Abstract
Abstract. We present a high-resolution P-wave velocity model of the sedimentary cover and the uppermost basement to ∼3 km depth obtained by full-waveform inversion of multichannel seismic data acquired with a 6 km long streamer in the Alboran Sea (SE Iberia). The inherent non-linearity of the method, especially for short-offset, band-limited seismic data as this one, is circumvented by applying a data processing or modelling sequence consisting of three steps: (1) data re-datuming by back-propagation of the recorded seismograms to the seafloor; (2) joint refraction and reflection travel-time tomography combining the original and the re-datumed shot gathers; and (3) full-waveform inversion of the original shot gathers using the model obtained by travel-time tomography as initial reference. The final velocity model shows a number of geological structures that cannot be identified in the travel-time tomography models or easily interpreted from seismic reflection images alone. A sharp strong velocity contrast accurately defines the geometry of the top of the basement. Several low-velocity zones that may correspond to the abrupt velocity change across steeply dipping normal faults are observed at the flanks of the basin. A 200–300 m thick, high-velocity layer embedded within lower-velocity sediment may correspond to evaporites deposited during the Messinian crisis. The results confirm that the combination of data re-datuming and joint refraction and reflection travel-time inversion provides reference models that are accurate enough to apply full-waveform inversion to relatively short offset streamer data in deep-water settings starting at a field-data standard low-frequency content of 6 Hz.
Highlights
Seismic methods are one of the most powerful existing geophysical tools to extract information on the structure and properties of the Earth’s subsurface
The proposed approach consists of the following steps: (1) data re-datuming by downward continuation (DC) of the recorded data to the seafloor, (2) joint refraction and reflection travel-time tomography (TTT) of the original and re-datumed shot gathers, (3) full-waveform inversion (FWI) of the original shot gathers using the model obtained in (2) as initial reference, and (4) pre-stack depth migration of the original shot gathers using the FWI VP model
We work under the premise that we have no a priori information on the structure and properties of the subsurface, so the goal is to recover all the possible information on the VP model from the multichannel reflection seismic (MCS) data alone
Summary
Seismic methods are one of the most powerful existing geophysical tools to extract information on the structure and properties of the Earth’s subsurface These techniques have been, and currently continue to be, widely used to obtain images of the sediments and crust and to map the variations in physical properties, P-wave velocity (VP). Whereas TTT is a robust, moderately non-linear technique that provides coarse VP models, FWI is computationally demanding and strongly non-linear, but it has the potential to provide higherresolution VP models. For their characteristics, TTT and FWI are considered to be complementary, so that they are often combined and applied together. Long-offset refracted waves containing longwavelength information are a key ingredient to obtain a kinematically correct reference macro-velocity model through
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