Krauklis wave initiation in fluid-filled fractures by seismic body waves

Abstract

Krauklis waves are a special wave mode that is bound to and propagates along fluid-filled fractures. They can repeatedly propagate back and forth along a fracture and eventually fall into resonance emitting a seismic signal with a dominant characteristic frequency. They are of great interest because this resonant behavior can lead to strongly frequency-dependent propagation effects for seismic body waves and may explain seismic tremor generation in volcanic areas or affect microseismic signals in fractured fluid reservoirs. It has been demonstrated that Krauklis waves can be initiated by a seismic source inside the fracture, for example by hydrofracturing. Here, the aim is to study Krauklis wave initiation by an incident plane P-or S-wave in numerical simulations. Both seismic body waves are reflected and scattered at the fracture, but also, two Krauklis waves are initiated with significant amplitude, one at each fracture tip (i.e., at the diffraction-points of the fracture). Generally, the incident S-wave initiates largeramplitude Krauklis waves compared to the incident P-wave case. For both incident wave modes, the initiation of Krauklis waves strongly depends on the fracture orientation. In the case of an incident P-wave, large-amplitude Krauklis waves are initiated at moderate (12°-40°) and high (>65°) inclination angles of the fracture with a distinct gap at approximately 50°. The dependency of Krauklis wave initiation on fracture orientation is almost inversed in the case of an incident S-wave and the largest-amplitude Krauklis waves are initiated at an S-wave incidence angle of approximately 50°. The initiation of large-amplitude Krauklis waves by both P-and S-waves has important implications for earthquake signals propagating through fluid-bearing fractured rocks (volcanic areas, fluidreservoirs) or for seismic exploration surveys in fractured reservoir situations.

Vibrational modes of hydraulic fractures: Inference of fracture geometry from resonant frequencies and attenuation

“…Following Chouet [1986] we refer to such waves as crack waves. Several recent studies refer to such waves as either Krauklis or Stoneley guided waves [Korneev, 2008[Korneev, , 2010Frehner and Schmalholz, 2010;Frehner, 2014], although other names have also been used [Korneev et al, 2012].…”

“…Chouet [1986,1988,1992], and Dunham and Ogden [2012] study the motion of a viscous fluid but assume fully developed flow. Deviations from full flow development have been explored in several recent analytical [Korneev, 2008[Korneev, , 2010Nakagawa and Korneev, 2014] and numerical [Frehner and Schmalholz, 2010;Frehner, 2014] …”

Subsurface fluid distribution and possible seismic precursory signal at the Salse di Nirano mud volcanic field, Italy

“…Previous authors (Frehner & Schmalholz 2010;Korneev 2011;Maksimov et al 2011) highlighted the effects generated by seismic waves propagating along fluid-filled fractures. Incident body waves may generate secondary seismic waves called Krauklis waves that may fall into resonance emitting a dominant frequency (Frehner 2014) while propagating back and forth along fluid-filled fractures. Krauklis waves may also be generated by fluid overpressure produced inside the fluid-filled fractures (Ferrazzini et al 1990).…”

Seismic responses of vertical fractures depending on their thickness

“…Наиболее полное исследование на базе численного моделирования волн типа Guided wave, распространяющих-ся вдоль трещин с конечной раскрытостью, их отражения и рассеяния от концов трещин приведено в [Frehner, Schmalholz, 2010]. Образование при падении волнового фронта на трещиноватый пласт, насыщенный флюидом, волн Крауклиса (их также иногда называют Stoneley guided waves), колеблющихся внутри трещины и при определенных условиях рассеивающихся наружу, рассмотрено в [Korneev, 2011; Frehner, 2013Frehner, , 2014. Однако все эти эффекты, связан-ные с трещинами конечной раскрытости, рассматривались на очень ограниченных фрагментах среды, не касались основных волн и не отражали полной картины волновых откликов на дневной поверхности.…”