Krauklis wave initiation in fluid-filled fractures by seismic body waves Marcel Frehner 1 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 char- acteristic 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 microseis- mic signals in fractured fluid reservoirs. It has been demon- strated 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 inci- dent plane P- or S-wave in numerical simulations. Both seis- mic 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 larger- amplitude 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 orien- tation 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, fluid- reservoirs) or for seismic exploration surveys in fractured res- ervoir situations. INTRODUCTION The presence of fluids in reservoir rocks has a major effect on seismic wave propagation behavior; for example, dispersion and frequency-dependent attenuation (Biot, 1962; White, 1975; Bour- bie et al., 1987; Carcione, 2001; Quintal et al., 2011). Research on fluid-related seismic effects faces some scientific challenges and is significant for various industrial applications. One particular chal- lenge is the interaction of simultaneous physical processes on dif- ferent length scales. Because not all scales can be modeled at once, microscale processes have to be upscaled and their macroscale ef- fects are described in effective medium models (e.g., Lambert et al., 2013). For the case of porous rocks such as sandstone, this is done successfully, for example in the Biot theory (Biot, 1962), the squirt-flow theory (Mavko and Jizba, 1991; Dvorkin et al., 1995), or the patchy-saturation model (White, 1975; White et al., 1975). Despite including many effects in porous rocks, existing effective medium models have more difficulties describing fracture-related phenomena. One such phenomenon of particular interest is the so-called Krauklis wave, which is a special wave mode that is bound to and propagates along fluid-filled fractures. They are highly dis- persive with a very low-phase velocity at low frequencies (Ferraz- zini and Aki, 1987; Ashour, 2000; Korneev, 2008). There has been some confusion about the terminology because they have been re- ferred to as Krauklis waves in Korneev (2011) and Frehner (2013), Stoneley-guided waves in Korneev et al. (2009), Frehner and Schmalholz (2010), and Korneev (2010), crack waves in Chouet (1986) and Yamamoto and Kawakatsu (2008), slow Stoneley waves in Ferrazzini and Aki (1987), or Stoneley waves in a fracture in Ashour (2000). Strictly speaking, a Stoneley wave is an interface wave propagating along an interface between two solid (elastic) half-spaces (Stoneley, 1924) and a Scholte wave is an interface wave propagating along an interface between a solid (elastic) Manuscript received by the Editor 6 March 2013; revised manuscript received 22 August 2013; published online 6 December 2013. 1 ETH Zurich, Geological Institute, Zurich, Switzerland. E-mail: marcel.frehner@erdw.ethz.ch. © 2013 Society of Exploration Geophysicists. All rights reserved. T27 GEOPHYSICS, VOL. 79, NO. 1 (JANUARY-FEBRUARY 2014); P. T27–T35, 7 FIGS., 1 TABLE. 10.1190/GEO2013-0093.1