Polarization Anomalies Research Articles

Core-mantle boundary structure investigated usingSKSandSKKSpolarization anomalies

Occasionally, SKS and SKKS waveforms in the same seismogram are affected differently by anisotropy. One source of this discrepancy may be structures in D″. In this study, we examine the discrepancy in order to determine where it arises in the propagation path and what it is due to. We find that D″ is the most likely source of the signal. The relatively minor differences in differential shear-wave splitting in SKS and SKKS limit large-scale azimuthal anisotropy in D″ to less than 2 per cent, though it may be locally stronger. The most effective way to develop the splitting differences is through polarization differences between SKS and SKKS imposed at the CMB. We examine how effective relief on and lateral gradients in CMB structure are at generating polarization anomalies and conclude that topography generates them best. Ramps and ridges are more effective generators of polarization anomalies than hills. Laterally extensive (500–1000 km) sloping CMB topography greater than 15° can develop the observed polarization anomalies in the data. The topography required exceeds constraints from other sources, so is unlikely to be the major factor explaining the anomalies. We produce a global map of SKS/SKKS exit points from the core where anomalous polarization behaviour is found, in relation to the velocity structure known to exist in D″: the anomalies appear to be restricted to faster than average areas of the CMB, suggesting a contribution from anisotropy related to the post-perovskite phase transition.

Low‐frequency electrical response to microbial induced sulfide precipitation

We investigated the sensitivity of low‐frequency electrical measurements to microbe‐induced metal sulfide precipitation. Three identical sand‐packed monitoring columns were used; a geochemical column, an electrical column and a control column. In the first experiment, continuous upward flow of nutrients and metals in solution was established in each column. Cells of Desulfovibrio vulgaris (D. vulgaris) were injected into the center of the geochemical and electrical columns. Geochemical sampling and post‐experiment destructive analysis showed that microbial induced sulfate reduction led to metal precipitation on bacteria cells, forming motile biominerals. Precipitation initially occurred in the injection zone, followed by chemotactic migration of D. vulgaris and ultimate accumulation around the nutrient source at the column base. Results from this experiment conducted with metals show (1) polarization anomalies, up to 14 mrad, develop at the bacteria injection and final accumulation areas, (2) the onset of polarization increase occurs concurrently with the onset of lactate consumption, (3) polarization profiles are similar to calculated profiles of the rate of lactate consumption, and (4) temporal changes in polarization and conduction correlate with a geometrical rearrangement of metal‐coated bacterial cells. In a second experiment, the same biogeochemical conditions were established except that no metals were added to the flow solution. Polarization anomalies were absent when the experiment was replicated without metals in solution. We therefore attribute the polarization increase observed in the first experiment to a metal‐fluid interfacial mechanism that develops as metal sulfides precipitate onto microbial cells and form biominerals. Temporal changes in polarization and conductivity reflect changes in (1) the amount of metal‐fluid interfacial area, and (2) the amount of electronic conduction resulting from microbial growth, chemotactic movement and final coagulation. This polarization is correlated with the rate of microbial activity inferred from the lactate concentration gradient, probably via a common total metal surface area effect.

Magnetostriction and electric polarization anomalies in GdFe 3(BO 3) 4 single crystals at phase transitions

Our experimental and theoretical investigation was aimed at the study of different gadolinium ferroborate single-crystal properties both at spontaneous and high magnetic field-induced phase transitions.

Pyroelectric properties of 0.7PbMg1∕3(Nb0.9Ta0.1)2∕3O3–0.3PbTiO3 ceramics

0.7 Pb Mg 1 ∕ 3 ( Nb 0.9 Ta 0.1 ) 2 ∕ 3 O 3 - 0.3 Pb Ti O 3 (PMNT–PT) ceramic was prepared with solid-state reaction method. The sample was characterized with x-ray diffraction and dielectric constant measurement, which ensured the high quality of the sample. The focus of this study is the pyroelectric properties of PMNT–PT ceramics. We measured the hysteresis loops at different temperatures and frequencies. The spontaneous and remnant polarizations as a function of temperature were obtained. An anomaly of remnant polarization as a function of temperature was found and discussed. Particularly, we measured the pyroelectric current with a Keithley 6485 Picoammeter. The maximum of the current is as high as 108nA∕cm2.

Diffraction, Interference, and Depolarization of Elastic Waves. Caustic and Penumbra

A theoretical study of polarization-spectral anomalies of wave fields, or their deviations from the predictions based on simple plane-wave models, is presented. A simple unified method of numerical simulation of anomalies of nonstationary wave fields near a caustic and in the penumbra is described. It uses both the leading and correcting terms in asymptotic expansions. Examples of calculation of displacements and average polarization ellipses are given. Qualitative properties of wave fields are discussed. A review of earlier research on polarization anomalies of elastic waves is given. Bibliography: 38 titles.

The geological setting of volcanogenic sulphide orebodies in Albanian ophiolites

Cu–Fe sulphides with minor Zn, Au and Ag occur as massive, disseminated and vein mineralizations in volcanosedimentary rocks, volcanics and occasionally associated intrusions of a sheeted dyke complex in ophiolites in Albania. The ophiolites form two sub-parallel units, the Western Ophiolite and Eastern Ophiolite Belts, stretching across the country from north-west to south-east, and have mid-ocean ridge and island arc (boninite supra-subduction type) affinities, respectively. The ores have been extensively prospected and were mined until the 1990s.The Western Ophiolite Belt comprises a volcanosedimentary series, pillow basalts, ultramafic rocks, gabbros and plagiogranites. The most significant orebodies are in volcanosedimentary rocks at Rubik, Palaj-Karme and Porave. The orebodies with massive chalcopyrite–pyrite ores vary in shape occurring between hematised radiolarian chert and tholeiitic basalt pillow lavas. Mapping the shape of the main orebody at Rubik shows a series of mounds comparable to the collapse of 'black smoker' chimneys. Massive and disseminated sulphides are hosted by MORB affinity basalts at Kachinar, Derven and Rehova. A geophysical induced polarisation survey has indicated further resources at depth at West Kachinar.The Eastern Ophiolite Belt consists of a basalt–andesite–rhyolite series cut by a sheeted dyke complex and underlain by quartz diorite–plagiogranite, gabbro, gabbronorite and harzburgite. It includes a 25-km long belt of a significant longitudinal continuity of mineralization between Chafe Mal and Reps. Massive sulphide ores underlain by disseminated veins and stockworks are hosted by a Middle–Upper Jurassic lower basalt–andesite succession at Reps, Spach, Maja e Made, Tuch, Lak Rosch, Paluce and Fushe Arres. Alteration includes chloritization, epidotization, silicification, argillisation, zeolitization and carbonatisation. The Upper Jurassic, upper dacite volcanoclastic, andesite–dacite, boninite succession hosts massive pyrite–chalcopyrite–sphalerite ore underlain by few or no disseminated veins or stockworks at Gurth Spach, Munelle and Chafe Bar. At Perlat, massive ores are in an andesite pillow lava. Induced polarisation anomalies indicate further orebodies at depth in this highly thrusted area. Sulphide veins and other quartz–lode sulphides occur in some gabbros. An unusual orebody at Gjegjan occurs in a tectonic mélange of volcanics, greenschists and amphibolites.Similarities between Albania and other ophiolitehosted volcanogenic deposits at Cyprus and Oman include a clustering of orebodies along the general strike of the ophiolite, the occurrence of massive ores at successive horizons, and the presences of vein, stockwork and disseminated mineralization surrounding and beneath the massive sulphide zones; but, in Albania, ochres and umbers are not found, the age is older and the age-span greater. The Albanian orebodies range in size from 0·3–10·0 Mt. Those in the Western Ophiolite Belt are comparable in size to modern seafloor deposits at ocean ridges. Eastern Ophiolite Belt orebodies are similar in size to others interpreted as having formed in a suprasubduction environment. The Eastern Ophiolite Belt offers the greatest potential for locating future orebodies, possibly beneath thrust planes.

Applications of large-power induced sounding in surveying deep Jiaojia-pattern gold mines

Jiaojia-pattern gold mines could be surveyed by utilizing large-power induced polarization sounding. Firstly, a multi-layered numerical model of a polarization body was established in sense of physical and mathematical derivation. The polarization field generated from the model was the stack of individual fields generated from multi-layered polarization units. The equation for eliminating strong shallow polarization anomalies from the sounding data was derived based on the principle of multiply eliminating the fields of individual polarization units. As a result, the deep polarization anomalies were prominent. The processed sounding data was used to quantitatively calculate the depths to the tops of deep altered zones of golden mineralization.

Comment on ‘The azimuthal dependence of surface wave polarization in a slightly anisotropic medium’ by T. Tanimoto

This comment points out that the simple azimuthal dependence of the polarization anomalies derived in a recent paper by Tanimoto may not be valid for the Love wave fundamental mode in oceanic or tectonic regions as a result of strong coupling with the first Rayleigh wave overtone. More generally, I also point out the fact that polarization anomalies reflect partly the anisotropy away from the station and that this usually complicates the azimuthal pattern observed at single stations.

Shear wave splitting and waveform complexity for lowermost mantle structures with low‐velocity lamellae and transverse isotropy

Shear waves that traverse the lowermost mantle exhibit polarization anomalies and waveform complexities that indicate the presence of complex velocity structure above the core‐mantle boundary. Synthetic seismograms for horizontally and vertically polarized shear waves (SH and SV, respectively) are computed using the reflectivity method for structures with low‐velocity sheets (“lamellae”), and for comb‐like models approximating long wavelength vertical transverse isotropy (VTI). Motivated by evidence for partial melt in the deep mantle, lamella parameter ranges include (1) δVP from −5 to −10%, δVS = 3δVP; (2) 100 to 300 km thickness of vertical stacks of lamella; (3) lamella spacing and thickness varying from 0.5 to 20 km; and (4) lamellae concentrated near the top, bottom, or throughout the D″ region at the base of the mantle. Such lamellae represent, in effect, horizontally emplaced dikes within D″. Excessively complex waveforms are produced when more than ∼20% of D″ volume is comprised of low‐velocity lamellae. Many lamellae models can match observed Sdiff splitting (1–10 s delays of SVdiff), but typically underpredict ScS splitting (1–4 s delays of ScSV). VTI model parameters are selected to address D″ observations, and include (1) 0.5 to 3% anisotropy; (2) discontinuous D″ shear velocity increases up to 3%; (3) D″ thicknesses from 100 to 300 km; and (4) VTI concentrated at the top, bottom, or throughout D″. VTI models readily match observed splits of ScS and Sdiff. We discuss lamellae and VTI model attributes in relationship to waveform complexities, splitting magnitude, triplications from a high‐velocity D″ discontinuity, and apparently reversed polarity SVdiff onsets. The possible presence of melt‐filled lamellae indicates that local chemical or thermal perturbations can produce regions that exceed the solidus within D″. Such melt could occur in the bulk of D″ because the melt is either close to neutral buoyancy, advective velocities exceed percolative velocities, or both.

A synthesis of seismicPandSanisotropy

Upper-mantle seismic anisotropy has been observed using a variety of methods, including S and SKS splitting, P and Pn traveltimes, P polarization anomalies and P to S conversions, and surface waves. Care must be taken when comparing the results from different methods because of bias introduced by depth sensitivity, frequency dependence, and simplifying assumptions concerning the form of anisotropy. We examine the differences and show that some apparent contradictions cited in previous studies can be reconciled using simple models. We perform forward modelling on a suite of anisotropic media, progressing from simple elastic symmetries to tensors obtained from laboratory measurements and numerical strain models. The results provide a systematic overview of the effect of a given anisotropy class and geometry on seismic observables. We simulate the full complement of body wave measurements—SKS and S splitting, Pn traveltimes, teleseismic P traveltimes and teleseismic P particle motion (Ppol)—to show any apparent differences between the phases. We also investigate depth and frequency sensitivity using reflectivity modelling in layered anisotropic media. Our principal findings are as follows. (1) No models, including low-order symmetries and multiple layers, exhibit a mean fast shear wave splitting direction nearly orthogonal to a consistent fast direction determined from P observables. For P delays, the azimuthal cos(2θ) variation is representative of the fast direction of anisotropy (rather than cos(1θ), which has led to a certain amount of confusion in the literature). (2) P times average linearly over the raypath; SKS weights toward the upper part of the model; and Ppol and Pn are even more sensitive to shallow anisotropy. Conclusive evidence in the literature for a disagreement between fast directions from SKS, on one hand, and Pn and Ppol, on the other hand, can be explained by layering. (3) The azimuthal dependence of SKS splitting results does not necessarily indicate layered or laterally heterogeneous anisotropy. The azimuthal dependence of SKS splitting is not observed for hexagonal symmetry with horizontal fast or slow axes, but has to be taken into consideration for dipping hexagonal and any orthorhombic and lower symmetry media. Teleseismic S shows a much stronger azimuthal dependence than SKS and SKKS. This makes procedures that stack splitting results over a wide range of incidence angles or azimuths questionable.

AVO polarization and hodograms: AVO strength and polarization product

An alternative approach to identifying amplitude‐variation‐with offset (AVO) anomalies is to consider the AVO polarization in the AVO intercept–AVO gradient (A‐B) plane. This method does not require deviations or separations from a background trend exhibited in traditional crossplots such as intercept‐gradient (A‐B) or near trace–far trace (N‐F). A benefit of the hodogram or polarization method is that the wavelet is taken into consideration. Crossplotted intercept and gradient are polarized along a “background trend” for nonanomalous events and at angles different from the “background trend” for anomalous events. This allows recognition of anomalous behavior otherwise buried in a background.Attributes resulting from this methodology include (1) the polarization angle, (2) the polarization angle difference, (3) the AVO strength, (4) the polarization product, and (5) the linear‐correlation coefficient. These different attributes can then be used to enhance AVO interpretation. Synthetic modeling for a succession of gas and brine layers encased in shale units indicates that the method can potentially be an effective hydrocarbon indicator. Application of the method to a real seismic dataset shows polarization anomalies associated with hydrocarbons.

PKP travel times at near antipodal distances: implications for inner core anisotropy and lowermost mantle structure

Previous studies from PKP(AB–DF) differential travel times at large distances suggest that the central part of the inner core is very anisotropic. These differential times, however, can be affected greatly by strong heterogeneity in the lowermost mantle. Here we examine a unique data set of PKP travel times from global digital and analog stations at near antipodal distances, where the effects of both inner core anisotropy and mantle heterogeneity are the greatest. Our results show that the AB–DF residuals for the polar paths are consistently larger than those of the equatorial paths by over 3–4 standard deviations. We also measured DF and AB absolute times, and found that the DF residuals are negatively correlated with the AB–DF residuals while the AB residuals have a much weaker correlation with the AB–DF residuals. We compare several mantle models with the data. Our results suggest that the mantle structure can explain part of the residuals of the equatorials paths, but cannot explain the polar path anomalies. These results strongly suggest that most of the AB–DF anomalies for the polar paths are likely from the inner core anisotropy and not from mantle heterogeneity. Assuming a uniform cylindrical anisotropy model, the average inner core anisotropy amplitude is about 2.5%. On the other hand, equatorial paths from events in the west Pacific and recorded at South America show a steep azimuthal change in AB–DF times (about 4 s over a 60° azimuthal range). The sharp change is well predicted by existing P and S tomographic models: the azimuthal change occurs as the AB paths sweep across the great slow anomaly in the central Pacific. The high correlation between the P and S velocities suggests that the central Pacific slow anomaly may be of thermal origin.

Lithospheric anisotropy on the Kerguelen hotspot track inferred from Rayleigh wave polarisation anomalies

Rayleigh waves recorded at the Geoscope station PAF on the Kerguelen Isles in the Indian Ocean, show strong polarisation anomalies in the period range 20–50 s, as demonstrated by dispersion analysis of 3-component recordings. The largest and most consistent anomalies are observed for events located in the southern part of the Java Trench. At 25 s the Rayleigh waves present transverse components with an amplitude of up to 55 per cent of the amplitude of the longitudinal components. The particle motion in the horizontal plane is largely elliptical. By comparison, very few and mostly small polarisation anomalies are detected at the nearby Geoscope stations AIS and CRZF on the Amsterdam and Crozet Isles, respectively. Wave path deviations from the epicentre–receiver great circle, as calculated in tomographic models of the Indian Ocean, cannot explain the polarisation anomalies. Using a multiple-scattering scheme for modelling surface waves in 3-D heterogeneous and anisotropic structures, we show that wavefield distortion due to the geometrical structure of the Kerguelen Plateau in the vicinity of the station cannot explain the anomalies either, but that anisotropy can. We infer the presence of an anisotropic structure in the lithosphere to the north of the Kerguelen Isles, containing 40 per cent oriented pyrolite, with fast axis tilting downwards in a north-north-east direction. The anisotropy may be caused by deformation of the lithosphere related to the Kerguelen hotspot.

Polarization anomalies of Love waves observed in and around Japan

Polarization anomalies of Love waves, sometimes called quasi-Love waves, are likely to be caused by lateral variations of azimuthal anisotropy in the upper mantle. Polarization anomalies of Love waves from the 9 October 1995 earthquake near the coast of Jalisco, Mexico, have been recorded by the Pacific21, IRIS, and Japan Meteorological Agency networks of broadband seismometers in and around Japan. The Love-to-Rayleigh conversion areas for the clearly detected quasi-Love waves are located by using the group velocities of the observed surface waves. In the cases of the stations in the northern part of Japan and Philippine, the locations of the Love-to-Rayleigh conversions mostly concentrate near the trenches, and suggest that the properties of the azimuthal anisotropies in the upper mantle have lateral variations across the trenches. We infer that the lateral variations of the azimuthal anisotropies may reflect the changes of the mantle flow due to the subducting slabs. The Love-to-Rayleigh conversion areas for the other stations mainly concentrate near the Emperor seamounts and the Mid-Pacific mountains. Several results of surface wave tomographic studies show that the azimuthal anisotropies in these regions are much weaker than those in the central part of the Pacific along the paths. The lateral variation in azimuthal anisotropy may cause the Love-to-Rayleigh conversions.

Southern Ocean Climate and Sea Ice Anomalies Associated with the Southern Oscillation

The anomalies in the climate and sea ice cover of the Southern Ocean and their relationships with the Southern Oscillation (SO) are investigated using a 17-year of data set from 1982 through 1998. We correlate the polar climate anomalies with the Southern Oscillation index (SOI) and examine the composites of these anomalies under the positive (SOI > 0), neutral (0 > SOI > -1), and negative (SOI < -1) phases of SOL The climate data set consists of sea-level pressure, wind, surface air temperature, and sea surface temperature fields, while the sea ice data set describes its extent, concentration, motion, and surface temperature. The analysis depicts, for the first time, the spatial variability in the relationship of the above variables and the SOL The strongest correlation between the SOI and the polar climate anomalies are found in the Bellingshausen, Amundsen and Ross sea sectors. The composite fields reveal anomalies that are organized in distinct large-scale spatial patterns with opposing polarities at the two extremes of SOI, and suggest oscillating climate anomalies that are closely linked to the SO. Within these sectors, positive (negative) phases of the SOI are generally associated with lower (higher) sea-level pressure, cooler (warmer) surface air temperature, and cooler (warmer) sea surface temperature in these sectors. Associations between these climate anomalies and the behavior of the Antarctic sea ice cover are clearly evident. Recent anomalies in the sea ice cover that are apparently associated with the SOI include: the record decrease in the sea ice extent in the Bellingshausen Sea from mid- 1988 through early 199 1; the relationship between Ross Sea SST and ENSO signal, and reduced sea ice concentration in the Ross Sea; and, the shortening of the ice season in the eastern Ross Sea, Amundsen Sea, far western Weddell Sea, and the lengthening of the ice season in the western Ross Sea, Bellingshausen Sea and central Weddell Sea gyre over the period 1988-1994. Four ENSO episodes over the last 17 years contributed to a negative mean in the SOI (-0.5). In each of these episodes, significant retreats in the Bellingshausen/Amundsen Sea were observed providing direct confirmation of the impact of SO on the Antarctic sea ice cover.

Magnetoelectric Effect in YMn 2 O 5 in Strong Pulsed Magnetic Fields

The electric polarization of single crystal YMn 2 O 5 was investigated in strong pulsed magnetic fields up to H = 250 kOe at low temperatures. It was shown that at T ∼ 40 K magnetic and structural phase transitions occur, with the structural changes taking place improperly (magnetically induced). We have also shown the existence of the structural phase transition at T ∼ 20 K with the onset of spontaneous polarization P b . The quadratic magnetoelectric effect at temperatures T < 40 K was observed (with b-component domination), the magnetoelectric susceptibility below T = 20 K greatly increased for all field orientations. When H Á a the magnetoelectric effect changed its sign which in analogue with EuMn 2 O 5 was attributed to non-commensurate to commensurate structural change. The polarization anomalies in strong magnetic fields were observed with the former being due to complex magnetic structure changes and the process of spontaneous polarization induction by magnetic field.

Body waves in a weakly anisotropic medium-II.Swaves from a centre of expansion andPwaves from a centre of rotation

A perturbation theory is presented, providing a simple description of the S wave from a centre of expansion and the P wave from a centre of rotation. The relationships between the directivities of these waves and polarization anomalies of plane waves are established. The effect of anisotropy on the generation of these waves is compared with that of a smooth inhomogeneity.

Mise‐a‐la‐masse survey for an auriferous sulfide deposit

A mise‐a‐la‐masse survey was carried out in Bhukia area, Banswara district, Rajasthan, India for auriferous sulfide occurrences. This area was originally surveyed for copper mineralization. Exploratory drilling, however, proved it to be economically not viable. The area was reopened for geophysical surveys when grab samples indicated the presence of gold. Initial geophysical surveys for copper mineralization showed electromagnetic, induced polarization, and resistivity anomalies. At first, one borehole was drilled for gold exploration on the basis of initial geophysical surveys. It encountered massive sulfide mineralization in association with gold. Borehole logging and a mise‐a‐la‐masse survey were carried out in this borehole. Three further boreholes drilled on the basis of the mise‐a‐la‐masse results encountered massive sulfide mineralization in association with gold. One of the three boreholes, 100 m from the first borehole along strike, was used for another set of mise‐a‐la‐masse measurements. A composite equipotential map was prepared using the results of mise‐a‐la‐masse results of both the boreholes. The equipotential contours show a north‐northwest‐south‐southeast trend of mineralization. The boreholes drilled on the basis of the mise‐a‐la‐masse results have delineated a strike length of more than 500 m of gold‐bearing sulfide mineralization. The sulfide content ranges from 10 to 40% and gold concentration ranges from 2 to 6 ppm. The dip and plunge of the lode, as anticipated from the mise‐a‐la‐masse results, are toward the west and north, respectively. Mise‐a‐la‐masse surveys are continuing in the adjoining areas.

Polarization and depolarization of relaxor ferroelectric strontium barium niobate

The anomalies of polarization (mismatch of hysteresis loops for several first repolarization cycles, the absence of specific coercive field, etc.) in quasi-static and dc electric fields have been found in the strontium barium niobate relaxor ferroelectric. The anomalies are associated with a highly inhomogeneous structure of the crystal, which is considered a clearly defined nonergodic system with a random distribution of strong local internal fields. The energy distributions of potential barriers for the polarization and the depolarization are obtained at different electric field strengths and temperatures.

Polarization anomalies in a relaxor ferroelectric

With a strontium barium niobate crystal used as an example, it is experimentally demonstrated that relaxor ferroelectrics exhibit pronounced polarization anomalies, which manifest themselves in the difference between the trajectories of the dielectric hysteresis observed in several repeated cycles of a varying external electric field. These anomalies originate from the nonuniformity of the relaxor composition, local symmetry lowering, local internal electric field, and a wide distribution of potential barriers in energy for the polarized regions. The anomalies can be observed only in slowly varying (quasistatic) or constant electric fields.