455 research outputs found
Groundwater in Jurassic carbonates
The Lincolnshire Limestone is an important regional aquifer. Pumping stations at Bourne and other locations along the
eastern edge of the Fens supply water to a large population in South Lincolnshire. Karst permeability development and rapid
groundwater flow raise issues of groundwater source protection, one of themes of this excursion. A second theme concerns the
influence of landscape development on the present hydrogeology. Glacial erosion during the Middle Pleistocene re-oriented
river patterns and changed the aquifer’s boundary conditions. Some elements of the modern groundwater flow pattern may be
controlled by karstic permeability inherited from pre-glacial conditions, whereas other flow directions are a response to the
aquifer’s current boundary conditions. Extremely high permeability is an important feature in part of the confined zone of the
present-day aquifer and the processes that may have produced this are a third theme of the excursion. The sites to be visited
will demonstrate the rapid groundwater flow paths that have been proved by water tracing, whereas the topography and
landscape history will be illustrated by views during a circular tour from the aquifer outcrop to the edge of the Fenland basin
and back. Quarry exposures will be used to show the karstification of the limestone, both at outcrop and beneath a cover of
mudrock
Groundwater in Cretaceous carbonates: KG@B field trip 21st June 2015
The Upper Cretaceous Chalk of southern England is the UK’s most important aquifer, providing
more than 75% of the public supply for southeast England, including London. The aquifer also
sustains rivers and wetlands, and their associated groundwater dependent ecosystems. However,
the aquifer is facing a multitude of threats including over-abstraction, nitrate pollution, and
climate change.
The Chalk is a complex aquifer in which groundwater flow is through the matrix, fractures and
karstic dissolutional voids. The Chalk matrix has a porosity of around 35% (Bloomfield et al.,
1995). The matrix is thought to provide an important contribution to storage, although the size of
the pore throats is very small, and therefore the permeability is very low (Price et al., 1993). The
average permeability of 977 core samples was only 6.3 x 10-4 m/day (Allen et al., 1997). The
matrix is particularly important in solute transport, because solutes move between the matrix and
the more permeable parts of the aquifer via diffusion (Foster 1975). The unmodified fracture
network provides an important contribution to storage and flow, and has a hydraulic conductivity
of about 0.1 m/d, and a transmissivity of about 20 m2/day (Price, 1987). However, it is the
dissolutionally enlarged fissures and conduits that make the Chalk such a good aquifer. The
median transmissivity from 2100 pumping tests is 540 m2/day, and the 25th and 75th percentiles
are 190 and 1500 m2/day respectively (MacDonald and Allen, 2001). Borehole packer testing,
logging and imaging have shown that most of this transmissivity comes from a small number of
dissolutional voids (e.g. Tate et al., 1970; Schurch and Buckley, 2002). Laterally extensive
lithostratigraphical horizons including marl seams, bedding planes, sheet and tabular flint bands,
and hard-grounds have an important influence on these groundwater flows. They are all horizons
where downward percolation of water may be impeded. Dissolution often occurs where flow is
concentrated along these horizons, creating conduits or fissures, especially where they are
intersected by joint sets
Probability of Physical Association of 104 Blended Companions to Kepler Objects of Interest Using Visible and Near-Infrared Adaptive Optics Photometry
We determine probabilities of physical association for stars in blended Kepler Objects of Interest (KOIs), and find that 14.5%_(-3.4%)^(+3.8%) of companions within ~ 4" are consistent with being physically unassociated with their primary. This produces a better understanding of potential false positives in the Kepler catalog and will guide models of planet formation in binary systems. Physical association is determined through two methods of calculating multi-band photometric parallax using visible and near-infrared adaptive optics observations of 84 KOI systems with 104 contaminating companions within ~ 4". We find no evidence that KOI companions with separations of less than 1" are more likely to be physically associated than KOI companions generally. We also reinterpret transit depths for 94 planet candidates, and calculate that 2.6% ± 0.4% of transits have R > 15R_⊕, which is consistent with prior modeling work
The Robo-AO KOI survey: laser adaptive optics imaging of every Kepler exoplanet candidate
The Robo-AO Kepler Planetary Candidate Survey is observing every Kepler planet candidate host star (KOI) with laser adaptive optics imaging to hunt for blended nearby stars which may be physically associated companions. With the unparalleled efficiency provided by the first fully robotic adaptive optics system, we perform the critical search for nearby stars (0.15" to 4.0" separation with contrasts up to 6 magnitudes) that dilute the observed planetary transit signal, contributing to inaccurate planetary characteristics or astrophysical false positives. We present 3313 high resolution observations of Kepler planetary hosts from 2012-2015, discovering 479 nearby stars. We measure an overall nearby star probability rate of 14.5±0.8%. With this large data set, we are uniquely able to explore broad correlations between multiple star systems and the properties of the planets which they host, providing insight into the formation and evolution of planetary systems in our galaxy. Several KOIs of particular interest will be discussed, including possible quadruple star systems hosting planets and updated properties for possible rocky planets orbiting with in their star's habitable zone
ROBO-AO KEPLER PLANETARY CANDIDATE SURVEY. II. ADAPTIVE OPTICS IMAGING OF 969 KEPLER EXOPLANET CANDIDATE HOST STARS
We initiated the Robo-AO Kepler Planetary Candidate Survey in 2012 to observe each Kepler exoplanet candidate host star with high angular resolution, visible light, laser adaptive optics (AOs) imaging. Our goal is to find nearby stars lying in Kepler's photometric apertures that are responsible for the relatively high probability of false-positive exoplanet detections and that cause underestimates of the size of transit radii. Our comprehensive survey will also shed light on the effects of stellar multiplicity on exoplanet properties and will identify rare exoplanetary architectures. In this second part of our ongoing survey, we observed an additional 969 Kepler planet candidate hosts and we report blended stellar companions up to that contribute to Kepler's measured light curves. We found 203 companions within ~4'' of 181 of the Kepler stars, of which 141 are new discoveries. We measure the nearby star probability for this sample of Kepler planet candidate host stars to be 10.6% ± 1.1% at angular separations up to 25, significantly higher than the 7.4% ± 1.0% probability discovered in our initial sample of 715 stars; we find the probability increases to 17.6% ± 1.5% out to a separation of 40. The median position of Kepler Objects of Interest (KOIs) observed in this survey are 11 closer to the galactic plane, which may account for some of the nearby star probability enhancement. We additionally detail 50 Keck AO images of Robo-AO observed KOIs in order to confirm 37 companions detected at a <5σ significance level and to obtain additional infrared photometry on higher significance detected companions
Karst hydrogeology of the Chalk and implications for groundwater protection
The Chalk is an unusual karst aquifer with limited cave development, but extensive networks of smaller solutional conduits and fissures enabling rapid groundwater flow. Small-scale karst features (stream sinks, dolines, dissolution pipes, and springs) are common, with hundreds of stream sinks recorded. Tracer velocities from 27 connections between stream sinks and springs have median and mean velocities of 4700 and 4600 m d−1. Tests to abstraction boreholes also demonstrate very rapid velocities of thousands of metres per day. Natural gradient tests from observation boreholes have rapid velocities of hundreds of metres per day. There is strong geological control on karst with dissolution focused on stratigraphical inception horizons. Surface karst features are concentrated near the Paleogene boundary, or where thin superficial cover occurs, but rapid groundwater flow is also common in other areas. The Chalk has higher storage and contaminant attenuation than classical karst, but recharge, storage and flow are influenced by karst. Point recharge through stream sinks, dolines, losing rivers, vertical solutional fissures, and soakaways enables rapid unsaturated zone flow. Saturated zone networks of solutional fissures and conduits create vulnerability to subsurface activities, and enable long distance transport of point source and diffuse pollutants, which may be derived from outside modelled catchment areas and source protection zones
ROBO-AO KEPLER PLANETARY CANDIDATE SURVEY. II. ADAPTIVE OPTICS IMAGING OF 969 KEPLER EXOPLANET CANDIDATE HOST STARS
We initiated the Robo-AO Kepler Planetary Candidate Survey in 2012 to observe each Kepler exoplanet candidate host star with high angular resolution, visible light, laser adaptive optics (AOs) imaging. Our goal is to find nearby stars lying in Kepler's photometric apertures that are responsible for the relatively high probability of false-positive exoplanet detections and that cause underestimates of the size of transit radii. Our comprehensive survey will also shed light on the effects of stellar multiplicity on exoplanet properties and will identify rare exoplanetary architectures. In this second part of our ongoing survey, we observed an additional 969 Kepler planet candidate hosts and we report blended stellar companions up to that contribute to Kepler's measured light curves. We found 203 companions within ~4'' of 181 of the Kepler stars, of which 141 are new discoveries. We measure the nearby star probability for this sample of Kepler planet candidate host stars to be 10.6% ± 1.1% at angular separations up to 25, significantly higher than the 7.4% ± 1.0% probability discovered in our initial sample of 715 stars; we find the probability increases to 17.6% ± 1.5% out to a separation of 40. The median position of Kepler Objects of Interest (KOIs) observed in this survey are 11 closer to the galactic plane, which may account for some of the nearby star probability enhancement. We additionally detail 50 Keck AO images of Robo-AO observed KOIs in order to confirm 37 companions detected at a <5σ significance level and to obtain additional infrared photometry on higher significance detected companions
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