Seismic velocity structure of the shallow Porcupine Basin, southwest of Ireland: new insights from travel-time tomography of long-streamer data
Abstract
The Porcupine Basin is a Mesozoic failed rift located in the North Atlantic margin, SW of Ireland. The post-rift sequence of the basin contains early Cretaceous to Cenozoic sediments deposited after several episodes of uplift/subsidence. Thermal subsidence has been recently associated with a new phase of faulting that implies reactivation of syn-rift basin-bounding faults. Normal faulting within the post-rift sequence controls the migration of fluids to the surface and plays a role in the initiation of carbonate seamounts in the basin. Little is known about the porosity and fluid migration processes associated with basin tectonics and sedimentary structure of the post-rift sequence. These petrophysical properties can be inferred from seismic properties such as P-wave seismic velocity (Vp) using empirical relationships.Here, we use long-streamer data to perform travel-time tomography of first arrivals and retrieve the 2D Vp structure of the post-rift sequence along a 130 km-long EW profile across the North Porcupine Basin. The tomographic model reveals two prominent vertical velocity anomalies located at the western margin of the basin, coinciding with the location of a reactivated basin-bounding fault. Based on synthetic tests, these anomalies are within the range of anomaly sizes that the tomographic method can retrieve. Comparing the corresponding time-migrated seismic section with the tomographic model, we observe that the hanging wall of the basin-bounding fault is not significantly affected by major normal faulting and yet is associated with comparatively lower seismic velocities.This result together with exploration well data suggest high effective porosities and fluid circulation within the hanging wall. Well data are used to build a new velocity-density relationship suitable for the post-rift sequence of the basin, and to calculate porosities across the sequence. The result shows that porosity increases towards the margins of the basin implying differential compaction between the center and the basin margins, where basin-bounding faults reveal higher porosity. Based on our results, we propose a new model of fluid circulation that has implications for fluid overpressure and fault reactivation. Further work will consider full waveform inversion to refine the tomographic results.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.T51C0489P
- Keywords:
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- 8105 Continental margins: divergent;
- TECTONOPHYSICS;
- 8109 Continental tectonics: extensional;
- TECTONOPHYSICS