On the Importance of Gravity in DNAPL Invasion of Saturated Horizontal Fractures

Invasion percolation (IP) models of dense non‐aqueous phase liquid (DNAPL) invasion into saturated horizontal fractures typically neglect viscous and gravity forces, as it is assumed that capillarity dominates in many situations. An IP model simulating DNAPL invasion into saturated horizontal fractures was modified to include gravity as a local effect. The model was optimized using a genetic algorithm, and demonstrated that the inclusion of gravity is important for replicating the architecture of the DNAPL invasion pattern. The optimized gravity‐included simulation showed the DNAPL invasion pattern to be significantly more representative of the experimentally observed pattern (80% accuracy) than did the optimized gravity‐neglected simulation (70% accuracy). Additional simulations of DNAPL invasion in 360 randomly generated fractures were compared with and without gravity forces. These simulations showed that with increasing fracture roughness, the minimum difference between simulations with and without gravity increases to 35% for a standard deviation of the mid‐aperture elevation field (SD_z) of 10 mm. Even for low roughness (SD_z = 0.1 mm), the difference was as high as 30%. Furthermore, a scaled Bond Number is defined which includes data regarding DNAPL type, media type and statistical characteristics of the fracture. The value of this scaled Bond Number can be used to determine the conditions under which gravity should be considered when simulating DNAPL invasion in a macroscopically horizontal fracture. Finally, a set of equations defining the minimum and maximum absolute percentage difference between gravity‐included and gravity‐neglected simulations is presented based on the fracture and DNAPL characteristics.     

Significance of the Toolebuc gamma ray anomaly in the search for and evaluation of oil shale in the Eromanga Basin

Chemical and radiometric analyses have been made on core samples from 15 drill holes intersecting the Toolebuc Formation at Julia Creek. The holes are located in the vicinity of the St Elmo Structure and include intersections of a potentially economic oil shale deposit west of this structure as well as non‐economic mixed coquinite/shale intersections to the east. The gamma ray activity of the formation is due largely to uranium in the shales. Wire‐line gamma ray logs, available for eight of these holes, reflect the distribution of organic carbon. Despite this, there is poor correlation between uranium and organic carbon in the oil shales as a result of remobilization of uranium from organic matter into phosphate during diagenesis. In consequence wire‐line gamma ray logs have no potential for evaluating oil yield. Density, on the other hand, is a good indicator of oil potential. On a hole by hole basis, the correlation between uranium and organic carbon in the formation ranges from 0.22 west of the St Elmo Structure (the potential oil shale resource) to 0.95 east of the structure. This reflects a systematic change in sedimentology from oil shale that is thick and well separated from coquinite (low correlation) to laterally equivalent but finely interbedded coquinite and shale (high correlations). Such trends could be used in exploratory drilling to indicate favourable areas elsewhere in the Toolebuc Formation.     

Wave impacts on coastal cliffs: Do bigger waves drive greater ground motion?

Coastal cliff erosion is caused by a combination of marine forcing and sub-aerial processes, but linking cliff erosion to the environmental drivers remains challenging. One key component of these drivers is energy transfer from wave–cliff interaction. The aim of this study is to directly observe cliff ground motion in response to wave impacts at an individual wave scale. Measurements are described from two coastal cliff sites: a 45-minute pilot study in southern California, USA and a 30-day deployment in Taranaki, New Zealand. Seismometers, pressure sensors and video are used to compare cliff-top ground motions with water depth, significant wave height (H_s) and wave impact types to examine cliff ground motion response. Analyses of the dataset demonstrate that individual impact events can be discriminated as discrete events in the seismic signal. Hourly mean ground motion increases with incident H_s, but the largest hourly peak ground motions occurred across a broad range of incident H_s (0.9–3.7 m), including during relatively calm conditions. Mean hourly metrics therefore smooth the short-term dynamics of wave–cliff interaction; hence, to fully assess wave impact energy transfer to cliffs, it is important also to consider peak ground motion. Video analyses showed that the dominant control on peak ground motion magnitude was wave impact type rather than incident H_s. Wave–cliff impacts where breaking occurs directly onto the cliff face consistently produced greater ground motion compared to broken or unbroken wave impacts: breaking, broken and unbroken impacts averaged peak ground motion of 287, 59 and 38 μm s⁻¹, respectively. The results illustrate a novel link between wave impact forcing and cliff ground motion response using individual wave field measurements, and highlight the influence of wave impact type on peak energy transfer to coastal cliffs. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.     

Current estimates of freshwater flux through Arctic and subarctic seas

As the world warms, the expectation is that the freshwater outflows from the Arctic Ocean to the North Atlantic will strengthen and may act to suppress the rate of the climatically-important Atlantic meridional overturning circulation. Hitherto, however, we have lacked the system of measurements required to estimate the totality of the freshwater flux through subarctic seas. Though observations remain patchy and rudimentary in places, we piece-together the results from recent large-scale observational programmes together with associated modelling, to establish preliminary maps of the rates and pathways of freshwater flux through subarctic seas. These fluxes are calculated according to two reference salinities, S = 34.8 to conform with the majority of estimates reported in the literature, and S = 35.2, the salinity of the inflowing Atlantic water, to calculate the freshwater balance of the ‘Arctic Mediterranean’. We find that 148 mSv of freshwater enters the Nordic Seas across its northern boundary. There it is supplemented by around 54 mSv of freshwater from Baltic runoff, Norwegian runoff, P − E and Greenland ice melt, so that the total freshwater contribution to the Nordic Seas from all sources is 202 mSv. Of this, around 51 mSv of freshwater is estimated to pass south to the deep Atlantic in the dense water overflows leaving an assumed balance of 151 mSv to leave the Nordic Seas in the upper water export through Denmark Strait. The corresponding estimate for the freshwater outflow west of Greenland is 103 mSv relative to 35.2 so that the total freshwater flux reaching the North Atlantic through subarctic seas is around 300 mSv.     

Quantitative measures of textural anisotropy resulting from magmatic compaction illustrated by a sample from the Palisades sill, New Jersey

Early in the crystallization of many tholeiitic basaltic magmas, plagioclase crystals cluster together into a 3-D cellular network, which forms a passive marker capable of recording the deformation that accompanies compaction of crystal mush. Although irregular in detail, the overall network is initially isotropic and only becomes anisotropic as a result of compaction. We have developed four independent methods to quantify the 3-D textural anisotropy of a basalt sample using at least three non-parallel thin sections. Three of the methods are based on the geometrical properties of digitized maps of the feldspar chain networks. One approach focuses on the angular variation of the mean intercept along parallel traverses through the network, another examines the orientation and size distribution of individual links, and the third considers the average shape of interstitial regions outlined by the plagioclase network. The fourth technique approximates the textural anisotropy by the variogram anisotropy of a scanned thin section image. We illustrate the methods using five oriented non-parallel thin sections from a sample of diabase 146 m above the base of the 300-m-thick Palisades sill of New Jersey. Compaction of crystal mush in this sill has previously been postulated on the basis of chemical evidence. The 3-D feldspar network anisotropy based on the first three approaches suggests nearly uniaxial compaction on the order of 8.6% in a direction within 3° of the intersection of the columnar joints at the sample site. A rigorous statistical test based on the statistics of elliptically contoured non-normal multivariate distributions documents that the link-vector distribution in vertical sections are statistically anisotropic at a 95% confidence level and that the overall compaction is 7.9±2.6%. The orientation and magnitude of the 3-D textural anisotropy determined by the image variogram of the non-opaque minerals is almost identical to the mean feldspar network anisotropy; 8.5% compaction in a direction 10° from the columnar joint intersections. The major silicate textural and feldspar network anisotropy axes both plunge almost directly down dip of the sill. On the other hand, the major axis of the variogram anisotropy of the opaque minerals is approximately parallel to the strike of the sill and to the major axis of the anisotropic magnetic susceptibility. The anisotropy of the silicate mineral fabric may reflect down-dip flow of a deformable melt-rich crystal mush, whereas the AMS and opaque textural anisotropy reflects the influence of gravitational stresses during the growth of magnetite in the final stages of melt crystallization. Evidently the Palisades sill was not originally horizontal but was intruded in an orientation close to its present attitude.     

Evaluation of lead isotopic methods for uranium exploration, Koongarra Area, Northern Territory, Australia

Analysis of radioactive (²¹⁰Pb) and stable lead isotopes in near-surface samples has been tested as a method of uranium exploration in the Pine Creek Geosyncline, Northern Territory, Australia. The lead isotopes were extracted from the samples by a mild leaching agent and were measured by alpha spectrometry for ²¹⁰Pb and by mass spectrometry for stable lead isotopes. The results are compared with those obtained by conventional methods utilizing measurements of radioactivity and radon (Track Etch) in situ and ²²⁶Ra, ²²⁸Ra and U contents of soils. The major problems addressed were whether the lead isotopic methods are more sensitive than the conventional methods and whether they can discriminate “real” anomalies from the common barren anomalies found in black soils and swamps which contain radium in excess of the uranium present.Four test areas, representing a range of exploration problems, were chosen in the vicinity of the Koongarra uranium deposits and 25 samples from each area were analyzed. Most samples have more ²²⁶Ra than uranium. Radium analyses of several water samples show the source of this radium to be non-uraniferous rocks within the Kombolgie sandstone. The results for soil ²²⁶Ra, radon, scintillometry and ²¹⁹Pb were generally closely correlated, and as a result, the ²¹⁰Pb method was not considered to have any advantages over the conventional methods.At the Koongarra X prospect, which has a weak surface expression, the ratio gave the strongest indication of the underlying uranium mineralization with an anomaly to background ratio of 12.5. However, this ratio is correlated with uranium content and does not offer any particular advantages over uranium analyses alone. More subtle indications of uranium mineralization were found by relating the radiogenic lead (²⁰⁶Pb) and the thorium-derived lead (²⁰⁸Pb) to the common lead content (²⁰⁴Pb). A plot of versus (horizontal axis) is linear for country rock samples, irrespective of the amount of more recently introduced ²²⁶Ra. Samples above uranium mineralization lie off this trend, along a line of near-zero slope. By the use of this plot, indications were found of the Koongarra No. 2 orebody, which is concealed by about 40 m of barren overburden; none of the other techniques detected this mineralization.