Buried Bedrock Research Articles

Simulating earth deformation evolution caused by groundwater pumping through ordinary state-based Peridynamics method

Earth deformation including land subsidence and the related earth fissures caused by the over-exploitation of groundwater resources is threatening countries all over the World. At present, the simulation of earth deformation of aquifer system is mainly based on continuum and contact mechanics. These approaches lead to two main challenges, i.e. the need of knowing the location of possible earth fissures in advance and using meshes that must be adjusted during the rupture propagation. Here, ordinary state-based Peridynamics method is firstly introduced to simulate the evolution of continuous and discontinuous earth deformation due to groundwater withdrawal. A failure criteria accounting for shear and tensile strain simultaneously is proposed. The approach is used to simulate the evolution of land subsidence and earth fissures in compressible sedimentary soils with buried rigid ridges. Two case studies are reproduced: one is a test carried out in laboratory and the other one concerns Guangming village, Yangtze River delta. In both cases, the presence of a shallow buried bedrock crossing the alluvial deposits is responsible for the initiation and growth of earth fissure that a satisfactory captured by the model. For the lab experiment, a 0.1 m deep fissure appearing above the rocky tip is computed. In Guangming village area, results show that some small fissure groups were initiated. Then, three main earth fissures developed rapidly with the maximum depth of 32.5 m. This study proposes a consistency modelling framework to simulate continuum earth deformation and discontinuous fissures due to groundwater withdrawal in specific geological settings, and supply a reliable scientific tool for early warning of this geo-hazard.

Laurentide Ice Sheet configuration in southern Ontario, Canada during the last glaciation (MIS 4 to 2) from stratigraphic drilling and LIDAR-based surficial mapping

Regional subsurface mapping of glacial depositional systems preserved in buried bedrock paleovalleys, and quantitative analysis of new LiDAR imagery of surface glacial landforms using machine learning techniques, when combined, are powerful tools for assessing the dynamics of the Laurentide Ice Sheet (LIS) during the last (Wisconsinan) glaciation in southern Ontario. While age dating of deposits preserved below Last Glacial Maximum tills (LGM: marine isotope stage (MIS) 2 < c.24 000 years B.P. (ybp)) is still sparse, newly available sedimentological data derived by cored drilling, combined with legacy outcrop data, identify thick (100 m+) successions of glaciolacustrine sediments and a lack of till(s), indicating that the ice sheet margin did not extend beyond the Niagara Escarpment at the western end of Lake Ontario, during the earliest phases of the glaciation (MIS 4) or the ensuing mid-Wisconsinan (MIS 3). Ice was able to extend into New York State blocking the Rome outlet to the Hudson Valley ponding deep proglacial lakes in the glacio-isostatically depressed Huron–Ontario–Erie basins recorded by thick glaciolacustrine sediments in paleovalleys. These were cannibalized by an expanding Late Wisconsinan ice sheet after ∼24 000 ybp recorded by extensive till sheets resting on a marked erosional unconformity, with drumlinized surfaces. Analysis and visualization of LiDAR data identifies discrete statistically validated flow sets of highly elongated streamlined bedforms (mega-scale glacial lineations (MSGLs)). These provide key evidence of a major reorganization of the ice sheet margin during deglaciation into lobate paleo ice streams shortly after 17 400 ybp. MSGLs are cut across earlier LGM drumlinized tills creating widespread “palimpsest” surfaces. At least two principal phases of fast ice flow can be identified, marked by large fluxes of sediment and the rapid building of large gravel and sand-dominated moraine complexes within interlobate depocentres, the largest glacial landforms in southern Ontario. Analysis of LiDAR data further reveals the common presence of DeGeer moraines where ice margins retreated in water, and iceberg scours. Future work using LiDAR mapping has the objective of fully documenting the number, extent, and timing of ice streams to enhance glaciological modelling when the ice sheet rapidly lost mass.

Tectonism and fracture-related dissolution induced the formation of the large-scale metamorphic granite reservoir: Implications from Bozhong 26–6 Precambrian bedrock trap, offshore Bohai Bay basin, Northern China

Crystalline rocks in basins are unconventional sites for hydrocarbon exploration. Owing to the lack of primary pores in the bedrock, it remains unclear how later tectonism and dissolution alter crystalline rock within large-scale reservoirs. Notably, the recent discovery of a 200 million-ton oil field of Precambrian metamorphic granite in the Bonan buried hill, Bohai Bay Basin, China provides an important opportunity for exploring this mechanism. Herein, the relationship between fractures and the regional tectonic evolution of this buried metamorphic granite mass, as well as the accompanying phenomena of dissolution and vein filling (which record related fluid interactions contributing to second pore formation from tight crystalline rocks) were investigated. The results indicated that the reservoir space was primarily composed of tectonic fractures and associated dissolution pores (maximum porosity ≤18%). The densely interlaced fractures with dominant NWW-trending flat surfaces indicated that the majority of Precambrian bedrock fractures were induced by Indosinian tectonic compression. The Cenozoic north–south tectonic extensional fault obliquely intersects these NWW-trending fractures, accounting for 73% of the previous dense shearing fractures transformed into open fractures. The dissolution pores were primarily distributed along the fractures and increased the porosity of these fracture-related reservoirs by ≤ 12%. The carbon and oxygen isotopes of the calcite fillings within the fractures suggested that atmospheric freshwater was the primary dissolution fluid, whereas some pores were formed by the dissolution action of deep hydrothermal and organic acid fluids. Thus, regional compressive stress is a necessary tectonic mechanism for the formation of densely interlaced fractures in tight crystalline rocks. The later extensional setting contributing to fracture release and fracture dissolution in the epi-diagenesis stage are two immediate factors in the formation of permeability and storage capacity within buried bedrock traps.

On the issue of improving forecasting and prospecting complexes for identifying new sources of acutely scarce mineral raw materials (using the example of placer boulder chromium ores)

Te purpose of the work is to improve forecasting and search complexes to identify new sources of extremely scarce mineral raw materials for the Russian industry. Research methodology. Analysis of the available information on the features of the geological structure and conditions for the placement of shallow slope placers of boulder chromium ores, reconstruction of the buried relief of the bedrock, study of the structural features of the bedrock and modern (daytime) surface of the placers using analytical methods (trend analysis, etc.), study of the spatial variability of the estimated parameters of placer objects, identifcation of factors controlling the distribution of placer productivity. Results. Te reconstruction of the bedrock surface of the most fully studied type placer of cobblestone chrome ores of the Saranovskaya group (placer no. 2) was performed, the spatial variability of the estimated parameters of the placer was studied in the plan, and the factors controlling the distribution of the productivity of loose deposits were determined. Te longitudinal profle of the bedrock surface of the placer is an alternation of slightly inclined and subhorizontal areas, the formation of which is probably due to the repeated manifestation of erosion cycles in the history of the development of the region. Te distribution of productive deposits thickness in the direction of placer elongation is periodic. Te increased thickness of the productive layer runs to areas where slightly inclined sections of the bedrock are replaced by more gentle (sub-horizontal) ones, to the bases of “terraced” steps in the relief. Overall, there is a general trend towards an increase in the thickness of the productive layer down the slope. Te distribution of placer productivity indicators is uneven; in the direction of the elongation of the placer, zones of increase, maximum and decrease in productivity are distinguished, which are typical features of the “elementary placer” according to N. A. Shilo. Tere is a clear confnement of the most productive areas to the subhorizontal morphostructural elements of the buried bedrock in the middle part of the placer. Te microforms of the buried relief, which are associated with the most productive areas of the slope deposits, manifest themselves as negative “remains” of the linear trend of the bedrock surface marks. Te morphostructural elements of the buried bedrock of the shallow placer are inherited in the modern relief. Conclusions. Te analysis of the presented materials emphasizes the importance of morphostructural studies in predicting and searching for placers of pebble chromium ores of close ablation. During prospecting, the task of ultra-precise surveying of the day surface relief with additional mathematical transformations of the obtained data seems to be an urgent task.

Combining infiltration hole and mulching techniques with fish-scale pits effectively improved soil water storage in semiarid areas with shallow buried bedrock (Pisha Sandstone) in China

Effectively regulating slope runoff and infiltrating maximum rainwater are essential to reduce soil and water loss, and promote vegetation restoration in the Pisha Sandstone areas of China, which are semiarid areas with shallow buried bedrock. However, the effects of individual or combined rainwater regulation measures, such as infiltration holes, mulch, and fish-scale pits (a semicircle-shaped pit), on soil water infiltration and restoration in such regions have not yet been fully explored. In this study, the responses of soil water to rainfall in the 0–1.6 m soil layer was investigated through in situ monitoring during the two main rainy seasons under four rainwater regulation treatments in the Pisha Sandstone area of China covered in thin soil layer (0–0.3 m): fish-scale pits with grass mulching and infiltration holes (PGH), fish-scale pits with grass mulching (PG), fish-scale pits without mulching (P), and slope land (control, CK). The results have shown that because of the relatively low permeability (0.08–0.17 m d−1) of Pisha Sandstone, individual torrential rainfall events (63.2 mm) increased the soil water only in the 0–0.3 m layer after 3 days under the CK, P, and PG treatments. However, the PGH treatment simultaneously and significantly increased the soil water in the 0–0.3 m and 0.7–0.8 m layers. Compared with the CK treatment, rainwater replenishment depths under the P, PG, and PGH treatments increased from approximately 0.4 m to 0.9 m, 1.0 m, and 1.1 m, respectively, and the average soil water storage increased by 27 mm, 34 mm, and 50 mm, respectively, in the 0–1.6 m soil layer. Moreover, the PGH treatment enhanced rainwater storage in deep soil (0.6–0.9 m), which decreased the potential for water loss by evaporation. We concluded that the PGH treatment was effective in increasing soil water storage and reducing soil erosion in areas with shallow buried bedrock.

An experimental approach to unravel 2D ground resonances: application to an alluvial-sedimentary basin

The study of ground resonances is important to assess seismic site amplification and to infer information on the geometrical and mechanical properties of the resonating structures. 1D- and 2D-type resonances imply different dynamic behavior that can be distinguished by inspecting the individual spectral components of single-station microtremor measurements. Typically, 2D resonance modes develop along cross-sections of deep sediment-filled valleys and consist of longitudinal, transverse and vertical modes that can be identified as spectral peaks when ground motion is recorded parallel to the axes of the valley. In the case of more complex geometries, such as sedimentary basins, resonance modes are more difficult to predict and depend on the unknown complexity of the buried bedrock geometry. We show how a simple signal rotation procedure applied to single-station microtremor recordings reveals the underlying 2D resonance pattern. The method allows assessing the axes of motion of buried geological structures and identifying 2D resonance modes along these axes. Their directionality, frequency and amplitude features are then analyzed to extract information on the bedrock geometry. We test our method in the Bolzano alluvial-sedimentary basin and we observe that apparently complicated resonance patterns may be simplified by locally referring to the simplest description of the phenomenon as 2D resonance of a valley slice. The bedrock morphology can be decomposed into 2D-like geometries, i.e., excavated channels, and the observed resonances develop within cross-sections of these channels.Graphical

Denudation of the Golan Heights basaltic terrain using in-situ 36Cl and OSL dating

While erosion rates of carbonates along the Dead Sea fault (DSF) and its margins have been determined in several studies, information about denudation of basalts along the DSF are absent. Here we report concentrations of in situ cosmogenic 36Cl in bedrock and sediment samples combined with OSL ages of terraces from the Meshushim basin, a typical basaltic basin in the Golan Heights, northern Dead Sea Rift (DSR).Denudation rates of exposed and soil-covered bedrock basalts, range from 12.3 ± 1.4 mm/ka to 176.1 ± 47.4 mm/ka, well within the range of DSR values, regardless of lithology and climate. In all locations where exposed and soil-covered bedrock were samples next to each other, the soil-covered basalt erodes faster. This is regardless of notable variations in average annual temperature and precipitation. This means that bedrock outcrops were not covered and exposed very frequently, and that chemical weathering at the bedrock-soil interface is a significant erosional factor.Sediments in the Meshushim basin are stored in soil pockets and blankets on the upper plateau, colluvial material on slopes, and alluvial terraces. The difference in 36Cl concentrations between soil-covered and exposed bedrock indicates coverage duration of ca. 3–6 ka. The residence time estimate of only several thousands of years for sediments stored in alluvial terraces is 170 ± 20 y to 4000 ± 290 y. Nowhere in the Meshushim drainage system did we find evidence for storage of sediment longer than a few thousands of years.36Cl concentrations in colluvial and alluvial sediments are similar to those measured in soil-covered basalts. This similarity suggests that sub-surface weathering of basalts is the major source for sediments in upper and central segments of the basin, concurrent with the higher denudation rates of buried bedrock. However, very low 36Cl concentrations measured in samples collected in the lower parts of the basin indicate involvement of landslides which instantaneously expose previously shielded sediments that were incorporated into the drainage system. The effect of landslides is also manifested by the high basin scale denudation rates calculated from alluvial samples in various locations, as well as the geomorphological profile of the Meshushim basin. Sediment yield calculated from 36Cl concentrations measured in this study suggest temporal changes in denudation rates over the Holocene and up to the last ∼60 ka, with higher past denudation rates and sediment yield even in most of the Holocene related to present rates and yields.Overall, the results of this study show that the fact that the Golan lithology is basaltic does not result in different denudation process and rate from other locations investigated along the Dead Sea Rift, which are mostly of carbonate lithology and that relief and climate are stronger parameters than lithology in controlling denudation.

The Sierra Nevada (California) Is a Relict Tropical Late Cretaceous Range: A Field Guide to the Evidence

ABSTRACT: The notion of late-Cenozoic uplift was implied in 1865 by Whitney, who assumed a buried bedrock canyon lay beneath the Table Mountain latite flow and above the modern Stanislaus River canyon, with the difference between the two longitudinal profiles suggesting post-latite uplift. However, only a series of buried bedrock ridges exists and no uplift is inferred. I briefly present thirty-three key sites of about three hundred remnants in the entire Sierra Nevada, from north of the Feather River south to the Kern River. These include Late Cretaceous strata, Eocene strata, Oligocene rhyolites, Mio-Pliocene andesites, and Quaternary basalts, some remnants on canyon floors but most on lower slopes, preserving topography at the time of deposition. I found no evidence of late-Cenozoic incision, which is required under the late-Cenozoic uplift paradigm. Instead, each site contains verifiable field evidence that challenges the uplift paradigm. Moreover, the results are consistent with the range having a common uplift history: the crest elevation, drainage topography, and relief developed in the Late Cretaceous, and only minor changes occurred in post-Eocene time. This supports uplift studies in the northern Sierra based on hydrogen and oxygen isotopes on auriferous deposits, and supports pediment ages of 40+ Ma in the southern Sierra. Uplift studies based on thermochronology and numerical modeling, on cave-sediment dating and its implied incision, and on significant denudation on benchlands cannot be reconciled with verifiable field evidence. Finally, migration of giant sequoias southwest into the Sierra Nevada was possible only from late in the Cretaceous, when the Nevadaplano came into existence and the range was high, negating any Sierran late-Cenozoic uplift paradigm.

Constraining the lithostratigraphic architecture of a buried bedrock valley using surface electrical resistivity and seismic refraction tomography

Buried bedrock valleys are common erosional features in northern mid-latitude environments that form through glaciofluvial or paleoalluvial processes and are typically infilled by Quaternary-aged sediments. The erosional extent and geometry of the valley including a weathered interface, along with sediment infill that can contain complex sequences of unconsolidated aquifer and aquitard sediments, mean these features may act as preferential pathways to deeper bedrock aquifers. Noninvasive geophysical tools can provide rapid, high-resolution subsurface characterization of these features. This study evaluates the application of electrical resistivity and seismic refraction tomography along two transects centred over a buried bedrock valley in Elora, Ontario, Canada. Geophysical measurements were combined with existing continuous core records and an electrofacies model based on downhole geophysical logs to constrain the morphology and infilled lithostratigraphic architecture of the valley. Bedrock competency associated with lithology may act as a control on depth and width of valley incision during erosion, with resistivity measurements of the bedrock revealing a potential association between interpreted mechanical properties and variations in the resolved valley morphology. Seismic velocity corroborated these contrasting valley widths but could not assess bedrock competency variability below the bedrock interface. This study reveals the sequence of events depositing sediments in the valley, yielding a revised architectural mapping that improves on previous regional scale lithostratigraphic interpretations. Results will be of use to groundwater practitioners requiring detailed conceptualization of this buried bedrock valley and its role on preferential zones of groundwater flow. Similar approaches can be used for delineation of these common and hydrogeologically significant features.

Detection of permafrost in shallow bedrock areas with the opposing coils transient electromagnetic method

The transient electromagnetic method (TEM) is a geophysical method for detecting underground geological bodies by following the principle of electromagnetic induction, which has been widely used in permafrost exploration. In the practical applications of the TEM to investigate permafrost, it is found that in certain areas with shallow buried bedrock, the electrical resistivity near the surface cannot be obtained, and both frozen soil and underground bedrock exhibit a high electrical resistivity, so it is difficult to determine the distribution characteristics of the permafrost thickness. Based on this background, by analyzing measured data, it is considered that the reason for this situation is that the noise superposition effect generated by the receiving coil under the action of the primary field forms a shallow detection blind area. This study uses equivalent anti-flux opposing coils to eliminate the abovementioned blind area and realize measurement in the permafrost area of Mahan Mountain in Lanzhou. The results showed that the opposing coils transient electromagnetic method (OCTEM) can clearly detect low-resistivity anomalies near the boundary and permafrost base in the Mahan Mountain area, solve the problem of the shallow detection blind area of the conventional TEM, effectively eliminate the interference caused by the primary field, and greatly improve the horizontal and vertical resolutions.

Buried bedrock valleys revealed in Michigan’s central Upper Peninsula using the horizontal-to-vertical spectral ratio passive seismic method

A network of north–south linear troughs occupies the eastern portion of the Lake Superior basin. Morphology suggests these features are tunnel valleys that formed through erosion as subglacial conduits and conveyed meltwater to a former glacial margin. We demonstrate the southern continuation of these valleys onshore into the central Upper Peninsula of Michigan by mapping bedrock topography using the Horizontal-to-Vertical Spectral Ratio (HVSR) passive seismic method. This is a single location, noninvasive seismic technique that records ambient signal. HVSR data are used to estimate subsurface elevation of the sediment-bedrock contact at each site. Regional bedrock topography was estimated using HVSR data from 347 locations. We delineated, for the first time, eight buried (or partially buried) bedrock valleys with geometric characteristics (width, depth, orientation, and continuity) suggesting a southward extension of the Lake Superior tunnel valley network. The largest valleys terminate near the crest of the Munising/Grand Marais upland. This association suggests the upland was constructed by deposition of sediment-laden meltwater at the terminal ends of these tunnel valleys as fans (or heads of outwash). Episodic erosion of the largest valleys likely began during the Two Rivers phase (∼13 ka) and were again active during the Marquette phase (∼11 ka). The smallest valleys developed later during final regional deglaciation following the Marquette phase.

Improving spatial characterization of buried bedrock valleys through airborne frequency-domain electromagnetic, residual magnetic, and surface resistivity measurements

The development of a three-dimensional (3D) hydrogeologic site model requires detailed knowledge of an area's hydrostratigraphy, information typically obtained from core logs, historical well records, outcrop mapping, and/or surface geophysics. Due to the high cost of drilling, a 3D model may rely on various statistical techniques to interpolate lithological boundaries between points located tens of kilometers or more apart. This situation can result in poorly constrained or simplified lithostratigraphic models, with under-sampled features such as karst and buried bedrock valleys. The non-invasive nature of airborne geophysical surveys combined with its dense sampling capability can fill critical data gaps. This study evaluates the capacity of a helicopter borne frequency-domain electromagnetic (FDEM) and residual magnetic survey using the Resolve™ system over a buried bedrock valley incised in a multi-layered dolostone aquifer within a complex glaciated environment in an urban/agricultural region of southern Ontario, Canada. The resulting airborne data was evaluated against two high-resolution ground-based electrical resistivity transects oriented orthogonal to the valley, alongside a published Quaternary geological model. A comparison between our local geophysically-based model and the regional geological model shows that the airborne electromagnetic surveys can markedly improve characterization of bedrock valley morphology and Quaternary sedimentary unit architecture that is comparable to that obtained using high-resolution surface-based electrical resistivity measurements with 10 m electrode spacing. The scale of heterogeneity that can be resolved by the airborne survey reveals the level of petrophysical variability that can be seen within the valley infill. Our study demonstrates the potential of airborne FDEM surveys in semi-urban agricultural environments where detailed characterization of Quaternary sediment architecture and bedrock topography is needed to inform 3D groundwater flow system variability.

The uses of FR and QWL for site effect

For an engineering site without strong motion records, the effect of full resonance (FR) and quarter-wavelength (QWL) to evaluate site response is discussed by comparing the predicted ground motion at each interface with the analytical solution of the frequency-wavenumber (F-K) method. We analyse the differences of time histories and amplitude spectra, with the inputs of full wave and upward wave on the buried bedrock. By comparing with the F-K result, the input of upward wave is reasonable. A solution to extract upward wave from full wave is presented and proved.

The Subsurface Geology and Landscape Evolution of the Volturno Coastal Plain, Italy: Interplay between Tectonics and Sea-Level Changes during the Quaternary

The Volturno alluvial-coastal plain is a relevant feature of the Tyrrhenian side of southern Italy. Its plan-view squared shape is due to Pliocene-Quaternary block-faulting of the western flank of the south-Apennines chain. On the basis of the stratigraphic analysis of almost 700 borehole logs and new geomorphological survey, an accurate paleoenvironmental reconstruction before and after the Campania Ignimbrite (CI; about 40 ky) eruption is here presented. Tectonics and eustatic forcing have been both taken into account to completely picture the evolution of the coastal plain during Late Quaternary times. The upper Pleistocene-Holocene infill of the Volturno plain has been here re-organized in a new stratigraphic framework, which includes seven depositional units. Structural analysis showed that two sets of faults displaced the CI, so accounting for recent tectonic activity. Yet Late Quaternary tectonics is rather mild, as evidenced by the decametric vertical separations operated by those faults. The average slip rate, which would represent the tectonic subsidence rate of the plain, is about 0.5 mm/year. A grid of cross sections shows the stratigraphic architecture which resulted from interactions among eustatic changes, tectonics and sedimentary input variations. On the basis of boreholes analysis, the trend of the CI roof was reconstructed. An asymmetrical shape of its ancient morphology—with a steeper slope toward the north-west border—and the lack of coincidence between the present course of the Volturno River and the main buried bedrock incision, are significant achievements of this study. Finally, the morpho-evolutionary path of the Volturno plain has been discussed.

Geology and Aquifer Sensitivity of Quaternary Glacial Deposits Overlying a Portion of the Mahomet Buried Bedrock Valley Aquifer System

To characterize the distribution of Holocene and Late Quaternary deposits and to assess the contamination potential of the Mahomet Aquifer, surficial geologic and aquifer sensitivity maps of the Gibson City East 7.5-Minute Quadrangle were created. Geologic data, extent, and thickness of the geologic materials were coupled with LiDAR topographic data and analyzed using ESRI’s ArcGIS 10.6.1. Aquifer sensitivity to contamination was calculated based on the depth to the first aquifer unit, aquifer thickness, and the lithology of the aquifer materials. The surficial geologic mapping identified five lithostratigraphic units: the Cahokia Formation, the Equality Formation, the Henry Formation, and the Yorkville and Batestown Members of the Lemont Formation. The southeast to northwest trending Illiana Morainic System is the most prominent feature in the study area and delineates the maximum extent of the glaciers during the Livingston Phase of glaciation. Postglacial deposits of the Cahokia Formation, alluvium, interfinger, and overlie with glacial outwash of the Henry Formation along channels and drainage ways downslope of the moraine. The areas of least sensitivity are located over the Illiana Morainic System, whereas the greatest potential to contamination occurs where the thickest deposits of the Henry Formation and Cahokia Formation lie at or just below the land surface.

Nonlinear broadband simulation of the Mw 6.0 May 29, 2012 Emilia earthquake in Northern Italy

We present the results of the numerical simulation of the May 29, 2012 Emilia Romagna earthquake in Northern Italy using an approach that computes synthetic seismograms associated to earthquake scenarios by combining broadband synthetic seismograms (0–10 Hz) obtained using the UCSB broadband code with nonlinear ground response analyses carried out using the program NOAH. The comparison with the recorded waveforms allowed validating the predictive capability of the adopted method in the proximity of the epicenter. The main limitation of the study is the inappropriateness of 1D modeling in a region characterized by a steep variation of the roof of the buried bedrock.

Study on Overlying Strata Motion Rule of Shortwall Mining Face of Shallow Seam with Simulation Experiment

Taking Shendong mining area as a research object, the roof strata moving law in the shallow buried deep thin bedrock was studied with 3D simulation experiment. The results showed that the surface displacement and roof pressure decrease little, and the roof does not appear the phenomenon of full thickness cutting. As the advancing distance of the working face increases, the displacement of the surface and roof increases. The old roof breaking is not easy to form a hinge structure, the roof when the pressure of mine pressure appearance is very intense.

Geophysical, geological, and hydrogeological characterization of a tributary buried bedrock valley in southern Ontario

Buried bedrock valleys infilled with Quaternary-aged sediment have the potential to become productive aquifers owing to prevalent sand and gravel deposits often associated with these topographic lows. In areas where groundwater is drawn from the underlying bedrock aquifer, buried bedrock channels may significantly affect the spatial distribution of recharge and localized contaminant pathways. Therefore, understanding the form, distribution, and the nature of Quaternary infill sediments within these buried bedrock river valleys, and their relationship to hydraulically transmissive bedrock features is an important aspect of groundwater resource management. Here, we evaluate the effectiveness of electrical resistivity and seismic refraction collected over a partially urbanized 150 ha area with variable vegetation, roads, and structures, to map the spatial distribution of sediments and delineation of a channel segment associated with a regional bedrock valley. Electrical resistivity and seismic refraction was performed along 13 (covering ∼11.6 km) and seven transects (covering ∼0.9 km), respectively, to map and characterize the bedrock surface morphology beneath a variable thickness of unconsolidated deposits. Three continuously cored holes and downhole geophysical logs, supplemented with four nearby water well records captured the in-channel as well as adjacent Quaternary stratigraphy (∼15–40 m). Cores recorded multiple glacial till deposits and ice-marginal processes associated with ice advances and retreats. Hydraulic transmissivity of the bedrock around the valley feature was evaluated using a FLUTe hydraulic transmissivity profiling technique. This study demonstrates the potential of combining several surface geophysical methods with sedimentological analysis of continuous cores and hydraulic data for characterizing tributary bedrock channel morphology and Quaternary infill at a scale relevant to localized studies of municipal production well recharge zones and contaminant transport and fate.

Evaluating the groundwater resource potential of the Dundas buried bedrock valley, southwestern Ontario: an integrated geological and hydrogeological case study

Population growth in the groundwater-dependent municipalities of southwestern Ontario has prompted interest in the exploration for new, previously untapped, groundwater resources. In this study, the groundwater resource potential of the sediments infilling a deeply buried bedrock valley network centred beneath the Region of Waterloo and the counties of Brant and Hamilton–Wentworth are explored. The objectives of this study are to further refine valley location and geometry, understand infilling sediments and their hydrogeological properties, and characterize waters contained within the aquifers to inform future water management decisions. Results of a regional ground gravity survey were instrumental in locating buried bedrock valleys and guided follow-up drilling. Continuous sediment coring and monitoring well installations were completed to target thick and coarse-grained sediment packages that, based on existing borehole data, showed aquifer potential. Hydraulic testing and groundwater sampling results provided valuable insights into groundwater quantity and quality. Highly transmissive aquifers, some worth investigating further, have been identified within portions of the valley network. The aquifers appear to occur at a number of stratigraphic positions and do not necessarily occur as the deepest unit overlying bedrock. Bedrock topography likely played a role, however, in their preferential preservation. They are commonly overlain by thick sequences of relatively impermeable sediments, providing excellent protection from anthropogenic contamination. Information from water chemistry, however, does suggest hydraulic connection to the surface at some locations. Groundwater quality and quantity information combined with a conceptual three-dimensional geologic model aids in the selection of groundwater resource exploration targets within the untapped resources of the deep, Dundas buried valley sediments.

The geology and hydrogeology of buried bedrock valley aquifers on Cape Breton Island, Nova Scotia: an overview

Buried bedrock valley aquifers are common across Canada, where multiple glaciations have buried both pre-glacial and Pleistocene valleys. These aquifers are becoming increasingly important as a supply of potable groundwater, for supporting aquatic habitat, and as part of strategies in adapting to a changing climate. However, in Canada, there are considerable knowledge gaps at national, regional, and local scales, such that many buried bedrock valleys remain unidentified or underexplored. Cape Breton Island provides a hydrogeological view into the roots of an ancient mountain range, now exhumed, glaciated, deglaciated, and tectonically inactive. Since the Cretaceous, a variety of geological processes have formed several buried bedrock valley aquifers over the island. These aquifers are important in providing municipal and commercial groundwater supplies, controlling mine dewatering, protection of salmonids, design and monitoring of waste disposal sites, and geotechnical investigations for infrastructure design. Of 150 sites assessed, 61% provided evidence of buried aquifers comprising unconsolidated sand and gravel of Cretaceous, Pleistocene, and Holocene ages. These sites provided the basis for five conceptual, 3-D hydrogeological block models. Three hydrogeological case studies provided further insight into the functioning of two of these models. Future studies should identify and characterize aquifers in high demand areas and/or those that support important riverine ecosystems. Research should focus on aquifer properties, groundwater-stream interaction, and the impact of changing climate with sea-level rise.