Multipathing and spectral modulation of the downgoing wavefield in a complex crust: an example from the KTB (Germany) borehole

VSP data collected in the KTB (Germany) borehole to a depth of 8.5 km in 1999 show a surprising spectral modulation of the downgoing wavefield. After filtering the data with the singular value decomposition technique it was found that below about 6.2 km there are two depth intervals where the modulation can be explained in terms of a basic wavelet plus two weighted and delayed copies of that wavelet, with the delay for each wavelet remaining almost constant in each interval. The boundary between the two intervals is at about 7.25 km depth and above and below this depth the delay for the second wavelet is almost the same, while the delay for the third wavelet is significantly different. Neither the modulation nor its depth variation are source related and cannot be explained in terms of multiple reflections in a subhorizontal low-velocity layer. On the other hand, finite difference synthetic data show that subvertical layering (which is prevalent in the borehole area) provides a mechanism that can explain the observations. This mechanism has analogies with the generation of the standard refracted (i.e. head) waves. When a plane wave front propagates perpendicular to the boundaries of a vertical low-velocity layer surrounded by two vertical high-velocity layers, refracted wave fronts are generated in the low-velocity layer, which in turn generate secondary wave fronts in the high-velocity layers. These wave fronts trail the primary wave fronts by a constant delay whose magnitude has a simple dependence on the thickness of the low-velocity layer and the velocities involved. This process creates multipath arrivals that in geological settings with steeply inclined and faulted layers may appear and disappear rather abruptly, which may contribute to a scattered appearance of the wavefield.     

The seismic structure of thinned continental crust in the northern Bay of Biscay

Summary. The stretching and thinning of the continental crust, which occurs during the formation of passive continental margins, may cause important changes in the velocity structure of such crust. Further, crust attenuated to a few kilometres' thickness, can be found underlying 'oceanic' water depths. This paper poses the question of whether thinned continental crust can be distinguished seismically from normal oceanic crust of about the same thickness. A single seismic refraction line shot over thinned continental crust as part of the North Biscay margin transect in 1979 was studied in detail. Tau— p inversion suggested that there are differences between oceanic and continental crust in the lower crustal structure. This was confirmed when synthetic seismograms were calculated. The thinned continental crust (β± 7.0) exhibits a two-gradient structure in the non-sedimentary crust with velocities between 5.9 and 7.4 km s⁻¹; an upper 0.8 s⁻¹ layer overlies a 0.4 s⁻¹ layer. No layer comparable to oceanic layer 3 was detected. The uppermost mantle also contains a low-velocity zone.     

The relationship between seismic velocity and density in the continental crust — a useful constraint?

Summary . Plots of seismic velocity and density of rock samples show that a range of densities is possible for rocks of each seismic velocity and vice versa. although a single linear relationship is often assumed in crustal gravity calculations. Because of the scatter, whenever rocks of known seismic velocity are converted to density using this relationship, a reduction is made to the resolving power of the resulting gravity calculation. If these rocks reach thicknesses of more than a few kilometres, then the uncertainties become significant when compared with the size of commonly observed gravity anomalies. Examples are considered from the North Sea, Mississippi and Carolina Trough. It is concluded that the use of a seismic velocity measurement as the only indication of rock density does not provide a useful constraint when attempting to reproduce observed gravity variations. An appropriate model for isostatic compensation is probably the most important factor for successful predictions of crustal structure on the basis of gravity data.     

Crustal structure of the French Guiana margin, West Equatorial Atlantic

Geophysical data from the Amazon Cone Experiment are used to determine the structure and evolution of the French Guiana and Northeast Brazil continental margin, and to better understand the origin and development of along-margin segmentation. A 427-km-long combined multichannel reflection and wide-angle refraction seismic profile acquired across the southern French Guiana margin is interpreted, where plate reconstructions suggest a rift-type setting.
The resulting model shows a crustal structure in which 35–37-km-thick pre-rift continental crust is thinned by a factor of 6.4 over a distance of ∼70  km associated with continental break-up and the initiation and establishment of seafloor spreading. The ocean–continent boundary is a transition zone up to 45  km in width, in which the two-layered oceanic-type crustal structure develops. Although relatively thin at 3.5–5.0  km, such thin oceanic crust appears characteristic of the margin as a whole.
There is no evidence of rift-related magmatism, either as seaward-dipping sequences in the reflection data or as a high velocity region in the lower crust in the P -wave velocity model, and as a such the margin is identified as non-volcanic in type. However, there is also no evidence of the rotated fault block and graben structures characteristic of rifted margins. Consequently, the thin oceanic crust, the rapidity of continental crustal thinning and the absence of characteristic rift-related structures leads to the conclusion that the southern French Guiana margin has instead developed in an oblique rift setting, in which transform motion also played a significant role in the evolution of the resulting crustal structure and along-margin segmentation in structural style.     

On the roughness of Mesozoic oceanic crust in the western North Atlantic

Seismic reflection profiles from Mesozoic oceanic crust around the Blake Spur Fracture Zone (BSFZ) in the western North Atlantic have been widely used in constraining tectonic models of slow-spreading mid-ocean ridges. These profiles have anomalously low basement relief compared to crust formed more recently at the Mid-Atlantic Ridge at the same spreading rate. Profiles from other regions of Mesozoic oceanic crust also have greater relief. The anomalous basement relief and slightly increased crustal thickness in the BSFZ survey area may be due to the presence of a mantle thermal anomaly close to the ridge axis at the time of crustal formation. If so, the intracrustal structures observed may be representative of an atypical tectonic regime.     

Geological and geophysical investigation of the Mid-Cayman Spreading Centre: seismic velocity measurements and implications for the constitution of layer 3

Summary . In this paper we present laboratory measurements of compressional and shear wave velocities of a diverse suite of gabbroic rocks collected from the walls of the Mid-Cayman Spreading Centre with DSRV Alvin. The degree of deformation and alteration affecting these gabbros is quite variable, and we believe that they are typical of plutonic rocks emplaced at shallow levels (upper portion of seismic layer 3 and shallower) of the oceanic crust. The compositional and textural variations are reflected in the wide range of laboratory velocities which span most of the range of seismic velocities reported for oceanic and ophiolite rock samples including basalts, gabbros, ultramafics, and their altered derivatives. Based upon the laboratory velocities and the geological setting of the Mid-Cayman gabbros, it is argued that no unique lithology, except anhydrous peridotite, can be unequivocally identified in the oceanic lithosphere from seismic velocity data alone. Furthermore, these data allow for the possibility of considerable lithologic heterogeneity within portions of the oceanic crust at the scale of a few centimetres to a few hundred metres. Such heterogeneities would go unrecognized because seismic refraction studies mask these variations resulting in a picture of apparent uniformity.