Reflectivity Synthetic Seismograms Research Articles

The Hatton Bank continental margin-II. Deep structure from two-ship expanding spread seismic profiles

SUMMARY The continent-ocean transition adjacent to Hatton Bank was studied using a dense grid of single-ship and two-ship multichannel seismic profiles. The interpretation of the explosive expanding spread profiles (ESPs) which were shot as part of this survey are discussed here in detail. Extensive seaward dipping reflectors are developed in the upper crust across the entire margin. These seaward dipping reflectors continue northwards on the Faeroes and Voring margins, where they have been shown to be caused by basaltic lavas, as well as on the conjugate margin of East Greenland. The dipping reflectors are an important feature of the rifting history of the margin and show that extensive volcanism was associated with the extension. The ESPs show clear seismic arrivals out to ranges of 100 km. Wide-angle Moho reflections can be seen on all the lines as well as good mid and lower crustal arrivals. The determination of seismic velocity structure was constrained by ray tracing and by amplitude modelling using reflectivity synthetic seismograms. The results from the ESPs show that there is a thick region of lower crustal material beneath the margin with an unusually high crustal velocity of 7.3-7.4 km s-'. This lower crustal material reaches a maximum thickness of 14 km beneath the central part of the margin and is terminated at depth by the Moho. The lower crustal lens of high-velocity material is interpreted as underplated or intruded igneous rocks associated with the large volumes of extrusive basaltic lavas, now seen as dipping reflectors on the margin.

The Hatton Bank continental margin-I. Shallow structure from two-ship expanding spread seismic profiles

SUMMARY The continent-ocean transition adjacent to Hatton Bank was studied using a dense grid of single-ship and two-ship multichannel seismic profiles. The interpretation of the explosive expanding spread profiles (ESPs) which were shot as part of this survey are discussed here in detail. Extensive seaward dipping reflectors are developed in the upper crust across the entire margin. These seaward dipping reflectors continue northwards on the Faeroes and Voring margins, where they have been shown to be caused by basaltic lavas, as well as on the conjugate margin of East Greenland. The dipping reflectors are an important feature of the rifting history of the margin and show that extensive volcanism was associated with the extension. The ESPs show clear seismic arrivals out to ranges of 100 km. Wide-angle Moho reflections can be seen on all the lines as well as good mid and lower crustal arrivals. The determination of seismic velocity structure was constrained by ray tracing and by amplitude modelling using reflectivity synthetic seismograms. The results from the ESPs show that there is a thick region of lower crustal material beneath the margin with an unusually high crustal velocity of 7.3-7.4 km s-'. This lower crustal material reaches a maximum thickness of 14 km beneath the central part of the margin and is terminated at depth by the Moho. The lower crustal lens of high-velocity material is interpreted as underplated or intruded igneous rocks associated with the large volumes of extrusive basaltic lavas, now seen as dipping reflectors on the margin.

Estimation of sediment shear Q using horizontal component OBS refraction data

Shear Q was estimated by amplitude comparison of horizontal component reflectivity synthetic seismograms with exceptionally high‐quality horizontal component ocean bottom seismometer (OBS) refraction data collected at DSDP site 581C in the Northwest Pacific using a Soviet air gun source. Computations were performed on a CRAY X‐MP/48 using an algorithm developed by Mallick and Frazer (1987). Sediment modeling was constrained by well log data and two‐way VP travel time from a normal incidence reflection profile. The velocity structure of the sediments and the upper 2 km of the basement was refined using ray tracing and reflectivity modeling. A strong shear conversion at the top of layer 2 constrains the shear‐wave travel time through the sediments to about 1.8 s, giving an average VS of about 0.2 km/s. Additional constraints on the modeling were imposed by matching the amplitudes of the primary as well as later arrivals. Preliminary results of a sensitivity analysis of the modeling indicate a minimum sediment shear Q of about 200 (α = 0.0136 N/wavelength) at 9 Hz is necessary to match amplitudes of later arrivals. A stability analysis to determine the sensitivity of the modeling to QP, QS, density, and velocity will be presented. [Work supported by ONR.]

A comparison of finite difference and reflectivity seismograms for marine models

Summary Finite difference and reflectivity synthetic seismograms are compared for laterally homogeneous seafloor models with step and ramp discontinuities in elastic properties. Given suitable numerical parameters in each case excellent agreement can be obtained. For step discontinuities between a liquid and a solid the explicit finite difference formulation of the elastic wave equation for heterogeneous media is unstable. It is necessary to introduce the boundary conditions specifically at the interface and suitable formulations correct to first and second order in the space increment are given. For ramp discontinuities between a liquid and a solid the finite difference formulation for heterogeneous media is stable and agrees with the reflectivity method when, in the latter case, the gradient is approximated by homogeneous layers thinner than one-fifth of the minimum compressional wavelength at the upper half-power frequency. For a step discontinuity between two solids the explicit finite difference formulation of the elastic wave equation for heterogeneous media, although stable, is inaccurate and boundary conditions must again be specifically introduced.