Toward improved DAS sensitivity for surface-based reflection seismics: Configuration tests at the Aquistore CO2 storage site

Alternative fiber configurations have been tested in an attempt to improve the sensitivity of surface-deployed DAS fiber cables for the purpose of recording steep-angle P-wave reflections. Four alternative fiber configurations were deployed at the Aquistore CO2 storage site to record 401 dynamite shots during a 3D VSP survey. The test cable comprised horizontal configurations (straight fiber, helixes and asymmetric helixes) buried in a shallow trench and vertical configurations (straight fiber and helixes) deployed in 3.5 m drillholes. Evaluation focused on deep reflections with two-way travel times of 0.8 to 1.8 s. All of the alternative fiber configurations increased the sensitivity relative to the horizontal straight fiber. Sensitivity was highest for the vertical straight fiber configurations and the asymmetric helixes with sensitivity increases of more than 10 dB and 5 dB, respectively, and AVO behavior similar to that of a vertical-component geophone for reflections with incidence angles of 0° to 15° at the surface and 0° to 34° at the reflector. Modeling of the DAS responses explains the general pattern of sensitivity variability amongst the different configurations, but does not explain the large range of observed sensitivities.

Evidence for a grounding line fan at the onset of a basal channel under the ice shelf of Support Force Glacier, Antarctica, revealed by reflection seismics

Curvilinear channels on the surface of an ice shelf indicate the presence of large channels at the base. Modelling studies have shown that where these surface expressions intersect the grounding line, they coincide with the likely outflow of subglacial water. An understanding of the initiation and the ice–ocean evolution of the basal channels is required to understand the present behaviour and future dynamics of ice sheets and ice shelves. Here, we present focused active seismic and radar surveys of a basal channel, ∼950 m wide and ∼200 m high, and its upstream continuation beneath Support Force Glacier, which feeds into the Filchner Ice Shelf, West Antarctica. Immediately seaward from the grounding line, below the basal channel, the seismic profiles show an ∼6.75 km long, 3.2 km wide and 200 m thick sedimentary sequence with chaotic to weakly stratified reflections we interpret as a grounding line fan deposited by a subglacial drainage channel directly upstream of the basal channel. Further downstream the seabed has a different character; it consists of harder, stratified consolidated sediments, deposited under different glaciological circumstances, or possibly bedrock. In contrast to the standard perception of a rapid change in ice shelf thickness just downstream of the grounding line, we find a flat topography of the ice shelf base with an almost constant ice thickness gradient along-flow, indicating only little basal melting, but an initial widening of the basal channel, which we ascribe to melting along its flanks. Our findings provide a detailed view of a more complex interaction between the ocean and subglacial hydrology to form basal channels in ice shelves.

Integrated reflection seismics, 3D-GPR, leveling, DInSAR to detect and characterize high-risk sinkholes in urban cover evaporite karst. A NE Italian case study.

<p>Sinkholes linked to cover evaporite karst in urban environments still represent a challenge in terms of clear identification and mapping considering the anthropic rehash and the presence of man-made structures.</p><p>We propose and tested a methodology to identify the subsiding features in an urban area within a cover evaporite karst environment, through an integrated and non-invasive multi-scale approach combining seismic reflection, DInSAR, leveling and full 3D GPR.</p><p>The analysis was conducted in a small village in the Tagliamento valley (Friuli Venezia Giulia region, NE Italy) named Quinis, where sinkholes are reported since a long time as well as the hazard linked to their presence: within the years, several houses have been demolished and at present many of them are damaged.</p><p>First we applied each methodology independently and after we compared, combined and integrated them to obtain more coherent and cross-validates results. Seismic reflection imagined the covered karst bedrock identifying three depocenters; DInSAR investigation allowed to identify an area with higher vertical velocities; leveling data presented a downward displacement comparable with DInSAR results; 3D GPR, applied here for the first time in the study and characterization of sinkholes, clearly defined shallow sinking features imaging also under a shallow dense pipe network. Combining all the obtained results with accurate field observations we identified and map the highest vulnerable zones.</p><p>The final result is the combining of the geophysical, DInSAR and leveling information, while also locating the damaged buildings, the local asphalt pavement breaks or renovation and the buildings which are nowadays demolished, by using vintage photographs and historical maps. The data are consistent, being the most relevant present damages and the demolished building within the zones with higher sinking velocity on the base of both leveling and DInSAR. Geophysically imaged depocenters lie within the most critical area and perfectly correlate with the local pavement damages.</p><p>In a complex geological and hydrological framework, as in the study area, a multidisciplinary and multi-scale approach is mandatory to identify and map the zone most affected by sinking phenomena. While punctual data such as borehole stratigraphy, local groundwater level variations with time, extensometers measurements and geotechnical parameters are useful to highlight local hazard due to occurring deformation, the proposed integrated methodology addresses a complete and quantitative assessment of the vulnerability of the area. It’s fundamental, especially in anthropized environments, using different integrated techniques, without forgetting the role of the fieldwork of the geologists who can detect the precursors or already occurred, even elusive, signs of the ongoing or incipient sinking.</p>

Combined on-fault and off-fault paleoseismic evidence in the postglacial lacustrine sediments of Achensee (Austria, Eastern Alps)

<p>Intraplate tectonic regimes such as the European Alps are characterized by low crustal deformation rates and thus long recurrence rates of severe earthquakes. High-quality paleoseismic archives are required to overcome our limited perspective of earthquake recurrence and maximum magnitude. However, especially on-fault paleoseismic evidence is scarcely found because of high erosion rates, gravitational slope processes and penetrative anthropogenic landscape modification, which often obscure geomorphic features related to surface ruptures.</p><p>Here, we present the inneralpine lake archive of Achensee in the Northern Calcareous Alps (6.8km² area; 133m water depth) cross-cut by a major fault and potentially holding a continuous paleoseismic archive since the last deglaciation at ~18 ka BP. This major fault is a Cretaceous-Paleogene relatively steep-dipping thrust, with at least 15km length and several hundreds of meters geological offset, located within the current area of enhanced seismicity and oriented to be preferentially re-activated in the current stress field. We used a high-resolution multi-beam bathymetry, a combination of a very dense grid of 3.5kHz “pinger” subbottom profiler and single-channel high-frequency (~0.8-2.0kHz) “sparker” reflection seismics to investigate the postglacial infill with high-resolution and image the deeper structures (e.g. the glacially scoured valley). The seismo-stratigraphic interpretation was ground-truthed and <sup>14</sup>C-dated by five, up to 11m long sediment cores from the two main subbasins.</p><p>We discovered at least eight strong earthquakes hitting the region in the past 11,000 years by off-fault paleoseismic evidence expressed by coeval, multiple mass-transport deposits (MTDs) and co-genetic turbidites. These earthquakes must have reached seismic intensity of >VI (EMS-98) at the lake site calibrated with the strongest known historical earthquake of the region (M<sub>L</sub> 5.2 in Hall CE1670). MTD size and extent corresponding to the CE1670 earthquake compared to the other earthquake imprints let us infer that at least four of the paleo-earthquakes reached higher intensities at Achensee.</p><p>Strikingly, Achensee has also recorded on-fault evidence expressed by steeply-dipping to vertical faults offsetting the lacustrine stratigraphy. These stratigraphic offsets can be traced downwards to the acoustic basement, which hints at faulting originating in the bedrock. For at least two stratigraphic levels, these faults are directly overlain by multiple MTDs indicating that fault activity and slope failures have occurred quasi-simultaneously. The faults observed on the seismic data, affecting the sedimentary infill of the lake, are located above the inferred trace of the major fault where it crosses the lake. Based on this rather unique combined on-fault and off-fault evidence we propose strong paleo-earthquakes documenting activity of this major thrust at ~8.5 ka BP and in the Late Glacial period (below reach of sediment cores). We suggest that these earthquakes have reached M<sub>L</sub>~5.5-6, which is within the magnitude capability of this thrust and at the lower limit of generating surface ruptures according to worldwide magnitude-surface rupture relationships. The other six event horizons lacking in on-fault evidence either represent earthquakes sourced from another fault in the region, earthquakes with a smaller magnitude not capable of surface rupturing like the M<sub>L</sub>5.2 earthquake in Hall CE1670 or on-fault evidence is blurred in seismic data by subsequent stacking of MTDs.</p>

Use of novel 3D seismic technology and machine learning for pothole detection, characterization, and classification — Case study in the Bushveld Complex (South Africa)

We demonstrate that integrating 3D reflection seismics with machine learning (ML) can bring many benefits for the future development of the mining industry. We use a serial integration of reflection seismics, which identifies economic horizon-depression structures known as potholes within the western Bushveld Complex. Thereafter, agglomerative clustering is applied to the resulting data, using features engineered from the physical characteristics of the potholes. Our results indicate that potholes can be divided into several classes based on characteristic features; e.g., large potholes are substantially less steep than small potholes. Furthermore, we model this empirical relationship and show that it can be used to predict average sizes of potholes given their typical in-mine exposures. We also demonstrate that pothole formation is likely to have been initiated depth-wise, followed by lateral increases in size. Lastly, we demonstrate that our serial application of seismically based data generation and ML-based data analytics is a viable alternative to conventional geostastistical analysis, especially for the classification, prediction, and modeling of geologic structures such as potholes.

Dynamics of hydrological and geomorphological processes in evaporite karst at the eastern Dead Sea – a multidisciplinary study

Karst groundwater systems are characterised by the presence of multiple porosity types. Of these, subsurface conduits that facilitate concentrated, heterogeneous flow are challenging to resolve geologically and geophysically. This is especially the case in evaporite karst systems, such as those present on the shores of the Dead Sea, where rapid geomorphological changes are linked to a fall in base level by over 35 m since 1967. Here we combine field observations, remote sensing analysis, and multiple geophysical surveying methods (shear wave reflection seismics, electrical resistivity tomography [ERT], self-potential [SP] and ground penetrating radar [GPR]) to investigate the nature of subsurface groundwater flow and its interaction with hypersaline Dead Sea water on the rapidly retreating eastern shoreline, near Ghor Al-Haditha in Jordan. Remote-sensing data highlight links between the evolution of surface stream channels fed by groundwater springs and the development of surface subsidence patterns over a 25-year period. ERT and SP data from the head of one groundwater-fed channel adjacent the former lakeshore show anomalies that point to concentrated, multidirectional water flow in conduits located in the shallow subsurface (

Acoustic Mapping of Submerged Stone Age Sites—A HALD Approach

Acoustic response from lithics knapped by humans has been demonstrated to facilitate effective detection of submerged Stone Age sites exposed on the seafloor or embedded within its sediments. This phenomenon has recently enabled the non-invasive detection of several hitherto unknown submerged Stone Age sites, as well as the registration of acoustic responses from already known localities. Investigation of the acoustic-response characteristics of knapped lithics, which appear not to be replicated in naturally cracked lithic pieces (geofacts), is presently on-going through laboratory experiments and finite element (FE) modelling of high-resolution 3D-scanned pieces. Experimental work is also being undertaken, employing chirp sub-bottom systems (reflection seismic) on known sites in marine areas and inland water bodies. Fieldwork has already yielded positive results in this initial stage of development of an optimised Human-Altered Lithic Detection (HALD) method for mapping submerged Stone Age sites. This paper reviews the maritime archaeological perspectives of this promising approach, which potentially facilitates new and improved practice, summarizes existing data, and reports on the present state of development. Its focus is not reflection seismics as such, but a useful resonance phenomenon induced by the use of high-resolution reflection seismic systems.

Non-Invasive Methodological Approach to Detect and Characterize High-Risk Sinkholes in Urban Cover Evaporite Karst: Integrated Reflection Seismics, PS-InSAR, Leveling, 3D-GPR and Ancillary Data. A NE Italian Case Study

Sinkholes linked to cover evaporite karst in urban environments still represent a challenge in terms of their clear identification and mapping considering the rehash and man-made structures. In the present research, we have proposed and tested a methodology to identify the subsiding features through an integrated and non-invasive multi-scale approach combining seismic reflection, PS-InSAR (PSI), leveling and full 3D Ground Penetrating Radar (GPR), and thus overpassing the limits of each method. The analysis was conducted in a small village in the Alta Val Tagliamento Valley (Friuli Venezia Giulia region, NE Italy). Here, sinkholes have been reported for a long time as well as the hazards linked to their presence. Within past years, several houses have been demolished and at present many of them are damaged. The PSI investigation allowed the identification of an area with higher vertical velocities; seismic reflection imagined the covered karst bedrock, identifying three depocenters; leveling data presented a downward displacement comparable with PSI results; 3D GPR, applied here for the first time in the study and characterization of sinkholes, defined shallow sinking features. Combining all the obtained results with accurate field observations, we identified and mapped the highest vulnerable zone.