Low‐frequency propagation through fine layering

Folstad, P.G.; Schoenberg, Michael

doi:10.1190/1.1821971

Cited by 20 publications

(9 citation statements)

References 2 publications

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“…However, the two peaks really start merging when the interbed sediment thickness is less than 5 m, as we see in the Brugdan I downgoing wavefield (Figure 4b). This observation is partly in agreement with Folstad and Schoenberg (1992), who find that scaling layers up to λ∕10 of the smallest wavelength yields signals very close to the exact response with insignificant changes in the arrival time, wave shape, or apparent attenuation. Here, the dominant wavelength within the top basalt flow is approximately 100 and 50 m for interbed sediments.…”

Section: Stratigraphic Filtering Of Waveforms D269supporting

confidence: 79%

Effect of flood basalt stratigraphy on the phase of seismic waveforms recorded offshore Faroe Islands

2015

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The generation of short-period multiples between highly heterogeneous layers of basalt flows can strongly alter transmitted seismic wavefields. These layers filter and modify penetrating waves, producing apparent attenuation and phase changes in the observed waveforms. We investigated the waveform and apparent phase changes of the primary seismic signal using mainly the maximum kurtosis approach. We compared the seismic recordings from two short-offset vertical seismic profiles (VSPs) with synthetic seismograms, generated from sonic logs in the same wells, and we found that short-period multiples cause a rapid broadening of the primary arrivals and strong apparent phase changes within a short depth interval below the top of the basalt flows. Relatively large uncertainties were associated with estimating constant phase shifts of the seismic arrivals within the topmost 250 m of the basalt sequences, where complex scattering occurred. Within this interval of the Brugdan I well, a phase-only compensation of the first arrivals with a frequency-independent, combined scattering, and intrinsic attenuation operator was unfeasible. At a greater depth, we found that the phase shifts, predicted by a VSP-derived effective Q value, were similar to those estimated from the VSP signals using the kurtosis method. Thus, phase-only compensation with a combined scattering and intrinsic attenuation operator could work well depending on the seismic signal bandwidth and the distribution, depth, and magnitude of the impedance contrasts in the basalt sequence.

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Section: Stratigraphic Filtering Of Waveforms D269supporting

confidence: 79%

Effect of flood basalt stratigraphy on the phase of seismic waveforms recorded offshore Faroe Islands

2015

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“…The logs are of good quality and allow the identification of many subaerially emplaced flows over much of the section. Figure 2 shows the P-and S-velocity and density logs after re-sampling the data to 3 m using a Backus average (Backus 1962;Folstad & Schoenberg 1992). There are wide variations in P-and S-velocities, in an asymmetric, quasi-periodic manner, though the mean velocities are very consistent over the well.…”

Section: Layer-induced Anisotropymentioning

confidence: 99%

Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic

Christie

Gollifer

Cowper³

2006

GEUS Bulletin

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Extruded basalt flows overlying sedimentary sequences present a challenge to hydrocarbon exploration using reflection seismic techniques. The Lopra-1/1A re-entry well on the Faroese island of Suðuroy allowed us to study the seismic characteristics of a thick sequence of basalt flows from well logs and borehole seismic recordings. Data acquired during the deepening operation in 1996 are presented here. The re-entry well found that the seismic event at 2340 m, prognosed from the pre-drill Vertical Seismic Profile (VSP) as a decrease in impedance, was not base basalt and the deepened well remained within the lower series basalts. Nonetheless, compressional and shear sonic logs and a density log were recorded over the full open hole interval. These allowed a firm tie to be made with the reflected wavefield from a new VSP. The sonic logs show a compressional to shear wavespeed ratio of 1.84 which is almost constant with depth. Sonic compressional wavespeeds are 3% higher than seismic velocities, suggesting dispersion in the basalt flows. Azimuthal anisotropy was weakly indicated by the shear sonic log but its orientation is consistent with the directions of mapped master joints in the vicinity of the well. The VSP downgoing compressional wavelet shows good persistence, retaining a dominant period of 28 ms at 3510 m depth. Average vertical velocity is 5248 m/s, higher than previously reported. Attenuation can largely be modelled by geometrical spreading and scattering loss, consistent with other studies. Within the piled flows, the effective Q from scattering is about 35. Elastic layered medium modelling shows some hope that a mode-converted shear wave may be observed at moderate offsets. Like its predecessor, the 1996 VSP indicates a decrease in impedance below the final depth of the well. However, it is unlikely to be basement or sediment and is probably an event within the volcanic sequence.

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“…It is known from geological and petrophysical studies (Gardiner, personal communication) that the porosity fluctuations between individual beds in this stack are not large and that the overall thickness, T, of the channel sand package is still small compared to the seismic wavelength, λ, such that λ/T < 5. It therefore follows that for the purposes of seismic wave propagation, the vertical heterogeneity arising from the internal thin layering in such a reservoir can be adequately replaced by a single saturated homogeneous layer representing the entire channel sand package, with no loss of accuracy (see for example Folstad and Schoenberg 1992). The elastic moduli κ eff sat and μ eff of this effective layer can be calculated by applying the theory of Backus (1962), who developed general averaging formulae for isotropic (or vertically transverse isotropic) layers.…”

Section: A P P E N D I Xmentioning

confidence: 99%

Seismic scale saturation relations in turbidite reservoirs undergoing waterflood

MacBeth

Stephen

2008

Geophysical Prospecting

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A B S T R A C TEstimates of the effective fluid modulus from seismic cannot be directly converted to the true pore-volume weighted mean saturation S w determined from fluid flow principles by using the saturation laws currently in use. One of the reasons is that seismic waves sample the reservoir geology and production induced saturation heterogeneity in a different way from the fluids. This mismatch prevents accurate quantitative evaluation of saturation changes from 4D seismic analysis. To tackle this problem, a reservoir-related saturation law is developed for a turbidite reservoir -this geology being chosen because the architecture for a single sand package can be modelled as a stack of horizontal beds. An effective medium and perturbation theory are applied to the determination of the seismic properties of this model. This calculation provides a relationship that connects the true saturation S w to the effective fluid modulus from seismic via statistical measures of the vertical spread of the porosity and saturation variations in the reservoir. These statistics can be extracted from the simulation model and if known, enable the new saturation law to deliver a significant improvement in accuracy when estimating S w compared to other well-known laws. The relationship that has been developed also captures the effect of inter-bedded shales and can therefore be used to estimate true saturation in regions of the reservoir with moderate to low net-to-gross, provided the fraction of the shale component is known. In practice, the final choice of saturation law depends upon the reservoir information available, the assumptions that can be tolerated and the accuracy required in any particular reservoir characterization study. I N T R O D U C T I O NDynamic reservoir management requires accurate monitoring of saturation values as production proceeds. Thus, understanding the lateral variation of saturation in the inter-well volumes is particularly important for evaluating the overall performance of a waterflood and potential encroachment on producing wells. In this context, there is a need to reliably determine the position and nature of the waterfront for the purposes of optimization of the oil displacement process and the associated well injectivity, to detect *

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Low‐frequency propagation through fine layering

Cited by 20 publications

References 2 publications

Effect of flood basalt stratigraphy on the phase of seismic waveforms recorded offshore Faroe Islands

Effect of flood basalt stratigraphy on the phase of seismic waveforms recorded offshore Faroe Islands

Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic

Seismic scale saturation relations in turbidite reservoirs undergoing waterflood

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