Non-standard P-and S-wave experiments at the KTB deep drilling site: Nature of reflections and seismic anisotropy of crystalline rocks

We relocate 303 microseismic events recorded in 1998 by sensors in a single borehole in the North Sea Valhall oil field. A semiautomated array analysis method repicks the P- and S-wave arrival times and P-wave polarizations, which are needed to locate these events. The relocated sources are confined predominantly to a [Formula: see text]-thick zone just above the reservoir, and location uncertainties are half those of previous efforts. Multiplet analysis identifies 40 multiplet groups, which include 208 of the 303 events. The largest group contains 24 events, and five groups contain 10 or more events. Within each multiplet group, we further improve arrival-time picking through crosscorrelation, which enhances the relative accuracy of the relocated events and reveals that more than 99% of the seismic activity lies spatially in three distinct clusters. The spatial distribution of events and wave-form similarities reveal two faultlike structures that match well with north-northwest–south-southeast-trending fault planes interpreted from 3D surface seismic data. Most waveform differences between multiplet groups located on these faults can be attributed to S-wave phase content and polarity or P-to-S amplitude ratio. The range in P-to-S amplitude ratios observed on the faults is explained best in terms of varying source mechanisms. We also find a correlation between multiplet groups and temporal variations in seismic anisotropy, as revealed by S-wave splitting analysis. We explain these findings in the context of a cyclic recharge and dissipation of cap-rock stresses in response to production-driven compaction of the underlying oil reservoir. The cyclic nature of this mechanism drives the short-term variations in seismic anisotropy and the reactivation of microseismic source mechanisms over time.

Download Full-text

Extending a thickened crustal bulge: toward a new geodynamic evolution model of the paleozoic NW Bohemian Massif, German Continental Deep Drilling site (SE Germany)

Earth-Science Reviews ◽

10.1016/s0012-8252(98)00028-2 ◽

1998 ◽

Vol 44 (3-4) ◽

pp. 95-145 ◽

Cited By ~ 14

Author(s):

Alexander Krohe

Keyword(s):

Bohemian Massif ◽

Evolution Model ◽

Deep Drilling ◽

Geodynamic Evolution ◽

Drilling Site ◽

Continental Deep Drilling

Download Full-text

Strain-Induced Fabric Transition of Chlorite and Implications for Seismic Anisotropy in Subduction Zones

Minerals ◽

10.3390/min10060503 ◽

2020 ◽

Vol 10 (6) ◽

pp. 503

Author(s):

Dohyun Kim ◽

Haemyeong Jung ◽

Jungjin Lee

Keyword(s):

Shear Strain ◽

Subduction Zones ◽

Seismic Anisotropy ◽

Shear Plane ◽

Shear Direction ◽

S Wave ◽

Lattice Preferred Orientation ◽

Back Arc

Seismic anisotropy of S-wave, trench-parallel or trench-normal polarization direction of fast S-wave, has been observed in the fore-arc and back-arc regions of subduction zones. Lattice preferred orientation (LPO) of elastically anisotropic chlorite has been suggested as one of the major causes of seismic anisotropy in subduction zones. However, there are two different LPOs of chlorite reported based on the previous studies of natural chlorite peridotites, which can produce different expression of seismic anisotropy. The mechanism for causing the two different LPOs of chlorite is not known. Therefore, we conducted deformation experiments of chlorite peridotite under high pressure–temperature conditions (P = 0.5–2.5 GPa, T = 540–720 °C). We found that two different chlorite LPOs were developed depending on the magnitude of shear strain. The type-1 chlorite LPO is characterized by the [001] axes aligned subnormal to the shear plane, and the type-2 chlorite LPO is characterized by a girdle distribution of the [001] axes subnormal to the shear direction. The type-1 chlorite LPO developed under low shear strain (γ ≤ 3.1 ± 0.3), producing trench-parallel seismic anisotropy. The type-2 chlorite LPO developed under high shear strain (γ ≥ 5.1 ± 1.5), producing trench-normal seismic anisotropy. The anisotropy of S-wave velocity (AVs) of chlorite was very strong up to AVs = 48.7% so that anomalous seismic anisotropy in subduction zones can be influenced by the chlorite LPOs.

Download Full-text