Combined Onshore Research Articles

Combined onshore and offshore wide-scale seismic data acquisition and imaging for carbon capture and storage exploration in Havnsø, Denmark

Strong global actions for climate change include carbon capture and storage (CCS) as a feasible solution to reach carbon neutrality and raise opportunities for detailed subsurface investigations. An acquisition set-up designed for onshore-offshore zones was maximized for wide-scale high-resolution structural imaging and implemented to cover a domal structure of interest for CCS utilization close to the town of Havnsø (Denmark). The challenges of the combined acquisition and processing of land and marine multisensor data along a 42 km seismic profile are analyzed, the suggested solutions are applied, and the limitations are discussed. On the onshore side, a nodal array and a seismic landstreamer system were simultaneously used, whereas along the transition zone, a marine seismic streamer and ocean-bottom seismometers were added to record the seismic response generated by two seismic vibrator sources. The adopted sensing domains (velocity, acceleration, and pressure) were studied, and different processing steps were evaluated to enable their processing and subsequent data set merging. Results suggest, as the best approach, a separate prestack processing of the different data sets and the computation of new geometries and new surface-consistent residual static correction after their merging. The data acquired in the transition zone illuminate, for the first time, the subsurface geology of the region, delineating an expected domal closure. The final seismic section shows high continuity of the reflections with good resolution along the entire profile, identifying the main reservoir structure and the surrounding areas, which are important to ensure reservoir integrity and safe exploitation over longer time scales. Shallow and deep reflections are consistent with the stratigraphic column from a well log near the profile. The presented study shows a comprehensive workflow for processing such a multisensor data set in onshore and transition zone settings.

Arc-crustal compression and its effects on the underlying mantle geometry as elucidated from the potential field signatures of the buckled Cretaceous Cebu lithosphere, Philippines

The occurrence of long wavelength Moho undulations in continental and oceanic lithospheres is attributed to lithospheric buckling. This has been reported for Cretaceous or older lithospheres under compressional regimes. However, there is very limited data on lithospheric buckling of oceanic island arcs. The region where the Cretaceous Cebu arc is under compression due to the impingement of the Palawan micro-Continental Block (PCB) to the Philippine Mobile Belt (PMB). The configuration of the inferred buckled Cretaceous Cebu arc lithosphere is derived from geologic and combined onshore and airborne gravity and magnetics datasets. Subsurface modeling using gravity data revealed the presence of mantle upwelling beneath Central Cebu. This could be considered as one of the crests of long-wavelength Moho undulations commonly attributed to a buckled lithosphere. The oceanic basement was also modelled to determine its configuration. Euler solutions derived from magnetic data showed that the contact between the arc crustal units of the Cansi Volcanics and Pandan Formation is affected by short wavelength folding with wavelengths ranging from 5 to 15 km. The younger arc crustal units are affected by tighter folding with wavelengths ranging from 1 to 5 km. The occurrence of a buckled arc lithosphere is being considered for the first time in this area. The consistent NE-SW folding axes inferred for the folding events strongly suggest that these are consequences of the Middle Miocene collision involving the Philippine Mobile Belt and the Palawan micro-Continental Block.

Trend decomposition aids short-term countrywide wind capacity factor forecasting with machine and deep learning methods

A straightforward decomposition technique applied to hourly wind capacity factor time-series data compiled at the country level can enhance short-term forecasting over the coming twelve hours. The method overcomes the limitations of relying upon limited exogeneous input variables and incorporates near-term impacts of system curtailments and market irregularities, potentially aiding system operators and regulators in optimizing ongoing supply–demand balances. The method establishes multiple attributes from the previous twelve hours of wind capacity data. It then uses those attributes as input variables for forecasting with multiple machine- and deep-learning methods. The method is evaluated using 43,824 hourly data records for combined onshore and offshore wind capacity for Britain from 2015 to 2019. Fourteen machine- and deep-learning models are trained and validated with 2015–2018 data and the trained models are applied to forecast the 8760 hourly records for 2019. 10-fold cross validation is used to verify the robustness of the validation procedure. Forecasting performance is benchmarked against autoregressive integrated moving average and multi-linear regression models. The best-performing models (extreme learning machine, support vector regression and extreme gradient boosting) outperform both autoregressive integrated moving average and multi-linear regression methods in forecasting t0 to t + 12 h ahead. Machine learning models substantially outperform the five deep learning models evaluated in forecasting accuracy and execution time. The transparency of the multi-linear regression model provides useful insight to the relative contributions of each input variable to the forecasts, highlighting that the trend attribute impacts vary significantly across the t0 to t + 12 time periods forecast.

Genesis, distribution and dynamics of lagoon marl extrusions along the Curonian Spit, southeast Baltic Coast

A unique geological process of extrusion of lagoon marl from beneath massive migrating sand dunes is characteristic for large segments of the Curonian Spit – a ~100‐km‐long sandy barrier that separates the Curonian Lagoon from the Baltic Sea. Exposures of a composite set of Holocene organic sediments such as gyttja, clayey gyttja and gyttja clay, collectively referred to as ‘lagoon marl’, are common along the northern half of the lagoon coast of the spit. The marl outcrops rise up to 3–4 m above the lagoon level and owe their origin to extrusion from their 7–8 m in situ depth beneath the present regional water table. New detailed investigations of the Baltic Sea bottom along the southern half of the Curonian Spit using side‐scan sonar, a multibeam echosounder, seismic imaging, sediment sampling and video observations allowed identification and mapping of a unique underwater landscape formed by extensive outcrops of laminated and folded lagoon marl at water depths of 5–15 m. The combined onshore–offshore database indicates that relict lagoon marl was deformed, compacted and dehydrated by a massive dune‐covered coastal barrier migrating landward (retrograding) over these sediments during the Littorina Sea transgression in a processes termed ‘dune tectonics’. Spatial analysis of the relict sediments traced in offshore geophysical data helped to constrain the rates of the southeast migration of the dune massif. A conceptual model is presented to explain the present context of marl exposures above the regional water table, as well as the occurrence of relict lagoon marl extrusions (diapirs) on the underwater marine slope of the Curonian Spit.

New insights into the tectonic inversion of North Canterbury and the regional structural context of the 2010–2011 Canterbury earthquake sequence, New Zealand

AbstractThe 2010–2011 Canterbury earthquake sequence highlighted the existence of previously unknown active faults beneath the North Canterbury plains and Pegasus Bay, South Island, New Zealand. We provide new insights into the geometry and kinematics of ongoing deformation by analyzing marine seismic data to produce new maps of regional faults and cross‐sectional reconstructions of deformation history. Active faulting and folding extends up to 30 km offshore, and involves reactivation of sets of Late Cretaceous‐Paleogene normal faults under NW‐SE tectonic compression. The active faults consist predominantly of NE‐SW striking, SE‐dipping reverse faults, and less commonly E‐W to NW‐SE faults suitably oriented for strike‐slip reactivation. Additionally, newly developing reverse faults obliquely segment and overprint the inherited basement fabric and impose geometric and kinematic complexities revealed by mapping and reverse displacement profiles of markers. The Quaternary reverse slip rates decrease from 0.1–0.3 mm/yr beneath northern Pegasus Bay to <0.05 mm/yr approaching Banks Peninsula. Fault growth modeling involving trishear fault‐propagation folding mechanisms successfully restores an evolutionary sequence of progressive fault inversion, revealing a history of reactivated individual faults. Tectonic inversion and overprinting processes beneath Pegasus Bay are immature and <1.2 ± 0.4 Ma old, with no evidence of systematic spatial migration of deformation. Our marine data analyses give insights into the structural context of the 2010–2011 Canterbury earthquake sequence, while the combined onshore to offshore data provide an excellent illustration of fault growth associated with immature inversion tectonics, in which selective fault reactivation results from compressive stress imposed across a complex network of inherited faults.

The insular shelves of the Faial‐Pico Ridge (Azores archipelago): A morphological record of its evolution

AbstractShelves surrounding reefless volcanic ocean islands are formed by surf erosion of their slopes during changing sea levels. Posterosional lava flows, if abundant, can cross the coastal cliffs and fill partially or completely the accommodation space left by erosion. In this study, multibeam bathymetry, high‐resolution seismic reflection profiles, and sediment samples are used to characterize the morphology of the insular shelves adjacent to Pico Island. The data show offshore fresh lava flow morphologies, as well as an irregular basement beneath shelf sedimentary bodies and reduced shelf width adjacent to older volcanic edifices in Pico. These observations suggest that these shelves have been significantly filled by volcanic progradation and can thus be classified as “rejuvenated.” Despite the general volcanic infilling of the shelves around Pico, most of their edges are below the depth of the Last Glacial Maximum, revealing that at least parts of the island have subsided after the shelves formed by surf erosion. Prograding lava deltas reached the shelf edge in some areas triggering small slope failures, locally decreasing the shelf width and depth of their edges. These areas can represent a significant risk for the local population; hence, their identification can be useful for hazard assessment and contribute to wiser land use planning. Shelf and subaerial geomorphology, magnetic anomalies and crustal structure data of the two islands were also interpreted to reconstruct the long‐term combined onshore and offshore evolution of the Faial‐Pico ridge. The subaerial emergence of this ridge is apparently older than previously thought, i.e., before ∼850 ka.

Combined onshore and offshore seismic investigations image fault structure at the geothermal field of Aedipsos-Yaltra, central Greece

Vassilis K. Karastathis and Evangelos Mouzakiotis delineated the hydrothermal mechanism associated with a large fault indicating the best exploitation point. The region of Aedipsos in the northern part of Evia island (Euboea), Greece (Figure 1), has been famous for its hot springs since the 4 th -century BC. It is notable that there was an ancient legend claiming that Hercules used to bathe in the hot waters of Aedipsos to renew his strength after his labours. Those springs have also been referred to by ancient writers such as Aristotle, Strabo, Plutarch, Claudius Ptolemaeus. The Roman general Syllas also visited Aedipsos and built public baths at the place. Today the region is a touristic attraction due to its thermal springs. The Yaltra Baths (or Gialtra), situated only 5 km to the west, also have high touristic development based on the local thermal springs. The hot springs of the wider region can be seen in Figure 1. In several sites the temperature of the springs reaches values higher than 40 o C and up to 84 o C (Gioni-Stavropoulou, 1983).

Effects of mass-wasting on the stratigraphic architecture of a fjord-valley fill: Correlation of onshore, shear-wave seismic and marine seismic data at Trondheim, Norway

A combined record of onshore, high-resolution, shear-wave seismic and marine, P-wave seismic data shows the near-shore stratigraphic organisation of a fjord-valley fill at Trondheim, central Norway. The overall stratigraphy shows a typical fjord-valley fill of up to 160m in thickness that is composed of glaciomarine and fjord-marine sediments overlain by deltaic deposits which, in the onshore section, are draped by an anthropogenic fill. An interval characterised by low shear-wave velocities below the onshore, anthropogenic fills is shown by sampled drillings to comprise part clay and part organic deposits. The seismic record reveals evidence of recurrent mass-wasting events that include landslide debris, turbidite deposits and channel features. A chronological framework for the stratigraphy is provided by existing radiocarbon dates for local deglaciation and mass-wasting events. The combined onshore–offshore datasets document the Lateglacial to Holocene development of an emerging fjord and delta system affected by major, mass-wasting events. The architectural development of the younger part of the stratigraphy is interpreted as particularly affected by two, known mass-wasting events each involving several million m3 of debris. A thick layer of debris from a 3500calyr event probably resulted in a blockage of the subaqueous channels, diversion of turbidity currents and deposition of lateral accretion packages, whereby local fjord sedimentation temporarily increased. A 2200calyr event resulted in a sudden reorganisation of the distributary system whereby distal delta sedimentation abruptly shifted north- to northeastwards to an area temporarily subjected to marine erosion during fall of relative sea level. The study adds to the understanding of landslides and low-stiffness zones in the area of importance for near-shore geohazard assessment. The study also provides insight into the stratigraphic variability of fjord-valley fills, and highlights the interplay between fjord marine sedimentation, marine abrasion, delta progradation and mass wasting during late stages of fjord filling.

New insights on the interseismic active deformation along the North Ecuadorian–South Colombian (NESC) margin

The North Ecuadorian–South Colombian subduction zone was the site of the 1906 Mw 8.8 megathrust earthquake. This main shock was followed by three large events in 1942, 1958, and 1979 whose rupture zones were located within the 500 km long 1906 rupture area. A combined onshore and offshore temporary seismic network covering from the trench to the Andes was deployed during 3 months in the area of large earthquakes, in order to obtain a detailed knowledge of the seismic background activity. Resulting earthquakes location and mechanisms bring new insights on interseismic active deformation distribution in the three main tectonic units of the margin, namely, the Interplate Seismogenic Zone, the fore‐arc region which is part of the North Andean Block and the downgoing oceanic Nazca plate. The interplate seismic activity presents along strike variations, suggesting that the seismicity and the associated stress buildup along the plate interface depend on the time elapsed since the last large earthquakes. According to our results, the updip and downdip limits of the seismogenic zone appear to be located at 12 and 30 km depth, respectively. Shallow to intermediate depth seismicity indicates a slab dip angle of ≈25°. North of the Carnegie Ridge, the Wadati‐Benioff plane is defined beneath the fore arc down to ≈100 km depth. Facing the ridge, the Wadati‐Benioff plane extends beneath the Andes, down to ≈140 km depth. This observation conflicts with the hypothesis of the presence of a flat slab at a depth of 100 km facing the ridge. In the overlying fore‐arc region, the crustal seismicity occurs down to 40 km depth and is mainly concentrated in a roughly NW‐SE 100 km wide stripe stretching from the coast, at about 1°N, to the Andes. The location of this active deformation stripe coincides with observed tectonic segmentation of the coastal domain as evidenced by the presence of an uplifting segment to the south and a subsiding segment to the north of the stripe. It also corresponds to a ≈30° change in the trend of the Andes, suggesting that the curvature of the volcanic arc might play an important role in the deformation of the fore‐arc region.

Arc‐parallel shear deformation and escape flow in the mantle wedge of the Central America subduction zone: Evidence from P wave anisotropy

The P wave anisotropy of the crust and uppermost mantle has been determined in two separate areas of the continental margin of Central America covering offshore and onshore parts of southern Nicaragua and northern and central Costa Rica. Our study is based on traveltime data from local earthquakes recorded with three combined onshore and offshore seismic networks that had been deployed in three half‐year intervals between 2002 and 2006. The P wave traveltime data have been tomographically inverted in terms of an elliptical anisotropic P wave velocity distribution. In the upper crust beneath Costa Rica, the seismic anisotropy is of the order of 2%–5% and locally strongly variable in orientation corresponding to the complicated tectonic structure. In contrast, the upper mantle beneath both study regions shows stronger anisotropy of the order of 5%–10% and a more coherent orientation of the axis of fast P wave velocity that varies systematically on regional scales: The upper mantle of the incoming oceanic plate is characterized by a trench‐normal orientation of the fast P wave velocity axis presumably corresponding to mineral alignment (lattice preferred orientation LPO) in transport direction. This pattern is corroded in the uppermost part of the subduction zone, possibly by the influence of bending‐related trench‐parallel faults and serpentinization that can overprint or annihilate the LPO anisotropy. The upper mantle of the overriding plate and mantle wedge shows a clear trench‐parallel orientation of the fast P wave velocity axis. The anisotropy is stronger in Nicaragua than in southern central Costa Rica (factor of 2 orders of magnitude). The development of a stronger anisotropy in the Nicaragua could be driven by a change in the stress regime from compressional in southern Costa Rica to transpressional in Nicaragua corresponding to a change from near‐orthogonal subduction in the SE to oblique subduction and slab retreat in the NW. From a comparison with S/SKS wave, GPS, and geochemical observations follows that the most likely explanation for the observed pattern of P wave anisotropy in the mantle wedge is LPO caused by a trench‐parallel shear deformation and/or NW oriented escape flow originating in the compressional zone near the Cocos Ridge collision area.

Segmentation of the eastern North Greenland oblique-shear margin — Regional plate tectonic implications

Late Cenozoic opening of the Fram Strait led to formation of the NW–SE striking, eastern North Greenland oblique-shear margin at transition from the Norwegian–Greenland Sea to the SW Eurasia Basin. Onshore geology exposed on large peninsulas in front of the major Greenland Inland Ice reveals a highly complex, Paleozoic–early Cenozoic pre-opening setting. However, due to extreme ice conditions, very little is known about the offshore areas seawards of – and between – the peninsulas. Consequently, prevailing structural-tectonic models of the margin tend to be significantly oversimplified and inadequate. We present the first, combined onshore–offshore, model of the margin integrating onshore outcrops with potential field data, new offshore seismic reflection data and receiver-function analysis of seismic broad band data. The results reveal a margin which is far more complex than previously anticipated. In particular, we interpret strong margin segmentation along N/NE-striking fault structures. The structures are likely to have formed by Late Mesozoic–early Cenozoic strike-slip tectonics and have continued to be active during the late Cenozoic. A more than 8 km deep sedimentary basin is interpreted to underlie the central Wandel Sea, confined by these N/NE-striking structures. We suggest that similar margin segmentation affected the conjugate Yermak Plateau–North Svalbard margin as well as parts of the NE Greenland margin to the SE. Hence, the results have important implications for the general understanding of the Mesozoic–early Cenozoic development of the intra-continental De Geer Mega-shear Region between North Greenland and Eurasia.

Flux and fate of small mountainous rivers derived sediments into the Taiwan Strait

High-resolution CHIRP sonar profiles across the Taiwan Strait reveal a large silt–sand-dominated deltaic clinoform, up to 50-m thick, overlying the postglacial transgressive sea floor across the southeastern, central, and northern strait. Delta-like configuration and internal depositional sequences indicate a northwestward progradation from western Taiwan, primarily from the Choshui (Zhuoshui) River. Grain-size and mineral data confirm the sediment's Taiwanese derivation. The CHIRP profiles, together with existing radiocarbon and geomagnetic dates, suggest that the clinoform has formed over the past 10 kyr. The estimated volume of 375 km 3 of sediment (mainly sand and silt) suggests a mean annual accumulation of 60 × 10 6 t/yr. Presumably much of fine mud delivered by Taiwanese rivers has been washed away by the local currents, and escaped either northeastward into the Southern Okinawa Trough or southward into the South China Sea. Numerous shallow borings onshore over the central western Taiwan coastal plain reveal an additional 350 km 3 of fluvial sediment that has accumulated over the past 10 kyr. The combined onshore–offshore Holocene accumulation, together with an unknown amount of finer sediment that escapes the system, indicates that the long-term sediment flux from Western Taiwanese rivers exceeds 100 × 10 6 t/yr, which is not different from the present-day combined annual discharges from the Choshui, Tsengwen, Ehrjen and Wu rivers into the Taiwan Strait.

Crustal-scale seismic profiles across Taiwan and the western Philippine Sea

We have used combined onshore and offshore wide-angle seismic data sets to model the velocity structure of the Taiwan arc–continent collision along three cross-island transects. Although Taiwan is well known as a collisional orogen, relatively few data have been collected that reveal the deeper structure resulting from this lithospheric-scale process. Our southern transect crosses the Hengchun Peninsula of southernmost Taiwan and demonstrates characteristics of incipient collision. Here, 11-km-thick, transitional crust of the Eurasian plate (EUP) subducts beneath a large, rapidly growing accretionary prism. This prism also overrides the N. Luzon forearc to the east as it grows. Just west of the arc axis there is an abrupt discontinuity in the forearc velocity structure. Because this break is accompanied by intense seismicity, we interpret that the forearc block is being detached from the N. Luzon arc and Philippine Sea plate (PSP) at this point. Our middle transect illustrates the structure of the developing collision. Steep and overturned velocity contours indicate probable large-scale thrust boundaries across the orogen. The leading edge of the coherent PSP appears to extend to beneath the east coast of Taiwan. Deformation of the PSP is largely limited to the remnant N. Luzon arc with no evidence of crustal thickening to the east in the Huatung basin. Our northern transect illustrates slab–continent collision—the continuing collision of the PSP and EUP as the PSP subducts. The collisional contact is below ∼20 km depths along this transect NE of Hualien. This transect shows elements of the transition from arc–continent collision to Ryukyu arc subduction. Both of our models across the Central Range suggest that the Paleozoic to Mesozoic basement rocks there may have been emplaced as thick, coherent thrust sheets. This suggests a process of partial continental subduction followed by intra-crustal detachment and buoyancy-aided exhumation. Although our models provide previously unknown structural information about the Taiwan orogen, our data do not define the deepest orogenic structure nor the structure of western Taiwan. Additional seismic (active and passive), geologic, and geodynamic modeling work must be done to fully define the structure, the active deformation zones, and the key geodynamic process of the Taiwan arc–continent collision.

Local earthquake tomography of shallow subduction in north Chile: A combined onshore and offshore study

Selected travel time data from the aftershock series of the great Antofagasta earthquake (Mw = 8.0) have been inverted simultaneously for both hypocenter locations and three‐dimensional VP and VP/VS structure. The data were collected with a dense 44‐station seismic network including ocean bottom hydrophones. We performed a series of inversions with increasing complexity: 1‐D, 2‐D, and 3‐D. This approach was chosen to account for the heterogeneous seismicity distribution and to obtain a smooth regional model in areas of low resolution. Special efforts were made to assess the solution quality including standard resolution estimates and tests with synthetic travel times. The subducted plate is imaged between 20 and 50 km in depth as an eastward dipping high VP feature. High VP/VS ratios within the oceanic crust possibly indicate elevated fluid content. Underplating of material eroded close to the trench is found beneath the Mejillones Peninsula. The lower crust of the overlying plate is characterized by an average VP of 6.8–6.9 km/s and an average to low VP/VS ratio. Large areas of anomalously high VP are found in the lower crust south of the city of Antofagasta; they are interpreted as remants of magmatic intrusions. A zone of high VP/VS ratios is found within the rupture area of the Antofagasta main shock, just above the subducted slab. Its location within the region of highest stress release from the main shock suggests that the main shock rupture causes the high VP/VS ratio. The high VP/VS ratio could indicate postseismic fluid migration from the subducted oceanic crust into the overlying lower crust.

Seasonal patterns of onshore/offshore sediment movement

Measurements of beach profiles from southern California spanning a 5‐year period have been examined for temporal changes in beach configuration. On an annual time scale the data suggest two distinct seasonal pivotal points separating eroding and accreting regions. Empirical eigenfunction analysis of combined onshore and offshore profiles shows a pivotal point for seasonal onshore/offshore sediment movement at a depth of 2–3 m below mean sea level and suggests another at a 6‐m depth. Analysis of accurate reference rod measurements at 4‐, 6‐, 10‐, 14‐, and 20‐m depths supports the presence of the 6‐m pivotal point. A simple model of depth‐dependent seasonal sand movement suggests that during initial winter storms, sand is eroded from both the foreshore and from depths of 6–10 m and is deposited in water depths from 2 to 6 m. During less energetic periods, sediment migrates both shoreward (to the beach face) and seaward (to depths of 10 m) from its winter site of deposition (water depths from 2–6 m). This observation of depth‐dependent motion contradicts the simple single pivotal point model previously suggested for nearshore seasonal onshore/offshore sediment motion and emphasizes the complexity of nearshore sediment transport. A sediment budget for seasonal onshore/offshore transport, based on the dual pivotal point model, consists of exchanges of 85 m3/m of beach length across the 3‐m pivotal point and 15 m3/m across the 6‐m pivotal point. Over a longer time scale (i.e., the entire 5 years of study) the beaches showed no net erosion or accretion, suggesting that this limited coastal region is stable over these short time scales.