The use of reference models from a priori data to guide 2D inversion of electrical resistivity tomography data

Clay-rich till plains cover much of the UK. Such sites are attractive locations for landfills, since clay aquitards lower the risk of landfill leachate entering groundwater. However, such tills often contain sand and gravel bodies that can act as leachate flow routes. Such bodies may not be detected by conventional site investigation techniques such as drilling boreholes and trial pitting. A method of guided inversion, where a priori data are used to construct structural reference models for use in inverting electrical resistivity tomography data, was proposed as a tool to improve the detection of sand and gravel bodies within clay-rich till sequences.
Following a successful 2D guided inversion synthetic modelling study, a field study was undertaken. Wenner 2D electrical resistivity tomography lines, resistivity cone penetrometry bores and electromagnetic induction ground resistivity data were collected over a site on the East Yorkshire coast, England, where sand and gravel lenses were known to exist from cliff exposures. A number of equally valid geoelectrical models were constructed using the electromagnetic and resistivity cone data. These were used as structural reference models in the inversion of the resistivity tomography data. Blind inversion using an homogenous reference model was also carried out for comparison.
It was shown for the first time that the best solution model produced by 2D inversion of one data set with a range of structural reference models could be determined by using the l ₂ model misfit between the solution models and associated reference models (reference misfit) as a proxy for the l ₂ misfit between the solution models and the synthetic model or 'best-guess' geoelectrical model (true misfit). The 2D methodology developed here is applicable in clay-rich till plains containing sand and gravel bodies throughout the UK.     

Soils and geomorphology of the chalk in south-east England

Soil mapping and soil profile studies on the Chalk of south-east England outline the distribution and origin of superficial deposits more clearly than hitherto, and do not support the widely accepted history of landscape development proposed by Wooldridge and Linton (1955). Deposits formed by weathering and periglacial disturbance of a thin cover of basal Tertiary deposits occur on all parts of the dip-slope interfluves, suggesting that the exhumed sub-Tertiary surface, somewhat lowered by dissolution of the Chalk, is much more extensive than supposed by Wooldridge and Linton. The evidence for high Plio-Pleistocene sea levels is reviewed, with particular reference to their limited effect on soils and superficial deposits and on the morphology of the dip-slope. Alternative explanations for the so-called Calabrian marine platform and cliff are considered. Emphasis is placed on periglacial processes in the later Pleistocene denudation of the Chalk, and on the protective role played by the cover of disturbed basal Tertiary sediment.     

A re-examination of the evidence for a Plio-Pleistocene marine transgression on the Chiltern Hills. III. deposits

A petrological re-examination of the deposits that Wooldridge and Linton used as evidence for a Plio-Pleistocene marine transgression on the Chiltern Hills indicates that many are fluvial and some have been reworked in a periglacial climate. However, those from Little Heath and Rothamsted, Hertfordshire and Lane End, Buckinghamshire are confirmed as marine and Plio-Pleistocene. Subsidence during the Pleistocene is suggested by the northeasterly decline in height of the few truly marine deposits.     

A re-examination of the evidence for a plio-pleistocene marine transgression on the Chiltern hills. I. structures and surfaces

Borehole and surface outcrop data are used to construct sub-Chalk, sub-Upper Chalk and sub-Tertiary contour maps for the Chiltern area. The results show a southwest-northeast monocline, which coincides with the change of slope at the lower boundary of the morphological bench on the Chalk dipslope that Wooldridge and Linton attributed to Plio-Pleistocene marine erosion. Younger northwest-southeast synclines coincide with the main transverse river valleys of the Chilterns. The monoclinal flexuring casts doubt on the value of the bench as evidence for a Plio-Pleistocene transgression.     

A re-examination of the evidence for a Plio-Pleistocene marine transgression on the Chiltern Hills. II. drainage patterns

The dry valley pattern of the Chiltern Hills is divided into 55 third order drainage basins which are grouped morphometrically into five groups. The character and distribution of the groups are determined mainly by the lithology of surface deposits and the structure of the underlying Chalk. No evidence has been found for different drainage patterns on the parts of the dipslope equivalent to the ‘Mio-Pliocene peneplain’ and ‘Plio-Pleistocene marine platform’ of Wooldridge and Linton (1955), nor for superimposition of drainage from covers of Plio-Pleistocene or Palaeogene sediments.     

No significant Alpine tectonic overprint on the Cimmerian Strandja Massif (SE Bulgaria and NW Turkey)

We provide the first comprehensive picture of the thermochronometric evolution of the Cimmerian Strandja metamorphic massif of SE Bulgaria and NW Turkey, concluding that the bulk of the massif has escaped significant Alpine-age deformation. Following Late Jurassic heating, the central part of the massif underwent a Kimmeridgian-Berriasian phase of relatively rapid cooling followed by very slow cooling in Cretaceous-to-Early Eocene times. These results are consistent with a Late Jurassic–Early Cretaceous Neocimmerian (palaeo-Alpine) phase of north-verging thrust imbrication and regional metamorphism, followed by slow cooling/exhumation driven by erosion. From a thermochronometric viewpoint, the bulk of the Cimmerian Strandja orogen was largely unaffected by the compressional stress related to the closure of the Vardar–?zmir–Ankara oceanic domain(s) to the south, contrary to the adjacent Rhodopes. Evidence of Alpine-age deformation is recorded only in the northern sector of the Strandja massif, where both basement and sedimentary rocks underwent cooling/exhumation associated with an important phase of shortening of the East Balkan fold-and-thrust belt starting in the Middle–Late Eocene. Such shortening focused in the former Srednogorie rift zone because this area had been rheologically weakened by Late Cretaceous extension.     

The origin of very flinty dry-valley deposits in the Marlow area,Buckinghamshire, England

Some of the soils on the floors of dry chalkland valleys north of the River Thames in the Marlow area have surface horizons of moderately to very flinty, weakly or non-calcareous, poorly sorted loams, which were deposited in the late Holocene. These overlie typical chalkland gelifluction and periglacial fluvial deposits. Fine calcareous colluvial loams also occur but are less common than in valleys elsewhere on the English chalk. Particle size and mineralogical analyses suggest that the loamy matrix of the flinty surface horizons was derived from loess and early Tnames terrace deposits or Clay-with-flints occurring on interfluves between the dry-valleys. On the evidence of their poorly sorted nature, lack of carbonate, abundance of large flints and irregularly ridged surfaces, it is suggested that they originated mainly as valley-side debris flows.     

The ‚loess’︁ section at Borden,Kent, SE England

A 5.3m profile originally described as loess with a buried interglacial soil is reinterpreted from petrographic evidence and thermoluminescence dating as a Holocene colluvial accretionary soil. Mineralogical analyses of coarse silt (16–63 μm) fractions suggest that most of the silty colluvium was derived from weathered Thanet Beds upslope, though some loess was incorporated during the final depositional phase. Thin sections show that clay illuviation occurred penecontem-poraneously with deposition of colluvium. Thermoluminescence properties suggest partial optical bleaching of the grains during rapid deposition, which is consistent with a Holocene colluvial origin.