Philip Christie

Imaging the pulsing Iceland mantle plume through the Eocene

The temperature of mantle plumes may vary on geologic time scales, from a few million years to tens of millions of years. In special circumstances such as near Iceland in the North Atlantic, where the plume underlies an oceanic spreading center, temporal variations in the oceanic crustal thickness provide a sensitive proxy for the mantle temperature if, as is likely, the crustal thickness is controlled primarily by passive decompression of mantle rising beneath the spreading center. We show from both seismic reflection imaging and wide-angle ocean bottom seismometer data from the Norwegian Sea that the temperature of the Iceland mantle plume decreased by ∼50 °C over the first 5 m.y. following continental breakup and then oscillated by ∼25 °C over an ∼3 m.y. period. Similar temperature variations on a 3–6 m.y. time scale, creating strong lineations in the gravity field, are inferred from the regional North Atlantic. They occur both in the period immediately following breakup and at the present-day Reykjanes Ridge south of Iceland, where they create V-shaped ridges as the mantle thermal anomalies propagate away from the center of the plume beneath Iceland. We propose that mantle plume temperature variations of ∼25 °C have occurred in the Iceland plume with a similar amplitude and frequency since at least 49 Ma, and are likely to be a feature of all mantle plumes.

Combination of multi-component streamer pressure and vertical particle velocity: theory and application to data

Summary In this paper, we generalize the optimal deghosting (ODG) method used for deghosting over/under data to combine pressure (P) and vertical velocity (Z) data recorded with a 4C multi-component streamer to reduce the impact of the noise on the deghosted data. The ODG approach uses pressure and velocity ghost models and the statistics of the residual noise to minimize, in a least-squares sense, the noise on the up-going/deghosted wavefield. ODG and the standard PZ summation (PZSUM) combinations are applied to pressure and velocity data recorded in the North Sea (2-D acquisition). We show that both methods attenuate the receiver ghost, fill in information at the pressure notch frequencies and that ODG has the least postcombination noise level. We also show pre- and post-stack vertical velocity data with encouraging signal-to-noise ratios. Some differences between the ODG and PZSUM combination results can be explained using the cross-line component measurement. Finally, in order to further improve the PZ deghosted data, we suggest a toolbox approach that takes advantage of both ODG and PZSUM combinations and accounts for the varying signal-to-noise ratios observed on multi-component streamer data.

Borehole study of compressional and shear attenuation of basalt flows penetrated by the Brugdan and William wells on the Faroes shelf

We investigated the seismic attenuation of compressional (P-) and converted shear (S-) waves through stacked basalt flows using short-offset vertical seismic profile(VSP) recordings from the Brugdan (6104/21–1) and William (6005/13–1A) wells in the Faroe-Shetland Trough. The seismic quality factors (Q) were evaluated with the classical spectral ratio method and a root-mean-square time-domain amplitude technique. We found the latter method showed more robust results when analysing signals within the basalt sequence. For the Brugdan well we calculated effective Q estimates of 22–26 and 13–17 for P- and S-waves, respectively, and 25–33 for P-waves in the William well. An effective QS/QP ratio of 0.50–0.77 was found from a depth interval in the basalt flow sequence where we expect fully saturated rocks. P-wave quality factor estimates are consistent with results from other VSP experiments in the North Atlantic Margin, while the S-wave quality factor is one of the first estimates from a stacked basalt formation using VSP data. Synthetic modelling demonstrates that seismic attenuation for P- and S-waves in the stacked basalt flow sequence is mainly caused by one-dimensional scattering, while intrinsic absorption is small.

Evaluating Fidelity and Repeatability of Wavefields Reconstructed from Multicomponent Streamer Data

In marine time-lapse (4D) seismic data processing, each wavefield snapshot is usually interpolated onto a common grid to compensate for differences in acquisition, especially receiver, geometry. Because crossline sampling is usually sparse compared to inline sampling, and most mispositioning results from poor crossline repeatability from variable feathering, crossline interpolation is often aliased and causes 4D errors that increase with interpolation distance. Recent developments in multicomponent wavefield reconstruction hold promise for good amplitude fidelity, even for data that are spatially aliased to high order. In this paper we examine repeated data acquired by an experimental 3D-4C towed-cable array. Data from six crossline samples are reconstructed 1:12 and placed on a rectangular grid uniformly sampled at 6.25 m inline and crossline. Wavefields are interpolated using pressure alone and compared with wavefields reconstructed from pressure, crossline gradient and vertical gradient. We find that 3C reconstruction has excellent fidelity, recapturing highly aliased diffractions which are lost by pressure-only interpolation. 3C reconstruction is also more repeatable, outperforming 1C reconstruction, where error correlates with midpoint mispositioning. Although the experimental 3D data have limited inline aperture, the level of repeatability achieved is extremely encouraging compared to full-aperture, pressure-only, 4D datasets at an equivalent stage of processing.

Fidelity and repeatability of wave fields reconstructed from multicomponent streamer data

Wave field reconstruction – the estimation of a three-dimensional (3D) wave field representing upgoing, downgoing or the combined total pressure at an arbitrary point within a marine streamer array – is enabled by simultaneous measurements of the crossline and vertical components of particle acceleration in addition to pressure in a multicomponent marine streamer. We examine a repeated sail line of North Sea data acquired by a prototype multicomponent towed-streamer array for both wave field reconstruction fidelity (or accuracy) and reconstruction repeatability. Data from six cables, finely sampled in-line but spaced at 75 m crossline, are reconstructed and placed on a rectangular data grid uniformly spaced at 6.25 m in-line and crossline. Benchmarks are generated using recorded pressure data and compared with wave fields reconstructed from pressure alone, and from combinations of pressure, crossline acceleration and vertical acceleration. We find that reconstruction using pressure and both crossline and vertical acceleration has excellent fidelity, recapturing highly aliased diffractions that are lost by interpolation of pressure-only data. We model wave field reconstruction error as a linear function of distance from the nearest physical sensor and find, for this data set with some mismatched shot positions, that the reconstructed wave field error sensitivity to sensor mispositioning is one-third that of the recorded wave field sensitivity. Multicomponent reconstruction is also more repeatable, outperforming single-component reconstruction in which wave field mismatch correlates with geometry mismatch. We find that adequate repeatability may mask poor reconstruction fidelity and that aliased reconstructions will repeat if the survey geometry repeats. Although the multicomponent 3D data have only 500min-line aperture, limiting the attenuation of non-repeating multiples, the level of repeatability achieved is extremely encouraging compared to full-aperture, pressureonly, time-lapse data sets at an equivalent stage of processing.

Survey Design and Modeling Framework for Towed Multimeasurement Seismic Streamer Data

Survey design and modeling is the process of evaluating prior data, if any, so as to optimize the acquisition of a fresh seismic survey. For conventional marine seismic acquisition comprising pressure-only data acquisition, the process of converting geological objectives into a realizable, cost-effective survey is reasonably well understood. However, for towed marine multimeasurement seismic acquisition recording collocating pressure and acceleration measurements, the process is still being explored. This paper proposes a framework of investigation, consisting of three modular workflows to optimize the acquisition geometry and its efficiency, focusing especially on 3D deghosting and wavefield reconstruction algorithms enabled by multimeasurement seismic, without compromising on the survey objectives.

P- and S-wave attenuation estimates of stacked basalt flows in the North Atlantic Margin

We present compressional (P-) and converted shear (S-) wave attenuation estimates (Q-values) obtained from near zero-offset VSP recordings through stacked basalt flows in the North Atlantic volcanic margin. The estimates are calculated with the classical spectral ratio method and root-mean-square amplitude technique. Effective attenuation estimates of 15–26 and 14–16 are obtained for P- and S-waves, respectively. This is one of the first shear wave attenuation estimates from a stacked basalt formation. Synthetic modeling reveals that seismic attenuation in the stacked basalt flow sequence is mainly caused by scattering, corresponding to Qscat = 27–29 for P- and S-waves where intrinsic energy absorption is small. ©2011 Society of Exploration Geophysicists