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31757 results in Solid Earth Geophysics

Page 1358 of 1588

  • Chapter 16 - Radioactive Surveying

  • from SUBPART 1.6: Radioactivity
  • Alan E. Mussett, University of Liverpool, M. Aftab Khan, University of Leicester
  • Illustrated by Sue Button
  • Book:
    Looking into the Earth
    Published online:
    05 June 2012
    Print publication:
    23 October 2000, pp 261-268

  • Summary

    Radioactivity surveying measures the natural radioactivity due to potassium, thorium, and uranium in near-surface rocks, which has applications in geological and geochemical mapping, and is used to find ores of uranium and thorium or other types of ore that have associated radioactivity. It also has environmental applications, mapping radon, a hazard to health, in surface rocks and waters.

    The most common surveying method detects γ rays, which can be used to identify the source element as well as detect the presence of radioactivity, and can be employed in ground or airborne surveys, but radon measurement often requires sampling below the surface.

    Radioactive radiations

    The previous chapter explained how radioactivity could be used to date rocks because isotopes decay from one element to another. This chapter is mainly concerned with the ‘radiations’ that accompany the decays, as a way of detecting and identifying the source elements. There are three principal types of radiation, all of which originate from the nuclei of radioactive atoms (Section 15.12.1). α-particles consist of two protons and two neutrons, and so have a positive electrical charge (ultimately, they each combine with two electrons to form helium atoms, and this is the origin of the helium used to inflate balloons). β-particles are electrons – produced when a proton converts to a neutron plus a electron – and so have a negative charge. Because of their electric charges, α-and β-particles cannot travel far through matter, no more than a few centimetres in air, or a few millimetres of rock, and so are little used in surveying.

  • Chapter 19 - Which Geophysical Methods to Use?

  • from Part II - EXAMPLES OF APPLICATIONS
  • Alan E. Mussett, University of Liverpool, M. Aftab Khan, University of Leicester
  • Illustrated by Sue Button
  • Book:
    Looking into the Earth
    Published online:
    05 June 2012
    Print publication:
    23 October 2000, pp 309-312

  • Summary

    Introduction

    In Part I, deciding which method to use in any of the examples given was not a problem, for they were chosen to illustrate the particular method being described, but when a geological problem is first encountered it is necessary to decide which – if any – geophysical methods to use and how best to employ them. Choosing the most suitable one or combination needs experience and perhaps some luck, but considering the following questions should narrow the choice.

    Does the problem have geophysical expression?

    Geophysical surveys do not respond to geological features as such, but to differences in physical properties, so the first requirement is that the geological situation has geophysical expression; that is, there must be some related subsurface body or structure that can be detected geophysically. For example, a granite pluton, which rose into place because of its low density, gives rise to a negative gravity anomaly (Fig. 8.16), and this may be used to locate it and estimate its size. In this example, the geophysical expression – the negative anomaly – is directly due to the body to be detected because its density is an intrinsic property of the granite, but sometimes geophysical expression is indirect. For example, a fault may be detectable by a seismic reflection survey if it has produced a vertical offset in subhorizontal layers (Fig. 7.10) but not if there are no layers or they are not offset vertically; or a concealed shaft may be directly detected by its negative gravity anomaly, but indirectly, for example, by a magnetic survey if it happens to contain ferrous objects (Section 27.2.3).

  • Evidence for a glacial origin of Neoproterozoic III striations at Oaibaččannjar'ga, Finnmark, northern Norway

  • A. H. N. RICE, C.-C. HOFMANN
  • Journal:
    Geological Magazine / Volume 137 / Issue 4 / July 2000
    Published online by Cambridge University Press:
    01 July 2000, pp. 355-366

  • The origin of the famous late Precambrian striated pavement at Oaibaččannjar'ga (Bigganjargga), northern Norway, remains controversial. Most investigators have accepted a glacial formation, but some prefer a soft-sediment mechanism. However, a newly discovered c. 2.5 mm thick zone of brecciation under rare polished striations indicates a hard substrate during formation and thus a glacial origin for the striations. Other points indicating that the striations formed in a hard substrate are: (1) the striated platform (in the Veidnesbotn Formation) is c. 150 Ma older than the overlying diamictite (Smalfjord Formation); having been buried to c. 2.5 km depth, cementation should have started before Smalfjord times; (2) the marked irregularity of the sub-Smalfjord Formation palaeotopography on Skjåholmen; (3) the presence of rounded Veidnesbotn Formation boulders in the diamictite above the striations. Imprints of clasts appearing to lie across the striations are re-interpreted as relicts of mud-flakes within the Veidnesbotn Formation which were cut across and quarried-out during pavement formation. The origin of the overlying diamictite (tillite vs. debris-flow) is not constrained by the presence of glacial striations and most probably was deposited some time after striation formation.

  • Shallow marine Lower and Middle Miocene deposits at the southern margin of the North Sea Basin (northern Belgium): dinoflagellate cyst biostratigraphy and depositional history

  • S. LOUWYE, J. DE CONINCK, J. VERNIERS
  • Journal:
    Geological Magazine / Volume 137 / Issue 4 / July 2000
    Published online by Cambridge University Press:
    01 July 2000, pp. 381-394

  • Detailed dinoflagellate cyst analysis of the Lower–Middle Miocene Berchem Formation at the southernmost margin of the North Sea Basin (northern Belgium) allowed a precise biostratigraphical positioning and a reconstruction of the depositional history. The two lower members of the formation (Edegem Sands and decalcified Kiel Sands) are biostratigraphically regarded as one unit since no significant break within the dinocyst assemblages is observed. The base of this late (or latest) Aquitanian–Burdigalian unit coincides with sequence boundary Aq3/Bur1 as defined by Hardenbol and others, in work published in 1998. A hiatus at the Lower–Middle Miocene transition separates the upper member (the Antwerpen Sands) from the underlying member. The greater part of the Antwerpen Sands were deposited in a Langhian (latest Burdigalian?)–middle Serravallian interval. The base of this unit coincides with sequence boundary Bur5/Lan1. Biostratigraphical correlation points to a diachronous post-depositional decalcification within the formation since parts of the decalcified Kiel Sands can be correlated with parts of the calcareous fossil-bearing section, up to now interpreted as Antwerpen Sands. The dinoflagellate cyst assemblages are dominated by species with a inner neritic preference, although higher numbers of oceanic taxa in the upper part of the formation indicate incursions of oceanic watermasses into the confined depositional environment of the southern North Sea Basin.

Page 1358 of 1588