High-resolution Seismic Reflection Data Research Articles

Geohazard features of the Ligurian Sea

ABSTRACT We present the characterization of geohazard-related features of the Ligurian Sea with the map of its physiographic domains (1:250,000 scale) and five maps (1:100,000 scale) of the morphological and morpho-bathymetric elements. These were realized in the framework of the MaGIC (Marine Geohazard along Italian Coasts) project, promoted by the Italian Civil Protection Department. The characterization of submarine geohazards along continental margins, together with mapping of marine geological features are critical for the management of coastal risks. Multibeam bathymetric data combined with high-resolution 2D reflection seismic data, allowed to identify and map the main tectono-sedimentary features related to mass movements along the Ligurian continental margin. A dense grid of seismic profiles allowed to improve the comprehension of the margin structure as well as its recent tectonic activity providing new insights concerning mass wasting processes and active tectonics relationships to plan further, focalized investigations in areas of maximum risk.

Hazardous explosive eruptions of a recharging multi-cyclic island arc caldera

Caldera-forming eruptions of silicic volcanic systems are among the most devastating events on Earth. By contrast, post-collapse volcanic activity initiating new caldera cycles is generally considered less hazardous. Formed after Santorini’s latest caldera-forming eruption of ~1600 bce, the Kameni Volcano in the southern Aegean Sea enables the eruptive evolution of a recharging multi-cyclic caldera to be reconstructed. Kameni’s eruptive record has been documented by onshore products and historical descriptions of mainly effusive eruptions dating back to 197 bce. Here we combine high-resolution seismic reflection data with cored lithologies from International Ocean Discovery Program Expedition 398 at four sites to determine the submarine architecture and volcanic history of intra-caldera deposits from Kameni. Our shore-crossing analysis reveals the deposits of a submarine explosive eruption that produced up to 3.1 km3 of pumice and ash, which we relate to a historical eruption in 726 ce. The estimated volcanic explosivity index of magnitude 5 exceeds previously considered worst-case eruptive scenarios for Santorini. Our finding that the Santorini caldera is capable of producing large explosive eruptions at an early stage in the caldera cycle implies an elevated hazard potential for the eastern Mediterranean region, and potentially for other recharging silicic calderas.

Fault structure, seismicity, and magmatism of the Littoral Fault zone, northern South China sea: Insights from high-resolution seismic reflection data

Four destructive M7+ earthquakes and 18 M6+ earthquakes demonstrate that the Littoral Fault Zone (LFZ) is one of the most seismically active regions in the offshore area of South China. However, the location, geometry, and structural characteristics of the LFZ, which spans over 1000 km, have not been properly examined. In this study, we investigate the precise position of the LFZ offshore East Guangdong (EGD) and map the fault structure using recently acquired high-resolution multi-channel seismic (MCS) reflection data. The LFZ consists of an array of normal faults that collectively form a terraced sidewall fault zone linked to the continental rifting of the South China Sea (SCS). This fault zone is approximately 35–75 km wide and runs parallel to the shoreline. The primary faults of the LFZ EGD segment correlate to a high-gravity gradient zone and serve as the northern boundary of the Zhu-I Depression. During the syn-rift phase, the LFZ experienced significant intra-plate deformation, resulting in fault throws of up to 1000 m. This deformation was locally influenced by pre-existing structures, giving rise to a listric geometry at depth. In the post-rift period, the LFZ exhibited inherited activity along its rift-related faults. The presence of growth fault structures suggests that the LFZ remained active throughout the Cenozoic, with numerous active faults extending practically to the seafloor. The negative flower structure confirms the strike-slip character of the LFZ, which is influenced by the modern tectonic stress field. Intense multi-phase Cenozoic magmatism, including igneous intrusion, volcanism, and lava flows, has been detected in the offshore area of Nan'ao Island. The frequent earthquakes in the study area are primarily attributed to far-field stress release resulting from the subduction of the Philippine Sea Plate beneath the Eurasian Plate. Additionally, the intense magmatism weakens crustal structures, further facilitating stress release and the frequent occurrence of earthquakes.

Out of sight, out of mind. Submarine springs in the Dead Sea — An underappreciated phenomenon

The drastic drop in Dead Sea lake levels during the last few decades has caused the formation of over 6000 dangerous sinkholes along its coasts. Concurrently, retreating shorelines have led to the exposure of an under-explored phenomenon – submarine spring systems. Once exposed on land, these features are labeled as the more familiar geological hazard – i.e. sinkholes, with no distinction between the two. While visually they may seem similar, the underlying formation mechanism is different, as may well be their hazard potential. This study utilizes high-resolution seismic reflection data collected in 1984 when lake levels were some 35 m higher, together with multibeam bathymetric data from 2014, visual observations and water chemistry data from a verified spring system in order to assess the underlying formation mechanisms of these features, their stability and morphology. Results show that the springs are relatively stable and long-lived systems. They “deserve” to be separated from the sinkholes and studied as a distinct phenomenon. The springs discharge into the lake a significant amount of freshwater from the adjacent aquifers and are therefore, a largely underestimated part of its hydrological budget and the connected fresh groundwater resources. The acceptance of submarine springs as a distinct geological phenomenon and their consideration as a major groundwater outlet into the lake will lead to more realistic groundwater resource models of the Judea and Samaria Eastern Mountain aquifer than exist today. This is extremely important given the increase aridification of the area and the increased demand for freshwater resources.

Formation of undulating seafloor bedforms during the Minoan eruption and their implications for eruption dynamics and slope stability at Santorini

The Minoan eruption of Santorini is one of the largest Holocene volcanic events and produced several cubic kilometers of pyroclastic flows emplaced on the submerged flanks of the volcano. Marine geophysical surveys reveal a multitude of undulating seafloor bedforms (USBs) around Santorini. While similar structures are known from other volcanoes worldwide, Santorini offers the unique opportunity to relate USB formation with volcanic processes during one of the best-studied volcanic eruptions worldwide. In this study, we combine high-resolution seismic reflection data with multibeam echosounder bathymetry to reveal the internal architecture of USBs around Santorini and to relate their morphological characteristics to formational processes. The USBs around Santorini were formed during the Minoan eruption and represent the seafloor expression of mass transport deposits. Three types of deposits differ in composition or origin. (1) Depositional USBs, which can only be found to the north of the island, where Minoan eruption ignimbrites reach their maximum thickness and the undulating topography is the result of thrusting within the deposit. (2) USBs related to slope failures of volcaniclastics from the entire Thera Pyroclastic Formation, which can be found east, south, and west of the island. (3) USBs associated with deep-seated deformation, which occurs on the southwestern flank along an area affected by rift tectonics and extends to a depth of more than 200 m below the seafloor. In cases (2) and (3), the USBs are formed upslope by block rotation and downslope by thrusting. Our study indicates that these processes may have contributed to the generation of the devastating Minoan tsunami. Since Santorini is located in one of the most tectonically active regions in the Mediterranean, capable of producing earthquakes with magnitude M7+, our study has important implications for hazard assessment. A strong earthquake located close to the island may have the potential to reactivate slope instabilities posing a previously undetected but potentially significant tsunami hazard.

Basement structures exert a crucial influence on the structural configuration of later rift development: Insights from the Shanan and Chengbei sag, Bohai Bay Basin

The Early Cretaceous and Paleogene rift structures of the Shanan and Chengbei sags were inherited from basement tectonic deformation of Indosinian and Yanshanian tectonism. However, the architecture of the basement structures and its influence on the structural configuration of later rift development are poorly understood, which impeded hydrocarbon exploration efforts in the study area. Based on 2D high-resolution seismic reflection data in the study area and onshore field data in the Luxi area, we recognize the Indosinian NW-striking low-angle fold-thrust belts and Yanshanian NE-striking high-angle thrusts in basement of the study area. The pre-rift NW- and NE-striking basement structures exert a crucial influence on the two-phase rift architecture and structural evolution of the study area: (1) Low-angle NW-striking Indosinian thrusts are dominantly reactivated as dip-slip major normal faults during Early Cretaceous, such as the Shanan fault and Chengbei faults. The high-angle Yanshanian NE-striking thrusts are primarily reactivated as left-lateral strike-slip faults during this stage, such as the Shaxi fault. (2) During the Paleogene, oblique extension took place across the pre-existing NW-striking reactivated normal faults. A set of half-grabens formed along the major NW-striking oblique-slip faults and large amount of ENE-striking secondary normal faults developed in the NW-striking sags. The pre-existing left-lateral NNE-striking faults shifted to become right-lateral and extensional faults. The Early Cretaceous rift was primarily controlled by NE-SW-directed extension, which was derived from the large-scale left-lateral strike-slip movement of the Tan-Lu Fault Zone. The Paleogene oblique rift was induced by NNW-SSE-directed extension related to the upwelling water-rich mantle wedge of the Pacific subduction zone.

Shallow deformation on the Kirby Hills fault, Sacramento–San Joaquin Delta, California (USA), revealed from high-resolution seismic reflection data and coring in a fluvial system

Abstract The Sacramento–San Joaquin Delta (Delta) in California (USA) is an important part of the state’s freshwater system and is also a major source of agricultural and natural resources. However, the Delta is traversed by a series of faults that make up the easternmost part of the San Andreas fault system at this latitude and pose seismic hazard to this region. In this study, we use new high-resolution chirp subbottom data1 to map and characterize the shallow expression of the Kirby Hills fault, where it has been mapped to cross the Sacramento River at the western extent of the Delta. The fault is buried here, but we document a broad zone of deformation associated with the eastern strand of the fault that changes in character, along strike, across ~600 m of the river channel. Radiocarbon dates from sediment cores collected in the Sacramento River provide some minimum constraints on the age of deformation. We do not observe evidence of the western strand as previously mapped. We also discuss difficulties of conducting a paleoseismologic study in a fluvial environment.

Constrained 2D inversion of radio-magnetotelluric and controlled-source audio-magnetotelluric data using high-resolution reflection seismic data: An example in groundwater surveying from Heby, Sweden

We advance a previously established method for 2D inversion of electromagnetic data, in which the smoothness constraints are locally reweighted according to the seismic envelope fractional gradients (SEFGs). As the first step of our modifications, seismic envelope values are edited and normalized. This results in the rejection of noise-contaminated parts, clipping the envelope outliers and increasing the reflectivity power of weak seismic signals. Second, we introduce a weighting matrix in the normalization process to incorporate prior information in the constrained inversion regarding the relative contrasts of electrical resistivity in the given parts of the model. Third, due to normalizing the SEFG, there is no need to search for an optimum stabilization factor to modify local smoothing weights, and hence, we set it to a constant value. Finally, an Occam inversion with additional Levenberg-Marquardt damping is used to mitigate possible artifacts in the resistivity model generated by reflection seismic constraints. In applying the proposed scheme to a synthetic example, interfaces of various geologic units are restored as sharp boundaries. In addition, artifacts generated in the original approach are effectively mitigated thanks to the applied normalization process. For the first time, we apply the method to radio-magnetotelluric (RMT) and controlled-source audio-magnetotelluric (CSAMT) field data acquired along a profile across a known aquifer in Heby, Sweden. Our inversion models compare favorably to previously presented results from seismic and geoelectric data. By modifying the smoothness weights based on SEFG, the depth to the bedrock is recovered well, being constrained by the corresponding interfaces in the seismic image. The resistivity models from seismically constrained inversions of RMT and CSAMT data reveal steeply dipping and possibly fractured bedrock underneath the valley-shaped aquifer in the area. This interpretation is verified by borehole logs.

Study of the wave scattering in geological environments for the propagation velocity estimation

Diffraction is a naturally physical phenomenon which plays an extremely key role in forming some theoretical foundations of analyzing geophysical data such as seismic reflection and high frequency electromagnetic, Ground Penetrating Radar to study geological objects. In this phenomenon, when a wave field propagates from a source to an object with small size or a rough surface, its wave energy receivers act as the secondary sources and send their energy parts back to geophones on the ground surface. The relationship between the wave propagation time and the receiver coordinates is formulated as a diffraction hyperbola. This paper reported examples of the high frequency electromagnetic and seismic scattering phenomena with their applications in the study of the wave propagation velocity, location of civil underground objects and seabed depth. The research on scattering phenomenon for the electromagnetic waves has been known and widely applied; however, studying the wave velocity model by the diffraction analysis from the high resolution reflection seismic data for a fixed source receiver distance can be firstly considered in Vietnam and in the world. To determine the two wave velocity types, two graphs of their energy and entropy attributes were built and then calculated from their Kirchhoff migrated data in velocity ranges in order to find out their attributes’ extremes versus velocity variables. Their migrated sections with correct wave velocities could correctly locate diffractors as underground anomalies and produced the maximum energy and minimum entropy values in the peaks of diffraction hyperbolae and the surrounding zones of the diffractors, respectively. These geophysical data were measured at the urban and coastal area of Can Gio district, Ho Chi Minh city, Vietnam.

Tunnel valleys in the southeastern North Sea: more data, more complexity

Abstract. Large Pleistocene ice sheets have produced glacial structures both at and below the surface in northern Europe. Some of the largest and most erosive structures are so-called tunnel valleys (TVs): large and deep channels (typically up to 5 km wide and up to 400 m deep, with lengths up to 100 km), which formed below ice sheets. Although the subject of many studies, the details of their formation and fill are still not well understood. Here, we present an update on the distribution of TVs in the southeastern North Sea between Amrum and Heligoland based on a very dense grid of high-resolution 2D multi-channel reflection seismic data (400 m line spacing). The known tunnel valleys (TV1–TV3) in that area can now be traced in greater detail and further westwards, which results in an increased resolution and coverage of their distribution. Additionally, we were able to identify an even deeper and older tunnel valley, TV0, whose orientation parallels the thrust direction of the Heligoland Glacitectonic Complex (HGC). This observation implies a formation of TV0 before the HGC during an early-Elsterian or pre-Elsterian ice advance. For the first time, we acquired high-resolution longitudinal seismic profiles following the thalweg of known TVs. These longitudinal profiles offer clear indications of an incision during high-pressure bank-full conditions. The fill indicates sedimentation in an early high-energy environment for the lower part and a subsequent low-energy environment for the upper part. Our results demonstrate that a very dense profile spacing is required to decipher the complex incisions of TVs during multiple ice advances in a specific region. We also demonstrate that the time- and cost-effective acquisition of high-resolution 2D reflection seismic data holds the potential to further our understanding of the incision and filling mechanisms as well as of the distribution, complexity and incision depths of TVs in different geological settings.

Delineation of Coal Bed Geometry using High Resolution Seismic Reflection Data for Uninterrupted Directional Drilling Exploration

Delineation of Coal Bed Geometry using High Resolution Seismic Reflection Data for Uninterrupted Directional Drilling Exploration

Latitudinal changes in submarine channel-levee system evolution, architecture and flow processes

Models of the sedimentary architecture of submarine channel-levee systems and their formative flow processes are predominantly based on studies from low latitude settings. Here, we integrate high-resolution seismic reflection, bathymetry and GLORIA side scan data to document the architecture and interpret the formative processes of a series of ultra-high latitude (72–76°N) submarine channel-levee systems that feed lobe complexes off the Greenland margin. We demonstrate that the sedimentary architecture of the channel-fills are dominated by vertical or near-vertical sediment accumulation, reflecting the lack of, or very limited nature of, lateral migration over time. All the Greenland channel-levee systems show significant cross-sectional asymmetry, and a peak sinuosity of 1.38, on a low gradient slope (∼0.3°). The bounding external levees are very thick (∼200 m) and wide relative to low latitude systems. Comparison of these channel-levee systems with other examples reveals that these characteristics appear to be common to systems in high and ultra-high latitudes, suggesting latitudinal controls in the sedimentary architecture of submarine channel-levee systems. The differences between high- and low-latitude systems is likely due to the interplay of physical forcing (i.e., Coriolis force) and climatic factors that control sediment calibre and flow type, both of which are latitudinally dependent. Several formative mechanisms for supressing the initial phase of lateral migration and subsequent asymmetrical development are proposed, including:i) rapid channel aggradation, (ii) Coriolis forcing causing preferred deposition on the right-hand side of the channel, and iii) variance in flow properties, with traction- and suspension-dominated flows deposited on opposing sides of the channel. We argue that a high latitudinal location of larger channel-levee systems may result in the dominance of vertical stacking of channels, the construction of large external levees, and the development of a low sinuosity planform.

Origin of the Palos Verdes Restraining Bend and Its Implications for the 3D Geometry of the Fault and Earthquake Hazards in Los Angeles, California

ABSTRACT The Palos Verdes fault zone (PVFZ) extends across the southwestern Los Angeles basin and Inner Continental Borderland, California, and is considered capable of generating large (Mw>7), damaging earthquakes with short recurrence intervals. The 110 km long fault zone is composed of vertical and moderately dipping segments that accommodate oblique, right-lateral reverse displacement. Onshore, there is a counterclockwise reorientation in the PVFZ’s strike, which produces a major restraining bend that generates the Palos Verdes Peninsula. Here, we use well and seismic reflection data to develop kinematic models that show folding of the PVFZ by the Wilmington blind thrust led to formation of the restraining bend. North of the peninsula in Santa Monica Bay, debate persists over the extent, geometry, and activity of the PVFZ. Here, we analyze a dense grid of high-resolution seismic reflection data and present a new mapping of the Santa Monica Bay segment of the PVFZ, including multiple active splays (e.g., Redondo Canyon fault zone) that occur within a broad damage zone at the northern termination of the fault system. Based on these insights and prior studies, we develop a new, comprehensive 3D model of the PVFZ including its Santa Monica Bay, San Pedro Bay, and Lasuen Knoll segments. The sizes of these segments indicate that PVFZ is capable of larger events than previously reported—Mw 7.1–7.4 for single-segment ruptures and Mw 7.4–7.8 for multisegment ruptures. Based on a reported slip rate of 1.1–5.9 mm/yr, average recurrence intervals for these single- and multisegment rupture scenarios are 580–610 and 760–1170 yr, respectively.

Bidirectional subduction of the Bangong-Nujiang ocean revealed by deep-crustal seismic reflection profile

It is well known that the Tibetan plateau exposes a series of terranes that were drifted northward and accreted progressively onto the south margin of Eurasia resulting from ocean closure. However, the processes associated with the ocean closure remains poorly unconstrained. One such issue is whether the demise of the Bangong-Nujiang ocean between the Lhasa and Qiangtang terranes were caused by northward subduction only or both northward and southward subduction. To address this issue, we analyze high-resolution seismic reflection data along a 275-km north-south traverse across the Bangong-Nujiang suture zone. Our work shows that the Moho of the Lhasa terrane lies at 22.5 s (two-way travel time) and the Moho of the Qiangtang terrane lies at 20 s. In the Lhasa terrane, the reflection profile reveals an X-shaped structure that segmented south-dipping reflectors are truncated by continuous north-dipping reflectors. Based on the existing geological and geophysical observations and the cross-cutting relationships among seismic reflectors, we propose that the fossil relics of south-dipping reflectors with relatively weak amplitude in Lhasa terrane were created by the Mesozoic southward subduction of the Bangong-Nujiang ocean, and the north-dipping reflectors were triggered by the younger Cenozoic deformation during the India-Asia collision. In the Qiangtang terrane, our seismic reflection profile displays an antiformal structure with a set of north-dipping reflections in the lower crust, which we interpret as triggered by the northward subduction of the Bangong-Nujiang ocean. The structures as revealed by our seismic observations as a whole indicate that the closure of the Bangong-Nujiang ocean was created by bidirectional oceanic subduction.

Sill swarms and hydrothermal vents in the Qiongdongnan Basin, northern South China Sea

Sills and hydrothermal vents are commonly observed in sedimentary basins. However, not all sills could generate overlying hydrothermal vents, and whether a sill generates a hydrothermal vent or not is still poorly constrained. In this study, a large number of sills and hydrothermal vents are observed, using high-resolution reflection seismic data in the Qiongdongnan Basin of northern South China Sea. These sills are characterized by saucer or linear shapes and present as isolated sills or sill swarms, roughly being larger with buried depth increase. More importantly, the hydrothermal vents are mainly related to the underlying sills intruding into the shallow strata (buried depth <1.4 km), which suggests that magma intrusion depth (from paleo-seabed to the tip of sill) plays an important role on the formation of hydrothermal vents. The duration of magmatism lasted for ∼5 Ma from ∼11.6 Ma to ∼5.7 Ma with peak occurring at ∼8.2 Ma, judging from the spans of hydrothermal vents in the sedimentary strata. This study reports the intense magmatism in the eastern Qiongdongnan Basin for the first time, which extends the distribution of post-rift magmatism in the South China Sea and benefits to explore the mysterious origin of post-rift magmatism. The intense magmatism would also influence the maturity of source rock and hydrocarbon migration in such a petroliferous basin. Moreover, this study also highlights that shallow emplacement of sill is advantageous for the occurrence of hydrothermal vents due to the heating of unconsolidated/less consolidated strata by magmatic sills.

Constraints on the Geometry and Gold Distribution in the Black Reef Formation of South Africa Using 3D Reflection Seismic Data and Micro-X-ray Computed Tomography

Geological and geophysical models are essential for developing reliable mine designs and mineral processing flowsheets. For mineral resource assessment, mine planning, and mineral processing, a deeper understanding of the orebody's features, geology, mineralogy, and variability is required. We investigated the gold-bearing Black Reef Formation in the West Rand and Carletonville goldfields of South Africa using approaches that are components of a transitional framework toward fully digitized mining: (1) high-resolution 3D reflection seismic data to model the orebody; (2) petrography to characterize Au and associated ore constituents (e.g., pyrite); and (3) 3D micro-X-ray computed tomography (µCT) and machine learning to determine mineral association and composition. Reflection seismic reveals that the Black Reef Formation is a planar horizon that dips < 10° and has a well-preserved and uneven paleotopography. Several large-scale faults and dikes (most dipping between 65° and 90°) crosscut the Black Reef Formation. Petrography reveals that gold is commonly associated with pyrite, implying that µCT can be used to assess gold grades using pyrite as a proxy. Moreover, we demonstrate that machine learning can be used to discriminate between pyrite and gold based on physical characteristics. The approaches in this study are intended to supplement rather than replace traditional methodologies. In this study, we demonstrated that they permit novel integration of micro-scale observations into macro-scale modeling, thus permitting better orebody assessment for exploration, resource estimation, mining, and metallurgical purposes. We envision that such integrated approaches will become a key component of future geometallurgical frameworks.

Cenozoic basement-involved rifting of the northern South China Sea margin

The northern South China Sea (SCS) margin developed on a thinning crust comprising basement-involved structures inherited from the post-orogenic extension of the Cathaysia Block. While, the pre-rift structural configuration and its influence on the geometry and development of the Cenozoic rift basins are ambiguous. Based on 3D high-resolution seismic reflection data in the western Pearl River Mouth Basin (PRMB) and onshore fieldwork, we delineated the intrabasement boudinage structure which is constituted by the low-angle E–W-striking Indosinian fold-thrust belts and the steeply dipping NE-striking Yanshanian thrusts. It is concluded that the pre-rift structure played a key role in controlling the architecture and two-phase early rifting of the northern SCS margin. (1) During the Early Eocene syn-rift phase, the Indosinian thrusts were rejuvenated first and controlled the formation of a series of ENE-striking grabens. (2) During the Late Eocene syn-rift phase, both the Indosinian and Yanshanian thrusts were reactivated. The Indosinian thrusts continued to maintain normal faulting and even evolved into the detachment faults. The Yanshanian thrusts were inverted into the NE-striking dextral strike-slip faults, which controlled the formation of the pull-apart basin group en échelon and even the SCS opening. The lithospheric thinning mechanism of the SCS continental margin experienced a major change from the Early Eocene pure shear to the Late Eocene simple shear.

High resolution seismic reflection PP and PS imaging of the bedrock surface below glacial deposits in Marsta, Sweden

Multi-component high resolution seismic reflection data were acquired at the Uppsala University field test site in Marsta, Sweden, in March 2019 with the aim of obtaining an improved understanding of the subsurface structure in the area. An advantage of the site is that a number of boreholes have been drilled there for both hydrogeological and geophysical purposes, allowing surface geophysics to be compared with downhole information. The presence of a low velocity layer above the water table generates significant processing challenges due to trapped waves. A further complication with the data is that the upper part of this layer was frozen at the time of the survey, resulting in the sediments just below the surface having a significantly higher velocity than those below. By analyzing and processing both the vertical and radial component data it was possible to build a velocity model that is consistent with the observed data. A strong PS converted reflection allows the bedrock to be imaged on the radial component stacks to much higher resolution than on the vertical component stacks. Both common conversion point binning and pre-stack depth migration were used to process the radial component data. We confirmed that our processing strategy was effective by generating synthetic data that were processed in a similar manner as the real data. The PS images indicate a step in the bedrock of about 2 m, with depth to it increasing from about 15 m to 17 m close to one of the boreholes. This step is not observed on the PP stacks, due to their lower resolution.

A Case Study Using Seismic Reflection and Well Logs to Reduce and Quantify Uncertainty During a Hydrogeological Assessment

Building a representative three-dimensional conceptual model of the hydrostratigraphic units is the critical first step when undertaking a hydrogeological assessment. The construction of such conceptual model requires integrating all geological data available for reducing the uncertainty of the conceptual model. In addition, the different types of data collected do not have the same level of uncertainty, and this has to be considered during the modeling. This manuscript presents a geostatistical workflow developed to integrate high resolution seismic reflection data with geological well markers and well logs. The proposed workflow allows reducing and quantifying the uncertainty when developing the conceptual model of hydrostratigraphic units and estimating the spatial distribution of hydraulic conductivity needed to simulate groundwater flow. The study involves the field investigation and numerical modeling for understanding the hydraulic connection between a local sand and gravel esker aquifer overlying a fractured bedrock regional aquifer. Seismic reflection data recorded in 2019 and 2021 significantly modify the conceptual model originally envisioned at site by discovering unknown deep esker extensions. Seismic data also help reducing the spatial uncertainty by delimiting the lateral extents, thickness, and depth of the esker aquifer between existing boreholes. Thickness maps of major hydrostratigraphic units are computed by first interpolating the data with higher uncertainty, which are then locally deformed by successive kriging with external drift to honor data with lower uncertainty. Well logs are then combined with seismic reflection data to better represent the spatial distribution of hydrofacies within the esker aquifer, and identify subunits in the bedrock aquifer. These modifications in the conceptual model of hydrostratigraphic units and the better understanding of the vertical heterogeneity of the hydraulic conductivity significantly impact the simulation of groundwater flow, both regionally and locally at site.

Paleolimnological reconstructions for Lake Arakhley (Central Transbaikalia, Russia) inferred from high-resolution reflection seismic data

Abstract. High-resolution seismic data (65 km profiles) have been used to study the sedimentary infill of Ivan-Arakhley lake system (Lakes Arakhley, Ivan and Shakshinskoe). It found that full limnological cycle bears only sediment cover of Lake Arakhley. Total sediment infill of Lake Arakhley is 6-7 m, and it was be divided into three sequences. The uppermost ~2.5 m of sediment cover was presented by a normal lacustrine filling. Lacustrine sediments began to form ca. 21 cal. ka BP. The lake was almost dried during the regression occurred between ca. 3 and 2.6 cal. ka BP.