Seismic attenuation in the continental crust SW of England

Scheirer, Daniel S.; Hobbs, Richard

doi:10.1111/j.1365-246x.1990.tb01789.x

Cited by 18 publications

(6 citation statements)

References 17 publications

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“…The density was calculated from the P ‐wave velocity using the Nafe–Drake empirical relationship (Ludwig et al 1970). Attenuation ( Q ) was included in the calculation assuming a two‐layer model: the upper layer, representing the sediments, had a thickness corresponding to a traveltime along the ray paths of 0.5 s and a Q ‐value of 100, and the lower layer, representing the crystalline basement, had a Q ‐value of 500 (after Scheirer & Hobbs 1990). The source wavelet for each OBH was obtained by summing 10 near‐offset traces windowed about the first arrival.…”

Section: Amplitude Modellingmentioning

confidence: 99%

Joint traveltime inversion of wide-angle seismic data and a deep reflection profile from the central North Sea

McCaughey¹,

Barton²,

Singh³

2000

Geophys. J. Int.

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We report results from the Seismic Wide‐Angle and Broadband Survey carried out over the Mid North Sea High. This paper focuses on integrating the information from a conventional deep multichannel reflection profile and a coincident wide‐angle profile obtained by recording the same shots on a set of ocean bottom hydrophones (OBH). To achieve this integration, a new traveltime inversion scheme was developed (reported elsewhere) that was used to invert traveltime information from both the wide‐angle OBH records and the reflection profile simultaneously. Results from the inversion were evaluated by producing synthetic seismograms from the final inversion model and comparing them with the observed wide‐angle data, and an excellent match was obtained. It was possible to fine‐tune velocities in less well‐resolved parts of the model by considering the critical distance for the Moho reflection. The seismic velocity model was checked for compatibility with the gravity field, and used to migrate and depth‐convert the reflection profile. The unreflective upper crust is characterized by a high velocity gradient, whilst the highly reflective lower crust is associated with a low velocity gradient. At the base of the crust there are several subhorizontal reflectors, a few kilometres apart in depth, and correlatable laterally for several tens of kilometres. These reflectors are interpreted as representing a strike section through northward‐dipping reflectors at the base of the crust, identified on orthogonal profiles by Freeman et al. (1988) as being slivers of subducted and imbricated oceanic crust, relics of the mid‐Palaeozoic Iapetus Ocean.

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Section: Amplitude Modellingmentioning

confidence: 99%

Joint traveltime inversion of wide-angle seismic data and a deep reflection profile from the central North Sea

McCaughey¹,

Barton²,

Singh³

2000

Geophys. J. Int.

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“…Hobbs & Snyder 1992). In most deep crustal environments, bandwidth is limited by attenuation to frequencies less than about 35 Hz (Scheirer & Hobbs 1990). Several combinations of airgun-array capacity, element configuration, and source depth were modelled using synthetic-source signatures and amplitude spectra.…”

Section: Source and Receivermentioning

confidence: 99%

Complex structure along a Mesozoic sea-floor spreading ridge: BIRPS deep seismic reflection, Cape Verde abyssal plain

McBride

White

Henstock

et al. 1994

Geophysical Journal International

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S U M M A R YAs part of an intensive study of a small area of oceanic lithosphere, the British Institutions Reflection Profiling Syndicate (BIRPS) acquired closely spaced deepseismic-reflection profiles over the Early Cretaceous crust of the Cape Verde abyssal plain off West Africa. The survey consisted of profiles spaced at 4 km arranged into strike lines parallel to the old sea-floor spreading axis ('isochron' profiles) and orthogonal dip lines oriented in the original direction of spreading ('flow' profiles). A large-capacity, well-tuned airgun source and very quiet shooting conditions ensured a high signal-to-noise ratio for deep reflection. Devising a strategy for mitigating contamination from 'wrap-around' multiples arriving from previous shots enabled us to use the minimum possible shot-point interval (50m) allowed for collecting long (18 s) records. Data processing was oriented towards a medium with low root-mean-square velocity, steeply dipping structure, and pervasive low apparent velocity noise from diffraction at the top of the igneous crust. The contrast between the isochron and flow profiles is striking. Isochron profiles are typically highly reflective throughout the igneous crust, consisting of bright, bidirectionally dipping reflection sets that extend in places from the top of the igneous basement down to the interpreted Moho reflection. These reflections do not offset intracrustal or top-basement structure and thus are not interpreted as faults: an igneous intrusive origin seems more likely. Flow profiles are more sparsely reflective but show individual steeply dipping reflections best developed in the upper igneous crust, continuing down in places to the Moho. Dipping reflections on the flow profiles are interpreted as major normal faults since they are clearly associated with offsets of the top of the basement as well as truncation of horizontal reflections within the igneous crust. The dominant dip of these reflections is to the west towards the spreading ridge axis. Reflections from the vicinity of the Moho, while well developed in some places, are not particularly prominent across the survey area. Moho reflections appear to show a different structural relation to crustal features on the isochron and flow profiles: on isochron profiles, dipping reflections occasionally flatten out into, and may merge with, the Moho reflection; on flow profiles, as dipping crustal reflections approach the Moho reflection, they are usually abruptly cut off by it without extending deeper. This survey shows how oceanic crustal structure can vary rapidly over relatively small areas, provides convincing evidence that a structurally complex fabric dominates oceanic igneous crust, and gives a conclusive observation of faults that penetrate the entire oceanic crust.

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“…They found no notable correlation between the variation in the single station values and surface geology. Scheirer & Hobbs (1990) studied crustal attenuation off the southwest coast of England using deep seismic reflection data. They observed that the thick granite plutons present in this region are associated with relatively low attenuation.…”

Section: Introductionmentioning

confidence: 99%

Lgwave attenuation in Britain

Sargeant

Ottemöller

2009

Geophysical Journal International

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S U M M A R YThe Lg wave quality factor (Q Lg ) in Britain has been modelled using data from the UK Seismic Network, operated by the British Geological Survey. The data set consists of 631 vertical, mostly short-period recordings of Lg waves from 64 earthquakes (2.7-4.7 ML) and 93 stations. We have inverted for both regional average Q Lg and tomographic images of Q Lg , and simultaneously a source term for each event and a site term for each station for 22 frequencies in the band 0.9-10.0 Hz. The regional average model is 266 f 0.53 between 1.0 and 10.0 Hz and indicates that attenuation in Britain is slightly higher than in France, and significantly higher than in eastern North America and Scandinavia. Tomographic inversions at each frequency indicate that Q Lg varies spatially. Broadly speaking, southeastern England, the Lake District and parts of the East Irish Sea Basin, and a small region between the Highland Boundary Fault and the Southern Uplands Fault are characterized by higher than average attenuation. Southwestern England, eastern central England and northwestern Scotland are regions of relatively low attenuation. To some extent, these regions correlate with what is known about the tectonics and structure of the crust in the UK.

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Seismic attenuation in the continental crust SW of England

Cited by 18 publications

References 17 publications

Joint traveltime inversion of wide-angle seismic data and a deep reflection profile from the central North Sea

Joint traveltime inversion of wide-angle seismic data and a deep reflection profile from the central North Sea

Complex structure along a Mesozoic sea-floor spreading ridge: BIRPS deep seismic reflection, Cape Verde abyssal plain

Lgwave attenuation in Britain

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