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Near Surface (near + surface)

Distribution by Scientific Domains

Earth and Environmental Science	75%

Selected Abstracts

Absolute S -velocity estimation from receiver functions

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2007
L. Svenningsen
SUMMARY We present a novel method to recover absolute S velocities from receiver functions. For a homogeneous half-space the S velocity can be calculated from the horizontal slowness and the angle of surface particle motion for an incident P wave. Generally, the calculated S velocity is an apparent half-space value which depends on model inhomogeneity and P -waveform. We therefore, suggest to calculate such apparent half-space S velocities from low-pass filtered (smoothed) receiver functions using a suite of filter-parameters, T. The use of receiver functions neutralize the influence of the P -waveform, and the successive low-pass filterings emphasize the variation of S velocity with depth. We apply this VS,app.(T) technique to teleseismic data from three stations: FUR, BFO and SUM, situated on thick sediments, bedrock and the Greenland ice cap, respectively. The observed VS,app.(T) curves indicate the absolute S velocities from the near surface to the uppermost mantle beneath each station, clearly revealing the different geological environments. Application of linearized, iterative inversion quantify these observations into VS(z) models, practically independent of the S -velocity starting model. The obtained models show high consistency with independent geoscientific results. These cases provide also a general validation of the VS,app.(T) method. We propose the computation of VS,app.(T) curves for individual three-component broad-band stations, both for direct indication of the S velocities and for inverse modelling. [source]

Near-surface models in Saudi Arabia

GEOPHYSICAL PROSPECTING, Issue 6 2007
Ralph Bridle
ABSTRACT A single-layer model of the near surface throughout the Kingdom of Saudi Arabia is available. While this simple model suffices for most areas and large subsurface structures, it fails in situations where the surface topography is complex, the base of weathering is below the datum, or where the time structural closure is less than the uncertainty in the static correction. In such cases, multiple-layered models that incorporate velocities derived from analysis of first arrivals picked from seismic shot records have proved to be successful in defining the lateral heterogeneity of the near surface. The additional velocity information obtained from this first-arrival analysis (direct as well as refracted arrivals) vastly improves the velocity,depth model of the near surface, regardless of the topography. Static corrections computed from these detailed near-surface velocity models have significantly enhanced subsurface image focusing, thereby reducing the uncertainty in the closure of target structures. Other non-seismic methods have been used either to confirm qualitatively or to enhance the layer models previously mentioned. Gravity data may be particularly useful in sandy areas to confirm general structure, while geostatistical modelling of vibrator base-plate attributes has yielded information that enhances the velocity field. In the global context, exploration targets of the oil and gas industry are seeking smaller and lower relief-time structures. Thus, near-surface models will need to enhance and integrate these methods, particularly in areas where the assumption of flat-lying near-surface layers cannot be met. [source]

Patterns of permafrost formation and degradation in relation to climate and ecosystems

PERMAFROST AND PERIGLACIAL PROCESSES, Issue 1 2007
Y. L. Shur
Abstract We develop a permafrost classification system to describe the complex interaction of climatic and ecological processes in permafrost formation and degradation that differentiates five patterns of formation: ,climate-driven'; ,climate-driven, ecosystem-modified'; ,climate-driven, ecosystem-protected'; ,ecosystem-driven'; and ,ecosystem-protected' permafrost. Climate-driven permafrost develops in the continuous permafrost zone, where permafrost forms immediately after the surface is exposed to the atmosphere and even under shallow water. Climate-driven, ecosystem-modified permafrost occurs in the continuous permafrost zone when vegetation succession and organic-matter accumulation lead to development of an ice-rich layer at the top of the permafrost. During warming climates, permafrost that has formed as climate-driven can occur in the discontinuous permafrost zone, where it can persist for a long time as ecosystem-protected. Climate-driven, ecosystem protected permafrost, and its associated ground ice, cannot re-establish in the discontinuous zone once degraded, although the near surface can recover as ecosystem-driven permafrost. Ecosystem-driven permafrost forms in the discontinuous permafrost zone in poorly drained, low-lying and north-facing landscape conditions, and under strong ecosystem influence. Finally, ecosystem-protected permafrost persists as sporadic patches under warmer climates, but cannot be re-established after disturbance. These distinctions are important because the various types react differently to climate change and surface disturbances. For example, climate-driven, ecosystem-modified permafrost can experience thermokarst even under cold conditions because of its ice-rich layer formed during ecosystem development, and ecosystem-driven permafrost is unlikely to recover after disturbance, such as fire, if there is sufficient climate warming. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Ground penetrating radar survey over a Roman building at Groundwell Ridge, Blunsdon St Andrew, Swindon, UK

ARCHAEOLOGICAL PROSPECTION, Issue 1 2004
N. T. Linford
Abstract A ground penetrating Radar (GPR) survey was conducted over well-preserved building remains revealed during a previous geophysical survey (1996) covering an apparent complex of Roman activity discovered at Groundwell Ridge to the north of Swindon, UK. Despite unfavourable, clay-rich soil conditions, the GPR survey provided a detailed plan of the Roman remains to a depth of approximately 1,m, confirming their survival in the very near surface. The GPR results complement the previous earth resistance and magnetic surveys and, together, the data suggest the presence of a high status Roman building, possibly incorporating thermoremanent features, for instance associated with a hypocaust system. Copyright © 2004 John Wiley & Sons, Ltd. [source]