Bedrock depth influences spatial patterns of summer baseflow, temperature, and flow disconnection for mountainous headwater streams

In mountain headwater streams the quality and resilience of cold-water habitat is regulated by surface stream channel connectivity and groundwater exchange. These critical hydrologic processes are thought to be influenced by the stream corridor bedrock contact depth (sediment thickness), which is often inferred from sparse hillslope borehole information, piezometer refusal, and remotely sensed data. To investigate how local bedrock depth might control summer stream temperature and channel disconnection (dewatering) patterns, we measured stream corridor bedrock depth by collecting and interpreting 191 passive seismic datasets along eight headwater streams in Shenandoah National Park (Virginia USA). In addition, we used multiyear stream temperature and streamflow records to calculate summer baseflow metrics along and among the study streams. Finally, comprehensive visual surveys of stream channel dewatering were conducted in 2016, 2019, and 2021 during summer baseflow conditions (124 total km of stream length). We found that measured bedrock depths were not well-characterized by soils maps or an existing global-scale geologic dataset, where the latter overpredicted measured depths by 12.2 m (mean), or approximately four times the average bedrock depth of 2.9 m. Half of the eight study stream corridors had an average bedrock depth of less than 2 m. Of the eight study streams, Staunton River had the deepest average bedrock depth (3.4 m), the coldest summer temperature profiles, and substantially higher summer baseflow indices compared to the other study steams. Staunton River also exhibited paired air and water annual temperature signals suggesting deeper groundwater influence, and the stream channel did not dewater in lower sections during any baseflow survey. In contrast, streams Paine Run and Piney River did show pronounced, patchy channel dewatering, with Paine Run having dozens of discrete dry channel sections ranging 1 to greater than 300 m in length. Stream dewatering patterns were apparently influenced by a combination of discrete deep bedrock (20 m+) features and more subtle sediment thickness variation (1–4 m), depending on local stream valley hydrogeology. In combination these unique datasets show the first large-scale empirical support for existing conceptual models of headwater stream disconnection based on underflow capacity and shallow groundwater supply.

Evaluation of Lake Sediment Thickness from Water-Borne Electrical Resistivity Tomography Data

Lakes are integrators of past climate and ecological change. This information is stored in the sediment record at the lake bottom, and to make it available for paleoclimate research, potential target sites with undisturbed and continuous sediment sequences need to be identified. Different geophysical methods are suitable to identify, explore, and characterize sediment layers prior to sediment core recovery. Due to the high resolution, reflection seismic methods have become standard for this purpose. However, seismic measurements cannot always provide a comprehensive image of lake-bottom sediments, e.g., due to lacking seismic contrasts between geological units or high attenuation of seismic waves. Here, we developed and tested a complementary method based on water-borne electrical-resistivity tomography (ERT) measurements. Our setup consisted of 13 floating electrodes (at 5 m spacing) used to collect ERT data with a dipole–dipole configuration. We used a 1D inversion to adjust a layered-earth model, which facilitates the implementation of constraints on water depth, water resistivity, and sediment resistivity as a priori information. The first two parameters were readily obtained from the echo-sounder and conductivity-probe measurements. The resistivity of sediment samples can also be determined in the laboratory. We applied this approach to process ERT data collected on a lake in southern Mexico. The direct comparison of ERT data with reflection seismic data collected with a sub-bottom profiler (SBP) showed that we can significantly improve the sediment-thickness estimates compared to unconstrained 2D inversions. Down to water depths of 20 m, our sediment thickness estimates were close to the sediment thickness derived from collocated SBP seismograms. Our approach represents an implementation of ERT measurements on lakes and complements the standard lake-bottom exploration by reflection seismic methods.

A gravity analysis of the Alpine Fault and the DFDP-2 drill site, Whataroa valley, South Westland, South Island, New Zealand

<p>The second phase of drilling into the Alpine Fault (DFDP-2), was completed in the Whataroa River valley, a former glacial valley located in central Westland, South Island, New Zealand. The site is located next to a steep hillside on the hanging-wall, ~1 km southeast of the mapped surface trace of the Alpine Fault. Projection of the hillside suggests a sediment thickness of 100 ± 40 m at the drill site; however, the sediment thickness was approximately double pre-drill estimates. Additionally, the surface expression and shallow geometry of the Alpine Fault in the Whataroa River valley, is not well-defined due to post-glacial burial of the fault zone. This thesis describes a gravity study designed to better constrain sub-surface structure beneath the DFDP-2 drill site and across the Alpine Fault. During this study, 466 new high-precision gravity observations were collected (standard error = 0.015 mGal) and amalgamated with 134 existing gravity stations, yielding comprehensive coverage of gravity data across the study area. A high density of observations was achieved within pre-determined zones, in addition to regional measurements so that residual gravity anomaly maps could be produced. The maps reveal: a negative residual gravity anomaly interpreted as a dextrally-offset glacial channel at least 350-450 m deep; steep localised gravity gradients near the Alpine Fault and DFDP-2 drill site that are interpreted as faulted and/or eroded boundaries; and a negative gravity anomaly adjacent to the DFDP-2 drill site that is interpreted as the deepest point of an over-deepened glacial lake. Gravity models were used to estimate the bedrock-sediment interface geometry near the DFDP-2 drill site and Alpine Fault. Structural inversion of the density boundary next to the drill site suggests either a moderately-dipping reverse fault or sub-vertical erosional wall exists beneath the hillside. Additional constraints on physical properties from direct density measurements or seismic velocity determinations and direct constraints on sediment thickness and layer geometry from seismic surveys will in future allow this new high-precision gravity dataset to be modelled more effectively.</p>

A gravity analysis of the Alpine Fault and the DFDP-2 drill site, Whataroa valley, South Westland, South Island, New Zealand

<p>The second phase of drilling into the Alpine Fault (DFDP-2), was completed in the Whataroa River valley, a former glacial valley located in central Westland, South Island, New Zealand. The site is located next to a steep hillside on the hanging-wall, ~1 km southeast of the mapped surface trace of the Alpine Fault. Projection of the hillside suggests a sediment thickness of 100 ± 40 m at the drill site; however, the sediment thickness was approximately double pre-drill estimates. Additionally, the surface expression and shallow geometry of the Alpine Fault in the Whataroa River valley, is not well-defined due to post-glacial burial of the fault zone. This thesis describes a gravity study designed to better constrain sub-surface structure beneath the DFDP-2 drill site and across the Alpine Fault. During this study, 466 new high-precision gravity observations were collected (standard error = 0.015 mGal) and amalgamated with 134 existing gravity stations, yielding comprehensive coverage of gravity data across the study area. A high density of observations was achieved within pre-determined zones, in addition to regional measurements so that residual gravity anomaly maps could be produced. The maps reveal: a negative residual gravity anomaly interpreted as a dextrally-offset glacial channel at least 350-450 m deep; steep localised gravity gradients near the Alpine Fault and DFDP-2 drill site that are interpreted as faulted and/or eroded boundaries; and a negative gravity anomaly adjacent to the DFDP-2 drill site that is interpreted as the deepest point of an over-deepened glacial lake. Gravity models were used to estimate the bedrock-sediment interface geometry near the DFDP-2 drill site and Alpine Fault. Structural inversion of the density boundary next to the drill site suggests either a moderately-dipping reverse fault or sub-vertical erosional wall exists beneath the hillside. Additional constraints on physical properties from direct density measurements or seismic velocity determinations and direct constraints on sediment thickness and layer geometry from seismic surveys will in future allow this new high-precision gravity dataset to be modelled more effectively.</p>

Dataset compilation by GRASS GIS for thematic mapping of Antarctica: Topographic surface, ice thickness, subglacial bed elevation and sediment thickness

This paper presents the GRASS GIS-based thematic mapping of Antarctica using scripting approach and associated datasets on topography and geophysics. The state-of-the art in cartographic development points at two important aspects. The first one comprises shell scripting promoted repeatability of the GIS technique, increased automatization in cartographic workflow, and compatibility of GRASS with Python, PROJ and GDAL libraries which enables advanced geospatial data processing: converting formats, re-projecting and spatial analysis. The second aspect is that data visualization greatly influences geologic research through improving the interpretation between the Antarctic glaciation and surface. This includes the machine learning algorithms of image classification enabling to distinguish between glacier and non-glacier surfaces through automatically partitioning data and analysis of various types of surfaces. Presented detailed maps of Antarctic include visualized datasets from the ETOPO1, GlobSed, EGM96 and Bedmap2 projects. The grids include bed and surface elevation, ETOPO1-based bathymetry and topography, bed, ice and sediment thickness, grounded bed uncertainty, subglacial bed elevation, geoid undulations, ice mask grounded and shelves. Data show the distribution of the present-day glacier, geophysical fields and topographic landforms for analysis of processes and correlations between the geophysical and geological phenomena. Advances in scripting cartography are significant contributions to the geological and glaciological research. Processing high-resolution datasets of Southern Ocean retrieved by remote sensing methods present new steps in automatization of the digital mapping, as presented in this research, and promotes comprehensive monitoring of geological, permafrost and glacial processes in Antarctica. All maps have been plotted using GRASS GIS version 7.8. with technical details of scripts described and interpreted.

Analysis and calculation of sediment scouring rate at different locations of storm sewer

To explore the migration differences of sediments at the front, middle, and end sections of a storm sewer when scoured by water, and further evaluate the pollution load, the scouring process of sediments at different locations of a storm sewer was simulated and mathematical models were built to calculate the scouring rate. Results show that scouring rate is affected by sediment particle size, pipeline slope, sediment thickness, and water flow velocity. As the slope increased, scouring rate at the end section increased more obviously. The scouring rate at the front section slightly decreased with increasing sediment thickness, but opposite trends were observed at the middle and end sections. When the particle size (0.33 mm–0.83 mm) and flow velocity (0.15 m/s–0.65 m/s) increased within their ranges, scouring rate increased across all three locations. Models for calculating scouring rate were established via two data fitting. The calculated values were compared with measured values at a scouring time of 1 min. Under different particle sizes, the difference between the calculated and measured values at front, middle, and end sections were in the ranges of −0.63% to 0.63%, −0.01% to 0.02%, and −0.13% to 0.16%, respectively, all of which showed good consistency.

2-dimensional modeling of high-resolution aeromagnetic data over Yola arm of the upper Benue trough, Northeastern Nigeria

2-D modeling of high-resolution aeromagnetic data over the area investigated was carried out with the aid of Oasis MontajTM software in order to get the total magnetic intensity map as well as the residual map from where the modeling was carried out. The data was acquired for Nigeria Geological Survey Agency (NGSA) in 2010 by Fugro international of Netherlands. The specifications of the data were terrain clearance of 80 m, flight line spacing of 500 m and a tie line spacing of 5000 m. The area investigated in this work is located between longitude 120 00' - 130 30' E and Latitude 80 00' – 100 00'N. GM-SYS module of the Oasis montaj was used for this research, with depocentres, structural patterns, basement geometry and morphology as well as estimates of the physical character of the cause to an investigated anomaly determined. The models reveal the horst and graben structures of the basement and the various faults that segmented the area into block patterns. The results revealed sediment thickness in the range of 2-5 km for the models which are in agreement with the results obtained by various researchers in the area. Horst and graben were identified in the models which confirmed the rifting nature of the Benue Trough. The authors of this research are of the view that these depocentres, structural patterns and basement geometry may be a promising site for hydrocarbon traps and reservoirs. Keywords: Aeromagnetic Data, GM-SYS, Modeling, Horst, Graben.