Natural hazard prediction and efficient crust exploration require dense seismic observations both in time and space. Seismological techniques provide ground-motion data, whose accuracy depends on ...sensor characteristics and spatial distribution. Here we demonstrate that dynamic strain determination is possible with conventional fibre-optic cables deployed for telecommunication. Extending recently distributed acoustic sensing (DAS) studies, we present high resolution spatially un-aliased broadband strain data. We recorded seismic signals from natural and man-made sources with 4-m spacing along a 15-km-long fibre-optic cable layout on Reykjanes Peninsula, SW-Iceland. We identify with unprecedented resolution structural features such as normal faults and volcanic dykes in the Reykjanes Oblique Rift, allowing us to infer new dynamic fault processes. Conventional seismometer recordings, acquired simultaneously, validate the spectral amplitude DAS response between 0.1 and 100 Hz bandwidth. We suggest that the networks of fibre-optic telecommunication lines worldwide could be used as seismometers opening a new window for Earth hazard assessment and exploration.
Understanding physical processes prior to and during volcanic eruptions has improved significantly in recent years. However, uncertainties about subsurface structures distorting observed signals and ...undetected processes within the volcano prevent volcanologists to infer subtle triggering mechanisms of volcanic phenomena. Here, we demonstrate that distributed acoustic sensing (DAS) with optical fibres allows us to identify volcanic events remotely and image hidden near-surface volcanic structural features. We detect and characterize strain signals associated with explosions and locate their origin using a 2D-template matching between picked and theoretical wave arrival times. We find evidence for non-linear grain interactions in a scoria layer of spatially variable thickness. We demonstrate that wavefield separation allows us to incrementally investigate the ground response to various excitation mechanisms. We identify very small volcanic events, which we relate to fluid migration and degassing. Those results provide the basis for improved volcano monitoring and hazard assessment using DAS.
Imaging the critical zone at depth, where intact bedrock transforms into regolith, is critical in understanding the interaction between geological and biological processes. We acquired a 500 m‐long ...near‐surface seismic profile to investigate the weathering structure in the Santa Gracia National Reserve, Chile, which is located in a granitic environment in an arid climate. Data processing comprised the combination of two seismic approaches: (1) body wave tomography and (2) multichannel analysis of surface wave (MASW) with Bayesian inversion. This allowed us to derive P‐wave and S‐wave velocity models down to 90 and 70 m depth, respectively. By calibrating the seismic results with those from an 87 m‐deep borehole that is crossed by the profile. We identified the boundaries of saprolite, weathered bedrock, and bedrock. These divisions are indicated in the seismic velocity variations and refer to weathering effects at depth. The thereby determined weathering front in the borehole location can be traced down to 30 m depth. The modelled lateral extent of the weathering front, however, cannot be described by an established weathering front model. The discrepancies suggest a more complex interaction between different aspects such as precipitation and topography in controlling the weathering front depth.
Combined seismic approaches of body wave tomography and MASW method revealed the upper 90 m of the critical zone down to the bedrock. The integration of the borehole data with the resulting P‐ and S‐wave velocity model, as well as the vertical velocity gradient model, provides a strong constraint in identifying the different lithologies. The resulting conceptual model shows the extent of the critical zone in Santa Gracia Reserve, Chile and its relation to topography.
ABSTRACT
The application of distributed acoustic sensing in borehole measurements allows for the use of fibre optic cables to measure strain. This is more efficient in terms of time and costs ...compared with the deploying of conventional borehole seismometers. Nevertheless, one known drawback for temporary deployment is represented by the freely hanging wireline cable slapping and ringing inside the casing, which introduces additional coherent coupling noise to the data. The present study proposes an explanation for the mechanism of noise generation and draws an analogy with similar wave propagation processes and phenomena, such as ghost waves in marine seismics. This observation allows to derive a ringing noise filter function, to study its behaviour and to consider known effects of the gauge length filter. After examining existing methods aimed at eliminating ringing noise and results of their application, we propose a two‐step approach: (1) developing a denoising method based on a matching pursuit decomposition with Gabor atoms and (2) subtracting the noise model for imaging improvement. The matching pursuit method focuses on decomposing the original input signal into a weighted sum of Gabor functions. Analysing Gabor atoms properties for frequency, amplitude and position in time provides the opportunity to distinguish parts of the original signal denoting noise caused by the vibrating cable. The matching pursuit decomposition applied to the distributed acoustic sensing‐vertical seismic profiling data at the geothermal test site Groß Schönebeck provides a versatile processing instrument for noise suppression.
SUMMARY
Low permeability, high retention capacity and self-sealing ability are advantageous characteristics that are attributed to argillaceous rocks. In contrast, other properties of clay, such as ...internal heterogeneities, strong attenuation and anisotropic behaviour, pose major challenges for underground exploration techniques. Although with regard to the underground storage of nuclear waste, the seismic exploration in the underground itself is of great importance to fill the gap between surface and borehole investigations. Furthermore, to prevent destruction of the host rock during exploration this demands low to non-invasive techniques. To approach these issues, a seismic survey was carried out in the Mont Terri Underground Rock Laboratory (Switzerland) using a gallery-based acquisition with an operating range up to several decametres. The seismic campaign included three-component borehole sensors and two different seismic source types (pneumatic impact and magnetostrictive vibroseis source). An executed source comparison analysed the characteristics of the different source types, for example frequency or amplitude behaviour of the generated wavefield, to assess their performance under similar conditions at the meso scale and to reveal their strengths and weaknesses in clay. Based on these findings, we performed traveltime and reflection analyses that demonstrate their potential to characterize clay formations and to map internal structures.
The highest seismic velocities are found in the carbonate-rich sandy facies (vPmax = 4000 m s−1, vSmax = 2050 m s−1), slower velocities are found in the sandy facies (vPmax = 3720 m s−1, vSmax = 1840 m s−1) and the slowest velocities are found in the shaly facies (vPmax = 3220 m s−1, vSmax = 1480 m s−1). The seismic velocity anisotropy is larger within the shaly facies (AvP = 23 per cent, AvS = 32 per cent) compared to the sandy facies (AvP = 9 per cent, AvS = 12 per cent) and it is more pronounced for S-waves than P-waves. Thus, non-invasive meso-scale seismic techniques are suited to characterize the Opalinus Clay in great detail.
SUMMARY
Ambient noise tomography on the basis of distributed acoustic sensing (DAS) deployed on existing telecommunication networks provides an opportunity to image the urban subsurface at regional ...scales and high-resolution. This capability has important implications in the assessment of the urban subsurface’s potential for sustainable and safe utilization, such as geothermal development. However, extracting coherent seismic signals from the DAS ambient wavefield in urban environments at low cost remains a challenge. One obstacle is the presence of complex sources of noise in urban environments, which may not be homogeneously distributed. Consequently, long recordings are required for the calculation of high-quality virtual shot gathers, which necessitates significant time and computational cost. In this paper, we present the analysis of 15 d of DAS data recorded on a pre-existing fibre optic cable (dark fibres), running along an 11-km-long major road in urban Berlin (Germany), hosting heavy traffic including vehicles and trains. To retrieve virtual shot gathers, we apply interferometric analysis based on the cross-correlation approach where we exclude low-quality virtual shot gathers to increase the signal-to-noise ratio of the stacked gathers. Moreover, we modify the conventional ambient noise interferometry workflow by incorporating a coherence-based enhancement approach designed for wavefield data recorded with large-N arrays. We then conduct multichannel analysis of surface waves to retrieve 1-D velocity models for two exemplary fibre subsegments, and compare the results of the conventional and modified workflows. The resulting 1-D velocity models correspond well with available lithology information. The modified workflow yields improved dispersion spectra, particularly in the low-frequency band (<1 Hz) of the signal. This leads to an increased investigation depth along with lower uncertainties in the inversion result. Additionally, these improved results were achieved using significantly less data than required using conventional approaches, thus opening the opportunity for shortening required acquisition times and accordingly lowering costs.
The Groß Schönebeck site in the North German Basin serves as research platform to study the geothermal potential of deeply buried Permian reservoir rocks and the technical feasibility of heat ...extraction. The structural setting of the site was investigated in more detail by a newly acquired 3D-seismic survey to improve the former conceptual model that was based on several old 2D seismic lines. The new data allow a revision of the geological interpretation, enabling the setup of a new reservoir model and providing base information for a possible further site development of Permo-Carboniferous targets. The 3D seismic allows for the first time a consistent geological interpretation and model parameterization of the well-studied geothermal site. Main reflector horizons and the corresponding stratigraphic units were mapped and the structural pattern of the subsurface presented in the 8 km × 8 km × 4 km large seismic volume. Attribute analysis revealed some fracture and fault patterns in the upper Zechstein and post-Permian units, while formerly hypothesized large offset faults are not present in the Rotliegend reservoir. However, a well-established graben-like structure at the top of the Zechstein succession is most likely related to broken anhydritic brittle intra-salt layers of some meter of thickness. Most reflectors above the salt show a rather undisturbed pattern. The main reservoir sandstone of the Dethlingen Formation (Rotliegend) was mapped and characterized. The base of the underlying Permo-Carboniferous volcanic rock sequence and hence its thickness could not be depicted reliably from the geophysical data. Based on the seismic data and the available reconnaissance drilling, logging, and laboratory data of the Groß Schönebeck research site, the thickness and distribution of the sedimentary Rotliegend (with emphasis of the sandy reservoir section) and of the volcanic rock sequence was modelled and stochastically parameterized with petrophysical properties guided by seismic facies pattern correlation, providing a more realistic reservoir description. Properties include total and effective porosity, permeability, bulk density, thermal conductivity, thermal diffusivity, and specific heat capacity. The data and interpretation constitute the basis for a better understanding of the thermo and hydromechanical processes at the site and for future measures. Further site development could include a deepening of one well to provide evidence on the volcanic rock sequence and consider deviated wells into favourable zones and the design of a fracture-dominated utilization approach.
In this study, we investigate the regional tectonic impact on salt movement at the northeastern margin of the intracontinental North German Basin. We discuss the evolution of salt pillows in the Bay ...of Mecklenburg in the light of thick‐ and thin‐skinned tectonics, including gravity gliding, and differential loading using seismic imaging. Stratigraphic and structural interpretation of a 170 km long, multichannel seismic line, extending from the Bay of Mecklenburg to northeast of Rügen Island, incorporates well information of nearby onshore wells. This new high‐resolution seismic line completely images the stratigraphic and tectonic pattern of the subsurface, from the base of the Zechstein to the seafloor. Our analysis reveals that subsidence during Late Triassic to Early Cretaceous at the northeastern basin margin was associated with transtensional dextral strike slip movement within the Trans‐European Suture Zone. We reinterpret the Werre and Prerow Fault Zones west of Rügen Island as an inverted, thin‐skinned normal fault system associated with the formation of the Western Pomeranian Fault System. Salt movement in the Bay of Mecklenburg was initiated in the Late Triassic and lasted until the Early Jurassic. A second phase of salt pillow growth occurred during the Coniacian until Cenozoic and correlates with compression‐related regional basin inversion due to the onset of the Africa‐Iberia‐Europe convergence. Thin‐skinned extensional initialization of salt pillow growth and compressional salt remobilization explains salt pillow evolution in the Bay of Mecklenburg. Additionally, we discuss an impact of gravity gliding on salt pillow evolution induced by basin margin tilt.
Key Points
A regional high‐resolution seismic profile continuously images the SW Baltic Sea subsurface from the Zechstein salt base to the seafloor
Late Triassic salt movement and Late Cretaceous‐Cenozoic remobilization correlate with regional tectonics at the North German Basin margin
The tectonic evolution of salt pillows is discussed in terms of thin‐skinned extensional/compressional deformation and gravity gliding
ABSTRACT
At the geothermal test site near Groß Schönebeck (NE German Basin), a new 3D seismic reflection survey was conducted to study geothermal target layers at around 4 km depth and 150°C. We ...present a workflow for seismic facies classification and modelling which is applied to a prospective sandstone horizon within the Rotliegend formation. Signal attributes are calculated along the horizon using the continuous Morlet wavelet transform. We use a short mother wavelet to allow for the temporal resolution of the relatively short reflection signals to be analysed. Time‐frequency domain data patterns form the input of a neural network clustering using self‐organizing maps. Neural model patterns are adopted during iterative learning to simulate the information inherent in the input data. After training we determine a gradient function across the self‐organizing maps and apply an image processing technique called watershed segmentation. The result is a pattern clustering based on similarities in wavelet transform characteristics. Three different types of wavelet transform patterns were found for the sandstone horizon. We apply seismic waveform modelling to improve the understanding of the classification results. The modelling tests indicate that thickness variations have a much stronger influence on the wavelet transform response of the sandstone horizon compared with reasonable variations of seismic attenuation. In our interpretation, the assumed thickness variations could be a result of variable paleo‐topography during deposition of predominantly fluvial sediments. A distinct seismic facies distribution is interpreted as a system of thicker paleo‐channels deposited within a deepened landscape. The results provide constraints for the ongoing development of the geothermal test site.
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