning source proved to provide useful first-break energy up to 600 m offset. A clear bedrock reflection can be correlated in P-P (vertical source – vertical component) and S-S (horizontal source – crossline component) shot gathers. Prominent reflections were also observed in the sediments, with a broadband frequency response (up to 200 Hz).

The FIN-EPOS and FLEX-EPOS are funded by Academy of Finland (Funding Decisions no. 328984, 328776, 328778, 328779, 328780, 328781, 328782, 328784 and 328786)

How to cite: Malinowski, M., Heinonen, S., Koskela, E., and Laakso, V.: Multicomponent seismic acquisition for the characterization of the groundwater system at Kurikka, western Finland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6997, https://doi.org/10.5194/egusphere-egu22-6997, 2022.

Subrosion, the dissolution of soluble rocks, e.g., sulfate, salt, or carbonate, and the resulting structures, such as sinkholes and depressions, are a great geohazard because they can cause damage to buildings and infrastructure, and lead to life-threatening situations. The process requires unsaturated water and fluid pathways that enable the water to flow through the subsurface and generate cavities.

In Germany, sinkholes are a widespread problem, because soluble rocks, such as gypsum and anhydrite, are located close to the surface in many areas. One such area is the federal state of Thuringia, where our study area Bad Frankenhausen is situated.

For a better understanding of the local subrosion processes and structures, a detailed subsurface characterization of sinkholes and small- and large-scale depressions was necessary. Therefore, we used P-wave and SH-wave reflection seismics for high-resolution imaging of the near-surface. We were able to identify covered subrosion structures and –zones, and faults and fractures, which serve as fluid pathways. The seismic investigations were supplemented by geoelectric and gravimetric surveys in order to validate the interpreted fluid pathways and areas of underground mass movement.

We conclude that tectonic movements during the Tertiary, which lead to the uplift of the Kyffhäuser hills north of Bad Frankenhausen and the formation of faults parallel and perpendicular to the low mountain range, were the initial trigger for subrosion. The faults and the fractured Triassic and Lower Tertiary deposits serve as fluid pathways for groundwater to leach the deep Permian Zechstein deposits, and subrosion is more intense near faults. The artesian-confined salt water ascends towards the surface along the faults and fracture networks, which formed an inland salt marsh over time. In the past, subrosion of the soluble Zechstein Formations formed several, now covered, sagging and collapse structures, and, since the entire region is affected by recent sinkhole development subrosion must be still ongoing.

Due to the results of this study, we suggest a combined approach using P- and SH-wave reflection seismics to identify and analyse subrosion structures, and to use additional geophysical methods like electromagnetic- and gravimetric surveys to develop a more comprehensive model explaining the local subrosion processes.

How to cite: Wadas, S., Buness, H., Rochlitz, R., Günther, T., Skiba, P., Grinat, M., Polom, U., Tanner, D., and Krawczyk, C.: Reflection seismic investigation of a subrosion area using a combined approach of P- and SH-waves, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11322, https://doi.org/10.5194/egusphere-egu22-11322, 2022.

Both sudden and continuous subsidence of the earth’s surface pose a geohazard to the population and infrastructure, especially in urban areas. In Northern Germany, sinkholes occur often at salt dome highs, where dissolution affects the caprock, creating mass deficits in the shallow subsurface. Gravitationally driven subsidence of the overburden subsequently leads to both slow and sudden deformations of the surface.

The city of Elmshorn, situated partly on the top of a shallow salt dome structure of nearly 30-40 m below surface, has to deal with such surface deformations which have been the motivation of several investigations in recent decades. Existing geologic data based on drillings has recently been extended by a shear wave seismic 2D profile grid, to support mapping of the spatial course of the salt structures and overlying sediments in high resolution, and to identify areas prone to subrosion more precisely. The seismic profiles were acquired using an Elvis shear wave vibrator (source signal: 20-160 Hz sweep) and a land streamer attached with 10 Hz horizontal geophones in 1 m spacing. High-resolution stacked sections of 0.5 m CMP spacing were generated using shot spacing of 2-4 m. The profile grid shows that the shear wave reflection methodology is suitable to image the heterogeneous caprock surface and the fine structure of the overlaying Quaternary sediments. Strong topographic variations in the caprock surface and strongly heterogeneous lithology of both caprock and overlying sediments occur over short lateral distances less than 100 m, reinforcing the requirements for a close-meshed profile grid. Different caprock lithologies can be distinguished by changes in reflectivity and wavelength. Further, derived physical parameters based on full waveform inversions enable the characterization of the caprock surface and the integrity of the overlying sediments to estimate areas affected by subrosion.

The results highlight the structural information capabilities of the shear wave reflection method for the investigation of subrosion-prone areas as well as the further potential of the methodology to improve the knowledge of subrosion process sequences.

How to cite: Mecking, R., Polom, U., Omlin, A., and Leineweber, P.: Characterization of the Elmshorn salt diapir caprock by SH-wave reflection seismic and Full Waveform Inversion, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5878, https://doi.org/10.5194/egusphere-egu22-5878, 2022.

The ancient city Miletus is located at the Mediterranean coast of present Turkey. Its former geographical position on a land tongue in the Gulf of Miletus with four bays, which can be used as harbours, made the city a place of great economical interest. During the last decades, several archaeological and geophysical investigations were carried out to reconstruct the cityscape of Miletus.
In 2018, geoelectrical measurements revealed a high-resistive anomaly near the ancient western market place. The aim of the presented study is a more detailed imaging of this anomaly with help of shear wave seismic methods. 
For this purpose, a 35.5m long profile was build up across the geoelectric anomaly. An overall of 72 S-geophones was used with a spacing of 0.5m. Shots were struck every 1m by the use of a hammer and a shear wave source. 
A "simple" velocity evaluation by the Wiechert-Herglotz method shows shear-wave velocities between 270-380 m/s in the first three meters. This depth gradient of velocities is verified by a refraction tomography using the first breaks of each channel. The tomography also shows a high velocity zone of about 470 m/s in the deepest part of the model. 
A Full Waveform Inversion (FWI) was calculated using the refraction tomography as a start model. The inversion model shows three distinct high velocity zones in a, apart of these anomalies, quite homogeneous model. In these zones, velocity reaches values of more than 500 m/s. 
The results are in accordance to geoelectrical measurements conducted on the same profile. High velocity zones strongly correlate with areas of higher electrical resistivity. Corings near these velocity and resistivity anomalies show massive layers of limestone starting at a depth of about 1.5m and thus verify the findings of the geophysical investigations.
In conclusion, the shear wave seismic measurements are capable of resolving small-scale features even in shallow depths, especially with help of FWI. Together with the geoelectrics and corings, the results deliver an important contribution for the further interpretation of the buried archaeological feature.

How to cite: Fischer, S. L., Erkul, E., Zolchow, M., Kaplanvural, I., Brückner, H., Berns, C., and Rabbel, W.: Investigating a small-scale archaeological feature in the ancient city of Miletus using shear wave seismics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7959, https://doi.org/10.5194/egusphere-egu22-7959, 2022.