Seismic Attenuation in Faroe Island Basalts

The main objective of this work is to establish the applicability of shallow surface‐seismic traveltime tomography in basalt‐covered areas. A densely sampled ∼1300‐m long surface seismic profile, acquired as part of the SeiFaBa project in 2003 (Japsen et al. 2006) at Glyvursnes in the Faroe Islands, served as the basis to evaluate the performance of the tomographic method in basalt‐covered areas. The profile is centred at a ∼700‐m deep well. VP, VS and density logs, a zero‐offset VSP, downhole‐geophone recordings and geological mapping in the area provided good means of control. The inversion was performed with facilities of the Wide Angle Reflection/Refraction Profiling program package (Ditmar et al. 1999). We tested many inversion sequences while varying the inversion parameters. Modelled traveltimes were verified by full‐waveform modelling. Typically an inversion sequence consists in several iterations that proceed until a satisfactory solution is reached. However, in the present case with high velocity contrasts in the subsurface we obtained the best result with two iterations: first obtaining a smooth starting model with small traveltime residuals by inverting with a high smoothing constraint and then inverting with the lowest possible smoothing constraint to allow the inversion to have the full benefit of the traveltime residuals. The tomogram gives usable velocity information for the near‐surface geology in the area but fails to reproduce the expected velocity distribution of the layered basalt flows. Based on the analysis of the tomogram and geological mapping in the area, a model was defined that correctly models first arrivals from both surface seismic data and downhole‐geophone data.

Section: T H E D a T Amentioning

confidence: 99%

Section: Q U a N T I T A T I V E C O R R E L A T I O N T O V E L O C mentioning

confidence: 99%

P‐wave velocity distribution in basalt flows of the Enni Formation in the Faroe Islands from refraction seismic analysis

Petersen

Brown

Andersen

2012

“…Other known parameters used in the modelling are listed in Table 2. An average V p/ V s ratio of 1.88 was obtained from an analysis of the VSP data (Shaw 2006). Values of crack radius, crack density and crack aspect ratio are not known.…”

Section: Estimation Of Intrinsic Qmentioning

confidence: 99%

Seismic attenuation in Faroe Islands basalts

Shaw

Worthington

White

et al. 2007

Self Cite

A B S T R A C TWe analysed vertical seismic profiling (VSP) data from two boreholes at Glyvursnes and Vestmanna on the island of Streymoy, Faroe Islands, to determine the magnitude and causes of seismic attenuation in sequences of basalt flows. The work is part of SeiFaBa, a major project integrating data from vertical and offset VSP, surface seismic surveys, core samples and wireline log data from the two boreholes. Values of effective seismic quality factor (Q) obtained at Glyvursnes and Vestmanna are sufficiently low to significantly degrade the quality of a surface reflection seismic image. This observation is consistent with results from other VSP experiments in the North Atlantic region. We demonstrate that the most likely cause of the low values of effective Q at Glyvursnes and Vestmanna is a combination of 1D scattering and intrinsic attenuation due to seismic wave-induced fluid flow within pores and microcracks. Tests involving 3D elastic wave numerical modelling with a hypothetical basalt model based on field observations, indicate that little scattering attenuation is caused by lateral variations in basalt structure.

“…They found that the dominant attenuation mechanism was caused by impedance scattering because their Q scat,P matched Q eff,P , within experimental error. In contrast, Shaw et al (2008) concluded that while impedance scattering is plausible to explain the Upper Basalt Series attenuation in the Glyvursnes borehole on the Faroe Islands, intrinsic attenuation is required to explain the losses estimated from the Middle Basalt Series of the Glyvursnes and Vestmanna boreholes.…”

Section: Introductionmentioning

confidence: 96%

“…However, Maresh et al (2006) showed that rough surface effects appear to be small compared to one-dimensional (1D) impedance scattering losses for the basalts penetrated in Well 164/07-1 in the nearby Rockall Trough, suggesting that a good estimate of scattering loss can be calculated with a 1D earth model. VSP-derived Q values of layered basalts have been reported from various igneous provinces (Rutledge and Winkler 1989;Pujol and Smithson 1991;Christie, Gollifer and Cowper 2006;Maresh et al 2006;Shaw et al 2008) with values of Q eff,P in the range of 15-48. The subscripts P and S for Q indicate compressional (P-) and shear (S-) wave measurements.…”

Section: Introductionmentioning

confidence: 99%

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

Schuler

Christie

White

2013

Self Cite

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.