Ambient seismic noise tomography at Ekofisk

Abstract

We have studied an approximately 40-hr recording from the Life of Field Seismic array installed over Ekofisk field to assess whether the recorded ambient seismic energy was suitable for passive seismic surface-wave interferometry. Passive seismic interferometry aims to retrieve virtual seismic sources by crosscorrelation. We have found that the noise recorded by the pressure sensors between 0.4 and 1.4 Hz consisted predominantly of Scholte-wave microseism energy. The noise incidence directions have an almost uniform distribution over the azimuth, enabling the synthesis of nearly symmetric estimated Green's functions between all stations in the array that formed virtual seismic sources. These sources were dominated by Scholte waves traveling along the seafloor, and we sought to determine which features in the subsurface can be imaged by straight-ray group-velocity tomography. We located a high-velocity anomaly in the center of Ekofisk's production-induced subsidence bowl, surrounded by lower velocities. This pattern seemed to result from production-induced seafloor subsidence that altered the near-surface shear strengths. A dispersion analysis showed that the Scholtewave virtual seismic source exhibited an approximate penetration depth to 600 m below the seafloor. These results are significant because they demonstrated that recordings made at the oceanbottom cable array at Ekofisk field in the absence of seismic shooting can be used to image and monitor the near surface.

Seismic Attenuation From Ambient Noise Across the North Sea Ekofisk Permanent Array

“…So compression of the sea floor appears to give higher Q values, possibly due to compressional closing of fractures at the sea floor. This relation between compression and fracture closure has also been suggested by Nagel (), who show in their Figure a compressional zone in the center of the Ekofisk subsidence bowl and a tensional zone on the lip of the subsidence bowl, and also by de Ridder and Biondi (), who saw related effects on the seismic velocities.…”

“…Their results fitted well with their seismically derived compaction map (which does not show the center of the array due to there being a seismically obscured area in the center of the array). The ringed structure of the velocity and Q at the Ekofisk array can also be observed in surface wave group velocity tomography (de Ridder et al, ; de Ridder & Biondi, ), surface wave phase velocity tomography (de Ridder & Biondi, ; de Ridder & Curtis, ), and compressional and shear wave velocity (Kazinnik et al, ) estimates at the site.…”

The future of passive seismic acquisition

“…de Ridder & Dellinger 2011). He emphasized that the information recorded by dense networks of seismometers is much greater than that from individual instruments or low-density arrays, thus allowing the use of the full wavefield for inversion (de Ridder & Biondi 2015). This was a message emphasized by many other speakers.…”

The Future of Broadband Passive Seismic Acquisition

“…Sjoerd de Ridder (Total E & P) showed how industry are pioneering this method, outlining the Ekofisk life of field seismic system of almost 4000 multicomponent seismometers deployed in the North Sea among other examples (e.g., de Ridder & Dellinger, 2011). He emphasised that the information recorded by these dense networks is much greater compared to individual instruments or low density arrays, thus allowing for the use of the full wavefield for inversion (de Ridder & Biondi, 2015). This was a message repeated by many other speakers.…”