Ray series modeling of seismic wave travel times and amplitudes in three‐dimensional heterogeneous anisotropic crystalline rock: Borehole vertical seismic profiling seismograms from the Mojave Desert, California

Abstract

A 600-m borehole vertical seismic profiling (VSP) survey conducted in crystalline rock in the Mojave Block stress province, southern California, yielded 100 oriented three-component seismograms from a fan-shooting geometry with radius of 100 m using both vertical and horizontal vibrators and a vertical impact source. The seismograms show up to 12 ms of shear wave splitting and 5% lateral velocity heterogeneity. First-and second-order ray tracing was used to model the travel times and amplitudes of P-, SV-and SH-waves in a three dimensional mildly heterogeneous medium with elastic anisotropy induced by aligned fractures. A least squares inverse procedure was used to improve model parameters determined by forward modeling; the final root mean square travel time residual was +_2 ms. For purposes of amplitude computation we assume that the wavelets observed in a VSP section derive from a smooth intermediate interface (base of weathering layer or sedimentary/basement interface) that is illuminated by a narrow range of ray angles originating at a point source on the surface. Source-derived amplitude complications are thus m'mimized. Our results strongly indicate a population of vertical cracks with an average crack density of 0.035 oriented in the N31.5øW direction. Such a fracture population is consistent with regional principal strain and stress determinations. Ray series geometric amplitudes were consistent with the observation that wave motion parallel to the aligned fractures decayed more slowly than wave motion transverse to the aligned fractures. We estimate that Q > 75-100; we were unable to measure a polarizationdependent Q. et al., 1970; Nur, 1971; Gupta, 1973; Anderson and Whitcomb, 1973; Kanamori and Hadley, 1975; Crampin, 1978]. Subsequently, the existence of fractures in basement rock was hypothesized by Smithson et at. [1980] and directly studied in the shallow basement by Green and Mair [1983]. Recently, shear wave splitting observations were obtained by threecomponent borehole vertical seismic profiling (VSP). Oriented three-component VSP data have provided a direct means of characterizing a fracture population in crustal rock [Martin et a/.,1986; Corrigan et a/.,1986; Leary et at., 1987; Majer et al., 1988; Li et al., 1988]. For instance, a crack density profile for oriented fractures was determined for a section of an active normal fault zone at Oroville, California [Leary et a/.,1987] and a Vsv/VSH ratio about 1.12 was estimated in the Geysers geothermal field, CA [Majer et al., 1988] from shear wave splitting observations in the plane of the local prevailing fracture set. The methods of three-component borehole seismic investigation of aligned fracture populations in crystalline rock are further explored in this paper. In contrast to the threecomponent borehole VSP work at Oroville [Leary et a/.,1987] Copyfight 1990 by the American Geophysical Union Paper number 89IB01170 0148-0227/90/89 IB-01170505.00 and at the Geysers geothermal field [Majer et al.,1988], where the principal fracture trends are ...

Deep borehole log evidence for fractal distribution of fractures in crystalline rock

“…Microscopic fractures can be stress sensitive as suggested by the EDA crack model of Crampin et al (1984) while mesoscopic and macroscopic fracture structures can arise in episodes of finite shear strain and brittle shear fracture failure of crustal rock (King 1983;King & Sammis 1991). In particular, the hypothesis of scale independence of crustal fracture structures allows EDA stress sensitive seismic anisotropy to exist in crust that is strained but has not failed (Booth et al 1985;Li, Leary & Aki 1990) while according with evidence for fault parallel fracture fabric in or near fault zones (Leary…”

Spatial distribution of shear wave anisotropy in the crust of the southern Hyogo region by borehole observations

“…Observed seismic anisotropy data are integrated effects from hypocentre to observer, similar to traveltime analysis, so we can obtain detailed images of the stress field or fluid distribution by a tomographic approach (e.g. Li et al 1990; Wu & Lees 1999) if huge amounts of anisotropy data are available. The resolution of the stress field imaged by the observation of anisotropy is expected to be higher than that studied by focal mechanisms or geodetic measurements such as GPS observations.…”