Reflectivity characteristics of the crust from modelling refraction/wide-angle and normal-incidence reflection data on BABEL line 1
1996 (English)In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 262, no 1-4, 173-193 p.Article in journal (Other academic) Published
Marine normal-incidence reflection and P- and S-wave refraction/wide-angle reflection data from the BABEL project, line 1, in the Bothnian Sea have been used for investigating the nature of the reflectivity in the area. The data are available along the same line, and therefore the same piece of crust is probed by two seismic methods. We use this favourable circumstance for model calculations concerning both data sets by analysing the waveforms of reflections from normal-incidence traveltimes at approximately 4, 12 and 16 s TWT. Strong sub-horizontal reflections are observed on the normal-incidence reflection section at about 4 s TWT. These reflections have been modelled using the reflectivity method (1-D modelling) utilising the water bottom reflection as an estimate of the source wavelet and show a positive polarity. Thickness and P-wave velocity consistent with these reflections are 50 m and 7 km/s, respectively, with a host rock velocity of 6 km/s. From the polarity, velocity and thickness, these reflections are attributed to dolerite sills in the upper crust. Strong lower-crustal reflections are observed on the wide-angle and normal-incidence reflection sections. However, the corresponding S-wave reflections from the lower crust are considerably weaker. The implied high positive impedance contrast for the P-wave lower-crustal reflection and the weak S-wave reflectivity suggests the presence of a highly mafic sheet at about 40 km depth. The wide-angle P- and S-wave sections show strong reflections at the Moho. Coincident normal incidence data show only weak Moho reflectivity indicating a gradient in velocity at the crust/mantle boundary. Dipping events onlap the strong sub-horizontal lower crustal and the P-wave Moho reflections on the wide-angle data. These are explained by dipping structures above the lower crustal reflector and below the Moho, with the latter also consisting of a gradient in velocity.
Place, publisher, year, edition, pages
Elsevier, 1996. Vol. 262, no 1-4, 173-193 p.
WIDE-ANGLE; SYNTHETIC SEISMOGRAMS; CONTINENTAL-CRUST; BLACK-FOREST; BENEATH; GERMANY
IdentifiersURN: urn:nbn:se:uu:diva-28036DOI: 10.1016/0040-1951(96)00002-9OAI: oai:DiVA.org:uu-28036DiVA: diva2:55932