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

Abstract

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.

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

“…In Figure 1b, the power spectrum of the reflection coefficients shows a deficiency at low frequencies, a deficiency also found also in other rock sequences (e.g., Walden and Hosken, 1985). Following Schuler et al (2014), the Brugdan I volcanic sequence is split into three main depth intervals for our analyses: a first interval (1150-1500 m), comprising high-impedance contrasts that lead to complex wave scattering; a second interval (1500-2300 m); and a third interval (2300-3150 m) that includes fewer high-impedance contrasts per depth unit compared with the first and second interval. We often give more accurate depth specifications in the text because we use selected VSP levels.…”

“…Such a scatterer could have generated diffractions and refractions that led to wave interference and a more pronounced second trough in our downgoing VSP wavefield. A surface seismic profile shown in Schuler et al (2014) may support the presence of a potential side structure further because the Brugdan I well was drilled at the edge of a horst-graben structure, where we can expect faults. No other curved reflector was identified down to the base basalt.…”

“…We measured a significant drop of the first-arrival amplitude as well as a decrease in the dominant and peak frequencies within this depth interval. Schuler et al (2014) analyze this in more detail. Although the dominant period of the wavelet above the volcanic rocks is 16 ms, it increases to 42 ms at 1355 m depth (approximately 180 m into the basalt) and changes very little thereafter.…”

“…We rotate the 3C geophones to account for the well deviation, but the wavefield still shows unexplained features, which possibly affects our phase measurements. Problems in calculating a seismic quality factor within this interval were also reported by Schuler et al (2014). Therefore, we disregard the 2607-2906 m depth interval.…”

“…In summary, the studied phenomenon of multiple scattering delays the arrival of the energy peak and leads to apparent attenuation and medium anisotropy (de Hoop et al, 1991). Schuler et al (2014) study the apparent attenuation effect of the stratigraphic filter formed by basalt flows. In that paper, a rapid loss of high frequencies with associated pulse broadening was observed with increasing depth below top basalt.…”