Slow Split Shear Waves Research Articles

Experimental study of the influence of fluids on seismic azimuthal anisotropy

Abstract Seismic characterization of subsurface fractures has important applications and implications to reservoir characterization. Recent studies have shown that characterization of azimuthal anisotropy can help delineate orientation and intensity of fractures, as well as indicate the fluid content of their embedded cracks. To explore the influence of fluids on seismic response in a transversely isotropic medium, an experimental study using vertically aligned and preconditioned polycarbonate material embedded in isotropic resin was conducted. Part of the theoretical motivation for this experimental study was the Hudson and Thomsen theories of the elastic response of penny-shaped cracks. Our composite model was designed to simulate a single set of aligned vertical cracks (HTI symmetry) filled with fluids. In a series of ultrasonic transmission and reflection experiments, the model was initially filled with air (a natural gas proxy) and then liquids (distilled water and glycerin as brine and oil proxies, respectively) were gradually injected into the composite material to simulate different rates of fluid saturation. Porosity in our assembled cracked media was about 2.5% and crack density was estimated to be 3.5%. Our results show changes in ultrasonic compressional and shear-wave velocities in alternate directions as a function of different saturating fluids. We also detected changes in delay between fast and slow split shear-waves with different saturating fluids. The NMO velocity measured from the ultrasonic reflection response, showed not only changes in velocity values on saturation, but also a difference in the trend of NMO velocities as a function of source–receiver azimuth. Anisotropic parameters also varied with different levels of saturation. Our experiments indicate that the anisotropic parameter e is reduced by 30% on full saturation with water (brine proxy) and 50% with glycerin (oil proxy). The anisotropic parameter γ increased by 50% from a gas-to-brine saturated medium and 60% from a gas-to-oil saturated medium. We conclude that the most significant changes in anisotropic and stiffness properties occur when gas is substituted for distilled water or glycerin. These changes are much smaller when brine is substituted for oil or vice versa .

Mantle lithosphere transition from the East European Craton to the Variscan Bohemian Massif imaged by shear-wave splitting

Abstract. We analyse splitting of teleseismic shear waves recorded during the PASSEQ passive experiment (2006–2008) focused on the upper mantle structure across and around the Trans-European Suture Zone (TESZ). Altogether 1009 pairs of the delay times of the slow split shear waves and orientations of the polarized fast shear waves exhibit lateral variations across the array, as well as back-azimuth dependences of measurements at individual stations. Variable components of the splitting parameters can be associated with fabrics of the mantle lithosphere of tectonic units. In comparison with a distinct regionalization of the splitting parameters in the Phanerozoic part of Europe that particularly in the Bohemian Massif (BM) correlate with the large-scale tectonics, variations of anisotropic parameters around the TESZ and in the East European Craton (EEC) are smooth and of a transitional character. No general and abrupt change in the splitting parameters (anisotropic structure) can be related to the Teisseyre–Tornquist Zone (TTZ), marking the edge of the Precambrian province on the surface. Instead, regional variations of anisotropic structure were found along the TESZ/TTZ. The coherence of anisotropic signals evaluated beneath the northern part of the Brunovistulian in the eastern rim of the BM and the pattern continuation to the NE towards the TTZ, support the idea of a common origin of the lithosphere micro-plates, most probably related to Baltica. Smooth changes in polarizations of the core-mantle boundary refracted shear waves (SKS), polarizations, or even a large number of null splits northward of the BM and further across the TESZ towards the EEC indicate less coherent fabrics and a transitional character of structural changes in the mantle beneath the surface trace of the TESZ/TTZ. The narrow and near-vertical TTZ in the crust does not seem to have a steep continuation in the mantle lithosphere. The mantle part of the TESZ, whose crust was formed by an assemblage of suspect terranes adjoining the EEC edge from the southwest, appears in our measurements of anisotropy as a relatively broad transitional zone in between the two lithospheric segments of different ages. We suggest a southwestward continuation of the Precambrian mantle lithosphere beneath the TESZ and the adjacent Phanerozoic part of Europe, probably as far as towards the Bohemian Massif.

Shear-wave splitting within the Southeastern Carpathian Arc, Transylvanian Basin, Romania

We present new splitting measurements for SKS and SKKS phases recorded at four broadband seismic stations located within the Carpathian Arc, in the eastern part of the Transylvanian Basin. We made 75 measurements, of which 58 are nulls and 17 show clear splitting. We used two methods to estimate splitting parameters, ϕ (the fast polarization direction) and δt (the delay time between fast and slow split shear waves): minimization of energy on the horizontal component corresponding to the minimum eigenvalue of the polarization matrix, and simultaneous linearization of split waves from a suite of earthquakes recorded at a given station. We consistently observed no splitting at station IACB, in the northern part of the study area, for a range of well-recorded earthquakes at different backazimuths; we consider the station splitting to be null. Null splitting here is consistent with several possible tectonic scenarios, including downwelling of asthenospheric material as a result of the sinking Vrancea body, asthenospheric upwelling beneath Ciomadul volcano, or lack of anisotropic fabric development in the large Ciomadul magma chamber beneath the station. Fast polarized shear waves trend NW–SE at CHDM, in the Transylvanian Basin, and at PMAR, which lies at the boundary between the Transylvanian Basin and the E–W-striking Southern Carpathians; these fast directions are consistent with a regional trend in splitting ϕ azimuths over much of the Eastern Carpathians, the East European and Moesian Platforms, and the northwestern Transylvanian Basin away from the Vrancea Bend Zone. The regional NW–SE splitting trend parallels the NW-to-SE temporal migration of Neogene volcanism along the Eastern Carpathians, and is likely due to a combination of southeastward rollback of the Vrancea high velocity body and simultaneous southwestwards absolute plate motion of the thick leading edge of the East European Platform (Tornquist-Teisseyre Suture Zone), which together constrain regional mantle flow. Fast shear wave polarization, ϕ, at station COMD, at the juncture between the Vrancea Bend Zone and the Transylvanian Basin, trends NE–SW, consistent with splitting at stations VOIR and MLR in the Southern Carpathians. Splitting at these stations reflects perturbed mantle flow in the vicinity of the Vrancea body, which entails an abrupt change from NE–SW at VOIR to NNW–SSE at PMAR, consistent with redirection of mantle flow along the northwestern side of the Vrancea body. Delay times at the three stations with clear splitting vary from 0.96 to 2.73s, indicating either the presence of an anisotropic layer ranging between ∼100km thick beneath CHDM and 300km beneath PMAR, or an unusually strong anisotropic fabric beneath the latter.

Shear‐wave splitting: A diagnostic tool to monitor fluid pressure in geothermal fields

An experiment on the uses of shear‐wave splitting as an imaging tool in fracture‐controlled geothermal reservoirs was conducted at Krafla, Iceland. Fifteen days after the beginning of the seismic recording the injection was stopped for eleven days and then restarted, a sequence designed to determine whether shear‐wave splitting measurements can detect the transient response of the subsurface crack system to changes in fluid pressure. It was observed that time delays between the fast and slow split shear waves changed significantly and promptly with the stoppage and resumption of injection. Large time delays occurred only during injection, decreased substantially during the stoppage phase, and increased again as injection restarted. Comparisons of these results with similar observations at the Coso geothermal field in California strongly suggest that the time delay of split shear waves can be a useful proxy to monitor fluid pressure in the cracks and changes in crack density.

The upper mantle anisotropy in Yunnan area, China

Shear wave phase SKS of 11 earthquakes, collected from 23 stations of Yunnan Digital Seismic Network, is analyzed by fitting the theoretical transverse component with the observed one for determining the orientation and extent of polarization seismic anisotropy of upper mantle. Shear wave splitting is obviously observed in all stations except Heqing station (HQ). The results show that the polarization of fast split S-wave of upper mantle in Yunnan area is north-northeast in general and the time delay between fast and slow split shear waves is 0.5–2.0 s. It suggests that the influence of faults upon anisotropy analysis could not be neglected in such a geologically complex area. As the transitional zone between Qinghai-Tibetan plateau and the block of southern China, in Yunnan area the orientation of fast shear wave polarization indicating the subduction of Indian plate into Eurasian plate is the fundamental background of earth dynamics. While the southeast or south-southeast movement of Sichuan-Yunnan rhomb block, formed by the uplift of Qinghai-Tibetan plateau, plays an important role in the composition of complicated structural and stress environment of Yunnan area. The divergence between the fast direction and the movement of upper mantle indicates in Yunnan area there exists complex coupling effect between lower velocity layer or asthenosphere and crustal block. The distribution of structure driving force looks like a palm extending to northeast. According to the time delay between fast and slow shear waves, it is deduced that the thickness of anisotropy layer is 60–225 km with variation range roughly equal to that of 104–260 km of the buried depth of lower velocity layer of the earth in Yunnan area. So it suggests the top of anisotropy zone starts from the bottom of crust or from the lower velocity layer varying with specific locations related to the tremendous variation of the Moho discontinuity in Yunnan area. Furthermore, it is deduced that the anisotropy of the upper mantle is mainly from the lithosphere rather than the whole upper mantle.

Seismic anisotropy in the region of the Chile margin triple junction

We have measured shear wave splitting at three temporary three-component short period stations that were deployed in southern Chile above the subducted Chile Rise spreading centre (Taitao Peninsula and environs). Subduction of the Chile Rise has been occurring beneath South America for at least the past 14 m.y. Previously published models of the ridge subduction posit the existence of ‘slab windows’, asthenosphere-filled gaps between subducted lithosphere segments of the spreading ridge, through which mantle might flow. Our preliminary results include two consistent fast polarization directions of splitting in the study region. Delay times between fast and slow split shear waves average around 1.0 s for all phases (ScS, PcS, SKS, and SKKS) that we measured. Fast-axis azimuths vary systematically among the three stations: near the coast, fast axes are parallel to the spreading ridge segments of the Chile Rise (approximately N-trending). This splitting fast-axis direction probably reflects either along-axis asthenospheric flow or results from the preferential attenuation effects of aligned pockets of melt at the subducted ridge segment. At one inland station above the slab window, we find two splitting fast-axis directions, one parallel to the subducted Chile Rise ridge segments, and a second trending NW–SE. We infer that upper mantle deformation in the vicinity of a well developed slab window is complicated and probably involves two superposed directions of upper mantle deformation. One of these directions (NW–SE) may indicate anomalous flow of asthenospheric mantle in the vicinity of the slab window gap.

Fractured reservoir delineation using multicomponent seismic data

The characteristic seismic response to an aligned‐fracture system is shear‐wave splitting, where the polarizations, time‐delays and amplitudes of the split shear waves are related to the orientation and intensity of the fracture system. This offers the possibility of delineating fractured reservoirs and optimizing the development of the reservoirs using shear‐wave data. However, such applications require carefully controlled amplitude processing to recover properly and preserve the reflections from the target zone. Here, an approach to this problem is suggested and is illustrated with field data. The proposed amplitude processing sequence contains a combination of conventional and specific shear‐wave processing procedures. Assuming a four‐component recording (two orthogonal horizontal sources recorded by two orthogonal horizontal receivers), the split shear waves can be simulated by an effective eigensystem, and a linear‐transform technique (LTT) can be used to separate the recorded vector wavefield into two principal scalar wavefields representing the fast and slow split shear waves. Conventional scalar processing methods, designed for processing P‐waves, including noise reduction and stacking procedures may be adapted to process the separated scalar wavefields. An overburden operator is then derived from and applied to the post‐stacked scalar wavefields. A four‐component seismic survey with three horizontal wells drilled nearby was selected to illustrate the processing sequence. The field data show that vector wavefield decomposition and overburden correction are essential for recovering the reflection amplitude information in the target zone. The variations in oil production in the three horizontal wells can be correlated with the variations in shear‐wave time‐delays and amplitudes, and with the variations in the azimuth angle between the horizontal well and the shear‐wave polarization. Dim spots in amplitude variations can be correlated with local fracture swarms encountered by the horizontal wells. This reveals the potential of shear waves for fractured reservoir delineation.

FRACTURE CHARACTERIZATION FROM NEAR‐OFFSET VSP INVERSION

A global optimization method incorporating a ray‐tracing scheme is used to invert observations of shear‐wave splitting from two near‐offset VSPs recorded at the Conoco Borehole Test Facility, Kay County, Oklahoma. Inversion results suggest that the seismic anisotropy is due to a non‐vertical fracture system. This interpretation is constrained by the VSP acquisition geometry for which two sources are employed along near diametrically opposite azimuths about the well heads. A correlation is noted between the time‐delay variations between the fast and slow split shear waves and the sandstone formations.

Linear‐transform techniques for processing shear‐wave anisotropy in four‐component seismic data

Most published techniques for analyzing shear‐wave splitting tend to be computing intensive, and make assumptions, such as the orthogonality of the two split shear waves, which are not necessarily correct. We present a fast linear‐transform technique for analyzing shear‐wave splitting in four‐component (two sources/ two receivers) seismic data, which is flexible and widely applicable. We transform the four‐component data by simple linear transforms so that the complicated shear‐wave motion is linearized in a wide variety of circumstances. This allows various attributes to be measured, including the polarizations of faster split shear waves and the time delays between faster and slower split shear waves, as well as allowing the time series of the faster and slower split shear waves to be separated deterministically. In addition, with minimal assumptions, the geophone orientations can be estimated for zero‐offset verticle seismic profiles (VSPs), and the polarizations of the slower split shear waves can be measured for offset VSPs. The time series of the split shear‐waves can be separated before stack for reflection surveys. The technique has been successfully applied to a number of field VSPs and reflection data sets. Applications to a zero‐offset VSP, an offset VSP, and a reflection data set will be presented to illustrate the technique.

Processing of seismic data in the presence of anisotropy

Processing techniques for extracting the polarization directions of the fast, and slower split shear waves, together with their traveltime splitting from vector wavefield data, form the basis of prospective procedures for imaging details of the internal structure of subsurface rocks. A recently developed technique involving the independent rotation of the source polarization direction, and geophone axes, provides an effective processing tool for extracting the effects of anisotropy from shear‐wave data. It is theoretically capable of handling nonorthogonal polarization directions of split shear waves. It is further developed, and its performance examined using vertical seismic profiling (VSP) shear‐wave datasets for different anisotropic earth models, and computed using full‐wave modeling. The procedure works well for shear waves propagating through an earth model with a uniform crack strike with depth, but produces inaccurate estimates of polarization and time‐delay when the crack strike varies abruptly with depth. It is unlikely that other techniques could do better under these circumstances, as the problems arise due to the complexities introduced by multiple shear‐wave splitting.

Origin of crustal anisotropy: Shear Wave splitting studies in Japan

Shear wave splitting manifested as leading shear wave polarization, that is, parallel alignment of leading shear wave particle motions from a variety of sources, has been observed at a number of seismograph stations in Japan. Detected on shear wave seismograms from crustal earthquakes over a wide range of source zones and source‐receiver azimuths, the shear wave splitting can be attributed to crustal anisotropy. This paper discusses the relation between leading shear wave polarization directions and tectonic features of Japan. To explain the observed shear wave splitting, we propose that at least three phenomena should be taken into account: stress‐induced microcracks primarily aligned in vertical or subvertical planes; cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes; and intrinsic rock anisotropy resulting from preferred orientation of minerals. Travel time differences between leading and slower split shear waves from crustal and upper mantle earthquakes analyzed for about one third of the stations suggest that the crustal anisotropy which causes the observed shear wave splitting may be limited to the upper 15–25 km. This implies that the density of nonhorizontally aligned cracks or fractures below 15–25 km and into the upper mantle is much smaller than that in the crust above 15–25 km.

Shear wave polarization anisotropy observed in a rift zone in Japan

We have found polarization anisotropy in shear waves from small earthquakes both in the upper crust and in the uppermost mantle beneath a rift zone in Central Kyushu, Japan. We have recognized two different directions in the polarization of the faster shear waves: about E-W directions at three stations located in the rift zone, and about N-S at two stations possibly located outside and close to the edge of the rift zone. The former case (E-W) is consistent with a hypothesis that shear-wave splitting is caused by vertical cracks or fractures striking in the E-W direction which have formed under the present tectonic stress inside a rift zone extending in the N-S direction, with the maximum horizontal compressive stress striking in the direction about E-W and the minimum compressive stress vertical. In contrast, the latter case (N-S) is inconsistent with the above hypothesis. This inconsistency in the polarization direction may be due to a complicated pattern of crack distribution around the stations outside the rift zone. Alternatively, it may result from a complicated stress regime within the crust close to the edge of the rift zone. Arrival-time differences between the faster and slower split shear waves from the upper crustal earthquakes are less than about 0.12 s, while those from the upper mantle events are less than about 0.25 s. The relatively small arrival-time differences from the upper mantle events with respect to their hypocentral distances could result from a small crack density in the lower crust and upper mantle compared with that in the upper crust.

Shear-wave splitting observed above small earthquakes in a geothermal area of Japan

The phenomenon of shear-wave splitting has been observed at two seismic stations with three-component seismographs above shallow microearthquakes which occurred at depths of 1–2 km in the Takinoue geothermal area of northern Honshu. An almost uniform alignment of the polarizations of the faster-split shear wave in the direction of ENE-WSW may result from the presence of non-randomly distributed liquid-filled microcracks. Two models of crack distribution reproduce the observed polarization pattern. In the first model vertical microcracks are aligned in the ENE-WSW direction, while the second supposes a distribution of microcracks striking in the ENE-WSW direction but dipping randomly. The ENE-WSW strike of the cracks in both models is consistent with the direction of the ENE-WSW maximum horizontal compressive stress inferred from the analysis of earthquake mechanisms in the study area. It differs slightly from the strike of vertical fractures (WNW-ESE) obtained by hydraulic fracturing experiments at sites about 10 km west of the study area. The tectonic stress regime obtained from an analysis of focal mechanisms of microearthquakes occurring in this area seems to be consistent with the latter model of the crack distribution from the shear-wave analysis, at least at depths less than about 700 m. An analysis of the delay times between the faster- and the slower-split shear waves has revealed a region with anomalously large delay times normalized to ray path lengths. A 3-D P-wave velocity inversion indicates that this region is a low P-wave velocity zone. The low P-wave velocities and high shear-wave birefringence is consistent with a relatively dense distribution of liquid-filled microcracks.

ScS polarization anisotropy around the Pacific Ocean.

ScS polarization anisotropy is investigated using WWSSN short-period seismograms of intermediate-depth and deep earthquakes recorded at 24 stations around the Pacific Ocean. Forty-one seismograms are selected from a period between 1966 and 1978. The data used in this study are restricted to epicentral distances less than 30° to avoid phase distortion at the core-mantle boundary. Shear-wave splitting is observed indicating some form of anisotropy in the mantle. In order to find the polarizations of the faster and slower split shear waves, each of the horizontal component seismograms is rotated. The polarizations of the split shear-waves are approximately NNE-SSE under Australia and North America, E-W under South America, and NWW-SEE under Japan. The polarization of the faster wave is generally found to be parallel to plate motion near the receiver points. The arrival time difference is 1.0±0.4s, corresponding to a velocity difference of 4 % when the vertical extent of an anisotropic zone is assumed to be 100 km in the upper mantle.