Variation in Saturated Hydraulic Conductivity at the Outcrop Scale, the Whanganui Basin, New Zealand
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Groundwater flow and contaminant transport are strongly influenced by hydrogeological spatial variation. Understanding the textural heterogeneity of aquifer and aquitard units is critical for predicting preferential flow pathways, but is often hindered by sparse hydrogeological data, widely spaced data points, and complex stratigraphy. Here, we demonstrate the application of a relatively new air permeameter technology, providing a cost-effective, rapid alternative for characterizing hydrostratigraphic units in the field. The aim of this research is to (1) characterize the variation of saturated hydraulic conductivity across shallow-marine hydrostratigraphic units of the Whanganui Basin, New Zealand, and (2) assess the variation of saturated hydraulic conductivity within individual hydrostratigraphic units and relate these changes to facies and depositional environments. Results suggest heterogeneity within fine-grained aquitard units is controlled by bioturbation, whereby burrowing, ingestion and defecation results in grain size segregation and differential micrite cementation. Coarse-grained heterolithic aquifer facies display sharp changes in permeability across planar to cross-bedded sets, related to current and wave energy fluctuations within shallow-marine depositional settings. Bedding plane orientation creates high permeability zones that promotes down dip subsurface flow. Down dip gradation of coarse-grained nearshore facies into fine-grained shelf facies along the paleo shoreline-shelf transect is suggested to promote lateral and vertical groundwater flow within the basin fill. Air permeameter techniques have potential for application within groundwater basins around the world, providing datasets that facilitate greater understanding of groundwater systems, informing practices and policies for targeted water quality management.Keywords:
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Digital outcrop model of the Echernwand and description of the methodologies used for digital outcrop construction and for cross-section construction and restoration.<br>
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Outcrop analogs play a central role in understanding subseismic interwell depositional facies heterogeneity of carbonate reservoirs. Outcrop geologists rarely utilize near-surface seismic data due to the limited vertical resolution and difficulty visualizing seismic signals as “band-limited rocks.” This study proposes a methodology using a combination of forward modeling and conditional generative adversarial network (cGAN) to translate seismic-derived acoustic impedance (AI) into a pseudo-high-resolution virtual outcrop. We tested the methodology on the Hanifa reservoir analog outcropping in Wadi Birk, Saudi Arabia. We interpret a 4 km long outcrop photomosaic from a digital outcrop model (DOM) for its depositional facies, populate the DOM with AI properties, and forward calculate the band-limited AI of the DOM facies using colored inversion. We pair the synthetic band-limited AI with DOM facies and train them using a cGAN. Similarly, we pair the DOM facies with outcrop photos and train them using a cGAN. We chain the two trained networks and apply them to the approximately 600 m long seismic-derived AI data acquired just behind the outcrop. The result translates AI images into a virtual outcrop “behind-the-outcrop” model. This virtual outcrop model is a visual medium that operates at a resolution and format more familiar to outcrop geologists. This model resolves subseismic stratigraphic features such as the intricate downlap-onlap stratal termination at scales of tens of centimeters and the outline of buildup facies, which are otherwise unresolvable in the band-limited AI.
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To bridge the sometimes large conceptual gap between what is observed in outcrop and what occurs in the subsurface, outcrop information ideally should be placed into the more familiar format used for exploration and development, that is, well logs, seismic, cross sections, and 3-D models. Borehole imaging logs from behind an outcrop, when calibrated to the outcrop (and core), can be particularly useful for identifying image log criteria that can be used for predicting stratigraphic (and structural) features away from the borehole, and sometimes for predicting well performance.
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Traditionally, the sourcing of prehistoric stone tools in Britain has been done most successfully by comparing the petrological and geochemical characteristics of individual stone tools with rock and debitage from known prehistoric quarry sites and stone tool production sites. However, this is a very rare occurrence because only a very small proportion of stone tools in Britain have a secure archaeological provenance, including those from prehistoric quarries or production sites. Substantial numbers of stone tools in the British archaeological record are chance finds; they lack a secure archaeological context. Through a case study of Carrock Fell and the Implement Petrology Group XXXIV, this article presents a new methodological and statistical model for assembling, analysing and interpreting fieldwork evidence, which combines petrological, geochemical portable X-ray fluorescence (PXRF) data, and geochemical inductively coupled plasma-atomic spectroscopy (ICP) data to establish a signature for 17 gabbroic prehistoric stone implements (Table 1). These results are then compared with similar data gathered from rocks at outcrop. Through qualitative and quantitative analysis, seven gabbroic implements could be securely provenanced to rock from particular outcrop locations. The model is transferable to other similar contexts where sources of implement rock are sought from apparently random distributions of stone tools.
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