regional observation of a nuclear test from a vertical hydrophone array abstract in order to evaluate the potential of water column vertical hydrophone arrays for the measurement of continental events at regional distances ocean acoustic data bases were examined for arrivals from these types of events in 1987 the nuclear test lockney m b 5 7 37 28 n 116 38 w was observed in the deep ocean water column at a range of 900 km by the vertical line array vla at 35 n 126 w although the hydrophone data were contaminated below 10 hz by array suspension noise and ocean acoustic noise adaptive array processing allows us to extract the p and t phases the root mean square rms direct p wave amplitude in the water column is 100 6 db referenced to 1 pa in the 4 to 8 hz band in a 20 sec window which suggests a detection threshold of m b 5 2 in a deep oceanic environment at this range for a single hydrophone the arrival time of the t phase i e the portion of seismic energy that has somehow coupled into and propagated within the deep ocean sound channel aki and richards 1980 is consistent with a p wave conversion at the continental margin assuming a velocity of 6 km sec from nts to the continental margin and acoustic propagation at 1 5 km sec in the water column from the continent to the array

hydroacoustic events located at the intersection of the atlantis 30 n and kane 23 40 n transform faults with the mid atlantic ridge we investigate the characteristics of t phase events located at the ends of two segments of the mid atlantic ridge our motivations for the study were to understand whether t phase locations represent earthquake epicenters and thus whether accurate geological inferences can be made from their spatial patterns and to understand further the relationship between t phase event characteristics and earthquake properties we examine the characteristics of 158 t phase events with respect to both event location water depth and source to receiver distance the propagation paths of the t phases are also modeled to study the effects of encountering seafloor topography we find that existing models for t phase excitation and propagation cannot explain adequately all of our observations the amplitudes received levels of t phases at the hydrophones show no dependence on event water depths in contrast to current excitation models which predict a decrease in event magnitude with increasing water depth the received levels are observed to decrease with increasing source to receiver distance and events from the two study areas exhibit different trends in relative received levels between hydrophones once attenuation is taken into account our acoustic ray trace model is able to reproduce similar trends in relative amplitudes at the hydrophones based on 1 d topography between the event and each hydrophone but the variances in both the observed data and model are high we observe a pattern of short t phase onset times for shallow water events and long onset times for deep water events where onset time is defined as the time interval between the appearance of the t phase envelope above the ambient noise and its first peak this suggests that the onset time may be a function of several variables including efficiency of energy conversion based on local topography efficiency of propagation based on event water depth and hypocentral depth in the crust the results of this study underscore the complexity of t phase excitation and propagation and argue that current models of t phase excitation and propagation need to be improved to explain the observed characteristics of t phase data

t phase spectra seismic moments and tsunamigenesis t phase spectrograms and spectra derived from deep ocean hydrophones for tsunamigenic and non tsunamigenic earthquakes in the pacific reveal the following 1 substantial amounts of t phase energy at high frequencies i e 1 hz to at least 35 hz 2 strong correlations of spectral strengths in the 10 hz to 35 hz frequency range to seismic moments and 3 large spectral strengths at high frequencies for tsunamigenic earthquakes these findings are consistent with the low attenuation of the so und fixing and ranging sofar channel which permits efficient propagation of high frequencies to great distances and with the perception that large earthquakes may generate high frequency signals at the water sediment interface which can best be observed by hydroacoustic sensors in the low noise environment of the deep oceans in addition these studies confirm that 1 source bathymetry can strongly influence the character of t phases 2 islands or seamounts near the source area can act as secondary radiators of energy which add to the complexity and duration of the coda 3 there appears to be no significant dependence of t phase strength on earthquake source mechanism and 4 the strength of the t phase is dependent on the purity of its oceanic travel path the strong correlation of t phase spectral strengths to seismic moments suggests that substantial improvements in the reliability of tsunami warnings may be possible by using t phases as an additional independent single station method for estimating seismic moments finally the high frequencies observed in the t phase may be associated with important alternate mechanisms for tsunamigenesis other than faulting i e slumpage compaction or liquefaction

narrow angle representations of the phase and group velocities and their applications in anisotropic velocity model building for microseismic monitoring it is usually believed that angular aperture of seismic data should be at least 20 to allow estimation of the subsurface anisotropy although this is certainly true for reflection data for which anisotropy parameters are inverted from the stacking velocities or the nonhyperbolic moveout traveltimes of direct p and s waves recorded in typical downhole microseismic geometries make it possible to infer seismic anisotropy in angular apertures as narrow as about 10 to ensure the uniqueness of such an inversion it has to be performed in a local coordinate frame tailored to a particular data set because any narrow fan of vectors is naturally characterized by its average direction we choose the axes of the local frame to coincide with the polarization vectors of three plane waves corresponding to such a direction this choice results in a significant simplification of the conventional equations for the phase and group velocities in anisotropic media and makes it possible to predict which elements of the elastic stiffness tensor are constrained by the available data we illustrate our approach on traveltime synthetics and then apply it to perforation shot data recorded in a shale gas field our case study indicates that isotropic velocity models are inadequate and accounting for seismic anisotropy is a prerequisite for building a physically sound model that explains the recorded traveltimes

synthesis of earthquake generated t waves t phases excited by earthquakes propagate with low transmission loss along the ocean sound channel and are characterized by long duration wavetrains often having multiple peaks associated with bathymetric highs in the source region although t waves were first identified almost fifty years ago their activation has not been adequately explained in this paper we show that for continental margin earthquakes realistic t phase coda can be computed by expressing the acoustic energy in terms of several low order modes excited by point sources distributed uniformly over the seafloor the conversion of seismic energy to acoustic energy is thus consistent with seafloor scattering

tilted ti models in surface microseismic monitoring abstracta recently published analytic technique for computing locations of microseismic events jointly with velocities of homogeneous isotropic media was extended to surface microseismic monitoring and transversely isotropic models with tilted symmetry axes tti the analysis of traveltimes of the direct p sv and sh waves conducted under the assumptions of homogeneity and weak anisotropy indicated that the sv wave data acquired in modern wide azimuth surface microseismic surveys yield uniquely solvable joint inverse problems for an arbitrary symmetry axis tilt whereas the tilts should be close to 90 from the vertical for the p waves propagating in anelliptically anisotropic media and strictly equal to 90 for the sh waves to maintain the uniqueness of the joint inversion these theoretical findings confirmed on ray tracing synthetic were applied to a field microseismic data set the p waves excited by microseismic events were found to exhibit significantly flatter moveouts and better focused st

comment on the directionality of acoustic t phase signals from small magnitude submarine earthquakes j acoust soc am 119 3669 3675 2006 in a recent paper chapman and marrett j acoust soc am 119 3669 3675 2006 examined the tertiary t waves associated with three subduction related earthquakes within the south fiji basin in that paper it is argued that acoustic energy is radiated into the sound channel by downslope propagation along abyssal seamounts and ridges that lie distant to the epicenter a reexamination of the travel time constraints indicates that this interpretation is not well supported rather the propagation model that is described would require the high amplitude t wave components to be sourced well to the east of the region identified along a relatively flat lying seafloor

3d waveform inversion of downhole microseismic data for transversely isotropic media abstract 3d anisotropic waveform inversion could provide high resolution velocity models and improved event locations for microseismic surveys here we extend our previously developed 2d inversion methodology for microseismic borehole data to 3d transversely isotropic media with a vertical symmetry axis this extension allows us to invert multicomponent data recorded in multiple boreholes and properly account for vertical and lateral heterogeneity synthetic examples illustrate the performance of the algorithm for layer cake and hydraulically fractured i e containing anomalies that simulate hydraulic fractures models in both cases waveform inversion is able to reconstruct the areas which are sufficiently illuminated for the employed source receiver geometry in addition we evaluate the sensitivity of the algorithm to errors in the source locations and to band limited noise in the input displacements we also present initial inversion results for a microseismic data set acquired during hydraulic fracturing in a shale reservoir

inversion of p wave vsp data for local anisotropy theory and case study we discuss improve and apply the slowness polarization method for estimating local anisotropy from vsp data although the idea of fitting a given anisotropic model to the apparent slownesses measured along a well and polarization vectors recorded by three component downhole geophones is hardly new we extend the area of applicability of the technique and make the anisotropic inversion more robust by eliminating the most operationally difficult and noisy portion of the data the shear waves we show that the shear wave velocity is actually unnecessary for fitting the slowness of polarization dependence of p wave vsp data for the most common geometry of a vertical borehole in a vertically transversely isotropic subsurface such data are governed by the p wave vertical velocity vp0 and two quantities vsp and vsp that describe the influence of anisotropy these quantities depend on conventional anisotropic coefficients and and absorb the s wave velocity we apply the developed theory to a 2d walka

relative location of microseismicity abstractapplying the two point paraxial ray tracing we develop a technique for relative location of microseismic events our technique assumes the availability of a perforation shot or an already located microseismic event termed the master for which the paraxial ray tracing has been performed the ray tracing output for the master makes it possible to compute the relative locations of adjacent microseismic events as many as a data set contains with an efficient algorithm that requires no additional ray tracing and reduces to solving a series of simple low dimensional and well behaved optimization problems the relative event location approach discussed in our paper is especially well suited for surface microseismic monitoring because the high accuracy of the paraxial ray approximation in the directions orthogonal to the reference rays typically spanning the stimulated horizons for surface microseismic geometries ensures the calculation of precise event hypocenters at appreciable distances from th

p and t wave detection thresholds pn velocity estimate and detection of lower mantle and core p waves on ocean sound channel hydrophones at the mid atlantic ridge since 1999 six sound fixing and ranging sofar hydrophones have been moored along the mid atlantic ridge mar 15 35 n these hydrophones 8 bit data resolution are designed for long term monitoring of mar seismicity using the acoustic t waves of seafloor earthquakes the completeness level of the mar t wave earthquake catalog estimated from size frequency constraints is mb 3 0 a significant improvement in detection compared to the mb 4 6 completeness level estimated from national earthquake information center magnitude frequency data the hydrophones also detect the acoustic phase of converted upper mantle p arrivals from regional earthquakes at epicentral distances of 374 1771 km and from events as small as mb 3 6 these regional p waves are used to estimate a pn velocity of 8 0 0 1 km sec 1 along the east and west mar flanks an unexpected result was the identification of p arrivals from earthquakes outside the atlantic ocean basin the hydrophones detected p waves from global earthquakes with magnitudes of 5 8 8 3 at epicentral distances ranging from 29 6 to 167 2 examination of travel times suggests these teleseismic p waves constitute the suite of body wave arrivals from direct mantle p to outer and inner core reflected refracted phases the amplitudes of the teleseismic p waves also exhibit the typical solid earth wave field phenomena of a p shadow zone and caustic at d 144 these instruments offer a long term relatively low cost alternative to ocean bottom seismometers that allows for obser vation of pn velocities and mantle core phases arriving at normally inaccessible deep sea locations

on the mechanism of conversion of seismic waves to and from t waves in the vicinity of island shores high frequency seismic records from the polynesian seismic network are used to investigate in detail the processes of conversion of acoustic t wave energy from and to seismic waves at island shores on the source side we study the seismic to acoustic conversion based on t phases from hawaiian events recorded at polynesian stations located on the coral platter on the receiver side we study the acoustic to seismic conversion based on t phases from marine sources recorded across the polynesian islands in both instances our results underline the importance of steep slopes typically 50 in allowing an efficient conversion between p waves in the island structure and t waves in the water column these slopes can be coral reefs or the heads of young presumably unconsolidated basalt flows under this geometry modeling based on raytracing indicates that the seismic record of the t phase consists of a p wave at distances from the conversion point greater than 9 km at shorter distances s waves and surface waves are generated in the absence of a steep slope only surface waves are present these models can be used to compute precise and predictable travel time station corrections allowing the use of seismometers located inland to exploit the superb detection and location capabilities of t waves

reply to comment on the directionality of acoustic t phase signals from small magnitude submarine earthquakes j acoust soc am 119 3669 3675 2006 d bohnenstiehl has raised important issues about the interpretation of the t phase directionality data presented in the paper by chapman and marrett n r chapman and r marrett j acoust soc am 119 3669 3675 2006 the central point in that comment is that the maximum component in the t phase signal is radiated directly from the earthquake source location this reply points out that the results in the original paper are not inconsistent with bohnenstiehl s model moreover the array directionality data contain significantly more information that can be useful in understanding the influence of bathymetric features in the radiation of t phase sound by an earthquake

joint inversion for vp vs and vp vs at safod parkfield california we refined the three dimensional 3 d vp vs and vp vs models around the san andreas fault observatory at depth safod site using a new double difference dd seismic tomography code tomoddps that simultaneously solves for earthquake locations and all three velocity models using both absolute and differential p s and s p times this new method is able to provide a more robust vp vs model than that from the original dd tomography code tomodd obtained simply by dividing vp by vs for the new inversion waveform cross correlation times for earthquakes from 2001 to 2002 were also used in addition to arrival times from earthquakes and explosions in the region the vp values extracted from the model along the safod trajectory match well with the borehole log data providing in situ confirmation of our results similar to previous tomographic studies the 3 d structure around parkfield is dominated by the velocity contrast across the san andreas fault saf in both the vp and vs models there is a clear low velocity zone as deep as 7 km along the saf trace compatible with the findings from fault zone guided waves there is a high vp vs anomaly zone on the southwest side of the saf trace that is about 1 2 km wide and extends as deep as 4 km which is interpreted to be due to fluids and fractures in the package of sedimentary rocks abutting the salinian basement rock to the southwest the relocated earthquakes align beneath the northeast edge of this high vp vs zone we carried out a 2 d correlation analysis for an existing resistivity model and the corresponding profiles through our model yielding a classification that distinguishes several major lithologies

the directionality of acoustic t phase signals from small magnitude submarine earthquakes acoustic transients radiated from small magnitude undersea earthquakes were studied in an experiment carried out using a towed horizontal line array operating in the south fiji basin the transient signals consisted of p s and t phases with the t phase signal from each earthquake lasting up to 7min the directionality of the t phase signal was determined by processing the array hydrophone data with a conventional beamformer the weakest part of the signal arrived first on a direct bearing between the earthquake source and the array however subsequent stronger components arrived from different directions farther south of the source in a region where a number of seamounts and ridges rose within the sound channel a simple model based on ray path travel times for elastic wave travel in the earth and acoustic wave propagation in the water suggests that the later components of the t phase signal were radiated into the water by downslope propagation from the seamounts and ridges the initial weaker compon

springtime reduction in mf mesospheric echo occurrence radar echoes from the upper mesosphere recorded by the dynasonde at halley 76 s 27 w antarctica over four years have been studied to reveal the diurnal and seasonal characteristics of echo occurrence in addition to the expected daily variation a distinct seasonal change in the number and altitude of the echoes is apparent the prevalence of echoes during the winter months is followed in all four years studied by a period during the antarctic spring when echo occurrence falls markedly for a period lasting for about three weeks this dearth of echoes coincides with the breakdown of the polar vortex circulation pattern in the lower atmosphere providing strong evidence of the neutral dynamical coupling between the stratosphere and mesosphere

inversion of multicomponent multiazimuth walkaway vsp data for the stiffness tensor vertical seismic profiling vsp an established technique can be used for estimating in situ anisotropy that might provide valuable information for characterization of reservoir lithology fractures and fluids the p wave slowness components conventionally measured in multiazimuth walkaway vsp surveys allow one to reconstruct some portion of the corresponding slowness surface a major limitation of this technique is that the p wave slowness surface alone does not constrain a number of stiffness coefficients that may be crucial for inferring certain rock properties those stiffnesses can be obtained only by combining the measurements of p waves with those of s or ps modes here we extend the idea of horne and leaney who proved the feasibility of joint inversion of the slowness and polarization vectors of p and sv waves for parameters of transversely isotropic media with a vertical symmetry axis vti symmetry we show that there is no need to assume a priori vti symmetry or any other specific typ

determination of s wave slowness from a linear array of borehole receivers summary locations of seismic events from a linear array of receivers may require determination of slowness vectors of arriving waves in an isotropic medium p waves are polarized along the slowness vector which enables direct determination of backazimuth i e azimuth to a source from a receiver from p wave polarization in contrast s waves usually have much larger signal to noise ratio than p waves but are polarized in a plane perpendicular to their slowness vectors which prevents direct determination of their backazimuth we have developed a novel technique to determine the slowness vector of s waves detected in a linear array of receivers in an isotropic medium we combine the s wave polarization measurement with the derivative of the s wave travel times along the array to obtain the full slowness vector and backazimuth the proposed method allows one to determine direction to sources of seismic events from a single linear array of receivers using only s waves this technique is not affected by sv waves which is shown by a test on a synthetic data set we also test the method on two real microseismic data sets from hydraulic fracturing treatments and show that it outperforms the backazimuth determination from p waves and from horizontal polarization of s waves

long term observations of the arctic mesosphere with the mst radar at poker flat alaska in this report we summarize the results of 22 months of observations of arctic mesosphere echoes with the 50 mhz radar at poker flat alaska operation with a partially complete system began in february 1979 and has continued on a nearly continuous basis to the present time the altitude range of the mesospheric echoes obtained during this period shows a pronounced seasonal variation during summer months the mesospheric echoes are relatively continuous in time and extend from about 80 to 100 km with an unexpectedly strong peak in signal to noise ratio at about 85 km in contrast in nonsummer months mesospheric echoes are less intense and occur at lower altitudes in the range from 55 to 80 km these lower altitude nonsummer echoes are observed primarily during daytime hours when energetic auroral particle precipitation as determined by the poker flat 30 mhz riometer is present

estimation of effective anisotropy simultaneously with locations of microseismic events passive seismic tomography in which the event locations and the velocity model are inferred simultaneously is seldom used to process microseismic surveys acquired in the oil and gas industry we discuss advantages of applying tomographic ideas to typical microseismic data recorded in a single nearly vertical well to monitor hydraulic stimulation of a shale gas reservoir microseismic events are conventionally located in the energy industry applications using a velocity model derived from sonic logs and perforation shots instead of fixing the model as is normally done we alter it while locating the events this added flexibility not only makes it possible to accurately predict traveltimes of the recorded p and s waves but also provides a convincing evidence for anisotropy of the examined shale formation while we find that velocity heterogeneity does not need to be introduced to explain the data acquired at each stage of hydraulic fracturing the obtained models are suggestive of possible time laps

a methodology for unstructured damped stress inversion of microseismic focal mechanisms application to the vaca muerta formation argentina current methodologies for stress inversion from microseismic focal mechanisms require the assignment of events to a regular grid and then solving for the stress state at each grid node this approach can lead to irregularities in the solution because some nodes may contain few or even no events to address this issue we modified the algorithm to solve for stresses on an irregular unstructured grid we first use the k means algorithm to split the data into suitably sized groups the centroids of these groups are then considered as the nodes of an unstructured grid and we simultaneously solve for the stress state in each group using damped inversion to account for the irregularity of the unstructured grid we use the reciprocal square distance between nodes as weights as opposed to the existing method in which a weight of unity is assigned between adjacent nodes on a regular grid focal planes are selected from the auxiliary plane using the fault instability criterion the method is first applied to synthetic data sets in which we simulate and subsequently invert for the stress field around a mode i fracture at depth in a strike slip and in a normal faulting stress regime results indicate a stress orientation error of 10 and a stress ratio error between 1 and 10 we then consider focal mechanism data from an unconventional shale play in the vaca muerta formation in argentina and our results suggest the presence of a preexisting strike slip faulting stress regime we also find that the unambiguous focal plane picks suggest that the apparent dip slip focal mechanisms are indeed dip slip movement along subvertical natural fractures which correlate well with image log data we suggest that these dip slip events are caused by stress changes induced by the opening of the hydraulic fractures

time clustering behavior of spreading center seismicity between 15 and 35 n on the mid atlantic ridge observations from hydroacoustic monitoring abstract an earthquake catalog derived from the detection of seismically generated t waves is used to study the time clustering behavior of moderate size 3 0 m earthquakes between 15 and 35 n along the mid atlantic ridge mar within this region the distribution of inter event times is consistent with a non periodic non random clustered process the highest degrees of clustering are associated temporally with large mainshock aftershock sequences however some swarm like activity also is evident temporal fluctuations characterized by a power spectral density p f that decays as 1 f are present within the time sequence with ranging from 0 12 to 0 55 for different regions of the spreading axis this behavior is negligible at time scales less than 5 103 s and earthquake occurrence becomes less clustered smaller as increasing size thresholds are applied to the catalog a power law size frequency scaling for mid atlantic ridge earthquakes also can be demonstrated using the distribution of acoustic magnitudes or source levels although fractal seismic behavior has been linked to the structure of the underlying fault population in other environments power law fault size distributions have not been observed widely in the mid ocean ridge setting

importance of borehole deviation surveys for monitoring of hydraulic fracturing treatments during seismic monitoring of hydraulic fracturing treatment it is very common to ignore the deviations of the monitoring or treatment wells from their assumed positions for example a well is assumed to be perfectly vertical but in fact it deviates from verticality this can lead to significant errors in the observed azimuth and other parameters of the monitored fracture system geometry derived from microseismic event locations for common hydraulic fracturing geometries a 2 deviation uncertainty on the positions of the monitoring or treatment well survey can cause a more than 20 uncertainty of the inverted fracture azimuths furthermore if the positions of both the injection point and the receiver array are not known accurately and the velocity model is adjusted to locate perforations on the assumed positions several millisecond discrepancies between measured and modeled sh p traveltime differences may appear along the receiver array these traveltime discrepancies may then be misinterpreted as an effect of anisotropy and the use of such anisotropic model may lead to the mislocation of the detected fracture system the uncertainty of the relative positions between the monitoring and treatment wells can have a cumulative nonlinear effect on inverted fracture parameters we show that incorporation of borehole deviation surveys allows reasonably accurate positioning of the microseismic events in this study we concentrate on the effects of horizontal uncertainties of receiver and perforation positions understanding them is sufficient for treatment of vertical wells and also necessary for horizontal wells

on the feasibility of inversion of single well microseismic data for full moment tensor the existing paradigm widely accepted in the microseismic community asserts the impossibility of unambiguous estimation of full seismic moment tensor from three component data recorded in a single straight borehole the statement of inversion nonuniqueness related to the lack of illumination of the off plane moment component in the zero solid angle ray apertures tacitly assumes the slowness vectors normal to the relevant wave fronts to be confined to the plane containing the ray trajectories although such arrangements of the rays and slownesses do take place for vertical wells drilled in horizontally layered isotropic and vti transversely isotropic with a vertical symmetry axis formations azimuthal anisotropy commonly expected for hydraulically fractured hydrocarbon bearing shales makes the slowness vectors deviate from a plane arrange themselves into nonzero solid angle apertures and yield theoretically unique inverse problems for seismic moment tensors this paper examines those problems for the anisotropy parameters characterizing fractured shales and siltstones in the bakken their strong azimuthal anisotropy entails not only formally unique but importantly for practical applications well posed inverse problems resulting in robust estimates of the complete moment tensors from single borehole data

full waveform based complete moment tensor inversion and source parameter estimation from downhole microseismic data for hydrofracture monitoring downhole microseismic monitoring is a valuable tool in understanding the efficacy of hydraulic fracturing inverting for the moment tensor has gained increasing popularity in recent years as a way to understand the fracturing process previous studies utilize only part of the information in the waveforms such as direct p and s wave amplitudes and make far field assumptions to determine the source mechanisms the method is hindered in downhole monitoring when only limited azimuthal coverage is available in this study we develop an approach to invert for complete moment tensor using full waveform data recorded at a vertical borehole we use the discrete wavenumber integration method to calculate full wavefields in the layered medium by using synthetic data we find that at the near field range a stable complete moment tensor can be retrieved by matching the waveforms without additional constraints at the far field range we discover that the off plane moment tensor component is poorly constrained

feasibility of joint 1d velocity model and event location inversion by the neighbourhood algorithm using a set of synthetic p and s wave onsets computed in a 1d medium model from sources that mimic a distribution of microseismic events induced by hydrofrac treatment to a monitoring geophone array s we test the possibility to invert back jointly the model and events location we use the neighbourhood algorithm for data inversion to account for non linear effects of velocity model and grid search for event location the velocity model used is composed of homogeneous layers derived from sonic logging results for the case of one and two monitoring wells are compared these results show that the velocity model can be obtained in the case of two monitoring wells if they have optimal relative position the use of one monitoring well fails due to the trade off between the velocity model and event locations

inferred quasi steady ionospheric neutral winds and electrical currents at 79 south latitude in austral summer conditions an analysis of the range of geomagnetic variations as measured at arrival heights and south pole stations in the antarctic over an 8 year interval during the present solar cycle shows that there are quasi dc level shifts in some of the magnetic components during austral summer conditions when referenced to austral winter the most dramatic of these level shifts in the signals occurs in the d component west east magnetic component at arrival heights geomagnetic latitude 79 s the onsets of the d component shifts occur during a period that lasts one to several days in late austral spring such geomagnetic variations would not appear to arise from magnetosphere processes coupling to the ionospheric e region the inferred quasi steady ionospheric current is estimated to be of the order of 105 amps above arrival heights it is speculated that neutral wind dynamics with quasi steady e region 110 115 km neutral wind speeds of 10 m sec directed east to west above ah are responsible for generating the austral summer electrical currents in the e region

passive seismic tomography using induced seismicity at a petroleum field in oman a borehole network consisting of five monitoring wells monitored the induced seismicity at a producing petroleum field for about 11 months nearly 5400 microseismic events were analyzed and used to image the reservoir based on a new double difference dd seismic tomography the dd tomography method simultaneously solved for event locations and vp vs and vp vs models using absolute and differential p s and s p arrival times microseismicity in the field was caused primarily by compaction of the reservoir in and above the gas bearing formation and was distributed along the two major northeast southwest faults in the field the model resolution analysis based on the checkerboard test and the resolution matrix showed that the central part of the model was resolved relatively well for the depth range of 0 7 to 1 1 km clear velocity contrasts were imaged across most parts of the two northeast southwest faults the vp vs ratio estimates from the tomographic inversion were low 1 75 in the shallow

upper mantle structure in the south central chilean subduction zone 30 to 36 s 1 regional p and s wave travel time data were used to obtain three dimensional seismic tomography models for vp vs and vp vs above the subducting slab in central chile and argentina in this region there is an abrupt change from a normal subduction geometry south of 33 s to a flat subduction geometry to the north we find low vp low vs and high vp vs ratios in the southern half of our study area directly beneath the modern active volcanic arc which we interpret as localized pockets of melt in the northern half of our study area above where the subducting nazca plate flattens at 100 km depth we find low vp high vs and low vp vs ratios these unusual results point to a lack of melt or hydrated mineralogies such as serpentine both of which are characterized by high vp vs values the only mantle rocks that have low vp vs and high vs are mg rich compositions such as dehydrated serpentinite or orthopyroxenite we suggest that significant portions of the mantle overlying the flat slab consist of orthopyroxenite formed by a transient fluxing of silica rich fluids such fluids may have come from sediments that were subducted during the initiation of flat subduction at this latitude at 10 ma this would imply that the hydration of mantle material above a flat slab can be a transient phenomenon which leaves little residual free water behind but significantly alters the mantle chemistry

joint inversion for effective anisotropic velocity model and event locations using s wave splitting measurements from downhole microseismic data abstractan accurate velocity model is essential in microseismic imaging layered sediments and subvertical cracks from tectonic stress and hydraulic fracturing make velocity models more complicated than isotropy or simple anisotropy such as vertical transverse isotropy downhole microseismic acquisition usually cannot achieve sufficient ray coverage that is required to develop a low symmetry anisotropic model from traveltimes alone to solve this problem we have developed a new type of data s wave splitting parameters the delay time of the slow s wave and fast s wave polarization direction to determine anisotropic models a genetic algorithm inversion is adopted to solve for the locations of events and the stiffness tensor of an anisotropic medium simultaneously we applied this method to synthetic waveforms from numerical modeling and successfully recovered the input event locations and the velocity model the effectiveness of this method is further demonstrated by using real microseismic data acqu

azimuthal anisotropy in microseismic monitoring a bakken case study abstracthydraulic fracturing routinely applied for enhancing the permeability of unconventional oil and gas reservoirs is one of the possible causes for azimuthal anisotropy of the treated formations accounting for both naturally occurring and completion induced azimuthal anisotropy leads to marked improvements in the results of microseismic data processing as illustrated on a data set acquired in the bakken field north dakota usa those improvements include the possibility of modeling the observed shear wave splitting reduction of misfit between the picked and modeled traveltimes of microseismic events and relocation and tightening of the spatial distribution of the event hypocenters in addition and perhaps most importantly for the development of microseismic technology the feasibility of joint inversion of field microseismic data for the event locations and azimuthally anisotropic velocity model containing triclinic layers is demonstrated

joint location of microseismic events in the presence of velocity uncertainty the locations of seismic events are used to infer reservoir properties and to guide future production activity as well as to determine and understand the stress field thus locating seismic events with uncertainty quantification remains an important problem using bayesian analysis a joint probability density function of all event locations was constructed from prior information about picking errors in kinematic data and explicitly quantified velocity model uncertainty simultaneous location of all seismic events captured the absolute event locations and the relative locations of some events with respect to others along with their associated uncertainties we found that the influence of an uncertain velocity model on location uncertainty under many realistic scenarios can be significantly reduced by jointly locating events many quantities of interest that are estimated from multiple event locations such as fault sizes and fracture spacing or orientation can be better estimated in practice using the proposed approach

microseismic joint location and anisotropic velocity inversion for hydraulic fracturing in a tight bakken reservoir abstractto improve the accuracy of microseismic event locations we developed a new inversion method with double difference constraints for determining the hypocenters and the anisotropic velocity model for unconventional reservoirs we applied this method to a microseismic data set monitoring a middle bakken completion in the beaver lodge area of north dakota geophone arrays in four observation wells improved the ray coverage for the velocity inversion using an accurate anisotropic velocity model is important to correctly assess the height growth of the hydraulically induced fractures in the middle bakken our results showed that 1 moderate to strong anisotropy exists in all studied sedimentary layers especially in the upper and lower bakken shale formations where the thomsen parameters and can be greater than 0 4 2 all the events selected for high signal to noise ratio and used for the joint velocity inversion are located in the bakken and overlying lodgepole formations i e no events a

modal scattering a key to understanding oceanic t waves the excitation mechanism of oceanic t waves has been a puzzle for almost fifty years with refraction from a sloping seafloor and seafloor scattering as two of the most commonly invoked mechanisms by representing the earthquake source field as a normal mode sum it can be seen that both mechanisms are very closely related strict modal orthogonality prohibits the existence of t waves in a laterally homogeneous semi infinite half space or radially symmetric sphere as energy cannot be transferred from one mode to another in an homogeneous medium deterministic non planar bathymetry random boundary roughness upper crustal heterogeneity or a combination of these provides a physical mechanism to break the strict orthogonality we show that modal scattering from the rough seabottom in the epicentral region converts energy from the directly excited ocean crustal water column modes to the propagating acoustic modes comprising the oceanic t wave submarine earthquake fault orientation also appears to be reflected in the t wave excitation

cross correlation triangulation and curved wavefront focusing of coral reef sound using a bi linear hydrophone array a seven element bi linear hydrophone array was deployed over a coral reef in the papahanaumokuakea marine national monument northwest hawaiian islands in order to investigate the spatial temporal and spectral properties of biological sound in an environment free of anthropogenic influences local biological sound sources including snapping shrimp and other organisms produced curved wavefront acoustic arrivals at the array allowing source location via focusing to be performed over an area of 1600 m2 initially however a rough estimate of source location was obtained from triangulation of pair wise cross correlations of the sound refinements to these initial source locations and source frequency information were then obtained using two techniques conventional and adaptive focusing it was found that most of the sources were situated on or inside the reef structure itself rather than over adjacent sandy areas snapping shrimp like sounds all with similar spectral characteristics originated from individual sources predominantly in one area to the east of the array to the west the spectral and spatial distributions of the sources were more varied suggesting the presence of a multitude of heterogeneous biological processes in addition to the biological sounds some low frequency noise due to distant breaking waves was received from end fire north of the array

approximate traveltime inversion in downhole microseismic monitoring abstract in downhole microseismic monitoring accurate event location relies on the accuracy of the velocity model the model can be estimated along with event locations anisotropic models are important to get accurate event locations taking anisotropy into account makes it possible to use additional data two s wave arrivals generated due to shear wave splitting however anisotropic ray tracing requires iterative procedures for computing group velocities which may become unstable around caustics as a result anisotropic kinematic inversion may become time consuming in this paper we explore the idea of using simplified ray tracing to locate events and estimate medium parameters in the simplified ray tracing algorithm the group velocity is assumed to be equal to phase velocity in both magnitude and direction this assumption makes the ray tracing algorithm five times faster compared to ray tracing based on exact equations we present a set of tests showing that given perforation shot data one can use inversion based on simplified ray tracing even for moderate to strong anisotropic models when there are no perforation shots event location errors may become too large for moderately anisotropic media

constrained tomography of realistic velocity models in microseismic monitoring using calibration shots the knowledge of the velocity model in microseismic jobs is critical to achieving statistically reliable microseismic event locations the design of microseismic networks and the limited sources for calibration do not allow for a full tomographic inversion we propose optimizing a priori velocity models using a few active shots and a nonlinear inversion suitable to poorly constrained systems the considered models can be described by several layers with different p and s wave velocities the velocities may be constant or have 3d gradients the layer interfaces may be simple dipping planes or more complex 3d surfaces in this process the p and s wave arrival times and polarizations measured on the seismograms constitute the observed data set they are used to estimate two misfit functions i one based on the measurement residuals and ii one based on the inaccuracy of the source relocation these two functions are minimized thanks to a simulated annealing scheme which decreases the risk of converging to a local solution within the velocity model the case study used to illustrate this methodology highlights the ability of this technique to constrain a velocity model with dipping layers this was performed by jointly using sixteen perforation shots recorded during a multi stage fracturing operation from a single string of 3c receivers this decreased the location inaccuracies and the residuals by a factor of six in addition the retrieved layer dip was consistent with the pseudo horizontal trajectories of the wells and the background information provided by the customer finally the theoretical position of each calibration shot was contained in the uncertainty domain of the relocation of each shot in contrast single stage inversions provided different velocity models that were neither consistent between each other nor with the well trajectories this example showed that it is essential to perform a multi stage inversion to derive a better updated velocity model

seismic anisotropy in a hydrocarbon field estimated from microseismic data the study of seismic anisotropy in exploration seismology is gaining interest as it provides valuable information about reservoir properties and stress directions in this study we estimate anisotropy in a petroleum field in oman using observations of shear wave splitting from microseismic data the data set was recorded by arrays of borehole geophones deployed in five wells we analyse nearly 3400 microearthquakes yielding around 8500 shear wave splitting measurements stringent quality control reduces the number of reliable measurements to 325 shear wave splitting modelling in a range of rock models is then used to guide the interpretation the difference between the fast and slow shear wave velocities along the raypath in the field ranges between 0 10 and it is controlled both by lithology and proximity to the ne sw trending graben fault system that cuts the field formations the anisotropy is interpreted in terms of aligned fractures or cracks superimposed on an intrinsic vertical transversely isotropic vti rock fabric the highest magnitudes of anisotropy are within the highly fractured uppermost unit of the natih carbonate reservoir anisotropy decreases with depth with the lowest magnitudes found in the deep part of the natih carbonate formation moderate amounts of anisotropy are found in the shale cap rock anisotropy also varies laterally with the highest anisotropy occurring either side of the south eastern graben fault the predominant fracture strikes inferred from the fast shear wave polarizations are consistent with the trends of the main faults ne sw and nw se the majority of observations indicate subvertical fracture dip 70 cumulatively these observations show how studies of shear wave splitting using microseismic data can be used to characterize fractures important information for the exploitation of many reservoirs

inversion of microseismic data for triclinic velocity models modern downhole microseismic surveys often employ geometries in which ray trajectories generated by a collection of locatable events provide full polar and azimuthal coverage making it possible to estimate the in situ seismic anisotropy we show that traveltimes and particle motions of the direct p and shear waves acquired in such geometries can constrain stiffness tensors of triclinic media while obtaining all 21 stiffness coefficients of a homogeneous triclinic space simultaneously with locating pertinent microseismic events from data recorded in a single vertical well is relatively straightforward the same methodology does not necessarily succeed in layered formations because the combination of their vertical heterogeneity and azimuthal anisotropy might invalidate the commonly adopted approximation of the event azimuths by those of the p wave polarization vectors when the event azimuths cannot be derived from the particle motions traveltimes observed in two or more wells are required to locate the events and build layered triclinic or higher symmetry azimuthally anisotropic velocity models as our numerical tests indicate the multi well event location methods are expected to perform better than their single well counterparts because they rely solely on triangulation and eliminate the usually pronounced azimuthal uncertainties in the event locations that stem from noises adversely affecting hodogram analysis

empirical and numerical modeling of t phase propagation from ocean to land t phase propagation from ocean onto land is investigated by comparing data from hydrophones in the water column with data from the same events recorded on island and coastal seismometers several events located on hawaii and the emerging seamount loihi generated very large amplitude t phases that were recorded at both the preliminary ims hydrophone station at point sur and land based stations along the northern california coast we use data from seismic stations operated by u c berkeley along the coast of california and from the pg e coastal california seismic network to estimate the t phase transfer functions the transfer function and predicted signal from the loihi events are modeled with a composite technique using normal mode based numerical propagation codes to calculate the hydroacoustic pressure field and an elastic finite difference code to calculate the seismic propagation to land based stations the modal code is used to calculate the acoustic pressure and particle velocity fields in the ocean off the california coast which is used as input to the finite difference code tres to model propagation onto land we find both empirically and in the calculations that t phases observed near the conversion point consist primarily of surface waves although the t phases propagate as p waves after the surface waves attenuate surface wave conversion occurs farther offshore and over a longer region than body wave conversion which has the effect that surface waves may arrive at coastal stations before body waves we also look at the nature of t phases after conversion from ocean to land by examining far inland t phases we find that t phases propagate primarily as p waves once they are well inland from the coast and can be observed in some cases hundreds of kilometers inland t phase conversion attenuates higher frequencies however we find that high frequency energy from underwater explosion sources can still be observed at t phase stations

estimating the horizontal and vertical direction of arrival of water borne seismic signals in the northern philippine sea conventional and adaptive plane wave beamforming with simultaneous recordings by large aperture horizontal and vertical line arrays during the 2009 philippine sea engineering test philsea09 reveal the rate of occurrence and the two dimensional arrival structure of seismic phases that couple into the deep ocean a ship deployed controlled acoustic source was used to evaluate performance of the horizontal array for a range of beamformer adaptiveness levels ninety t phases from unique azimuths were recorded between yeardays 107 to 119 t phase azimuth and s minus p phase time of arrival range estimates were validated using united states geological survey seismic monitoring network data analysis of phases from a seismic event that occurred on yearday 112 near the east coast of taiwan approximately 450 km from the arrays revealed a 22 clockwise evolution of t phase azimuth over 90 s two hypotheses to explain such evolution body wave excitation of multiple sources or in water scattering are presented bas

reservoir characterization using surface microseismic monitoring microseismic monitoring of reservoir processes can be performed using surface or near surface arrays we review the published technical basis for the use of the arrays and the historical development of the method beginning with locating earthquakes through geothermal exploration to the growing field of hydraulic fracture monitoring practical considerations for the array deployment and data processing are presented the road ahead for the technology includes a move toward life of field buried arrays as well as opportunities for extended interpretation of the data particularly inversion for source mechanism estimation and measurement of anisotropy in the monitored subsurface

extended wave equation imaging conditions for passive seismic data abstractthe information obtained from seismic monitoring at injection sites is often limited to seismic event properties e g location origin time moment tensor the accuracy of which greatly depends on the assumed or estimated elastic velocity models current traveltime based methods to calibrate velocity models rely on having distinct arrivals in surface seismic monitoring however the signal to noise ratio of arrivals is often too poor for pick based methods and migration techniques are needed wave equation migration can image microseismic sources without the need for picking or knowledge of the onset time by reconstructing the wavefield through the model space and applying an imaging condition we have devised extended imaging conditions for passive seismic wave equation imaging algorithms and we have found that they represent a key step toward verifying and updating elastic velocity models by extending imaging conditions away from zero lag in time and space we can better evaluate the focu

influence of models on seismic event localization source localization is a fundamental problem in seismology current localization techniques often rely on homogeneous isotropic models even if a survey region is known to be geologically complex or anisotropic we investigated a model s influence on localization using data from a hydraulic injection experiment at the continental deep drilling kontinentale tiefbohrung or ktb site in germany we performed the localization with a grid search algorithm and two additional methods for verification from previous work a homogeneous isotropic model based on a borehole check shot and a heterogeneous 3d isotropic model were available vertical seismic profiling vsp and borehole data as well as laboratory data from rock samples provided homogeneous anisotropic models although localization with the isotropic and anisotropic models led to the same magnitude of residuals and therefore to a comparable quality of fit the distribution of the event cloud differed significantly for isotropic and anisotropic media

uncertainties in passive seismic monitoring the use of passive seismic techniques to monitor oil field completion and production processes is on the rise stress changes induced by such reservoir activities as hydraulic fracturing water injection or fluid extraction will often result in failure of the rocks with a concurrent release of seismic energy in the form of compressional p and shear s waves passive seismic monitoring is based on recording these emitted waves and then using their arrival times to estimate the location of the failure events

simultaneous inversion of multiple microseismic data for event locations and velocity model with bayesian inference abstractwe have applied bayesian inference for simultaneous inversion of multiple microseismic data to obtain event locations along with the subsurface velocity model the traditional method of using a predetermined velocity model for event location may be subject to large uncertainties particularly if the prior velocity model is poor our study indicated that microseismic data can help to construct the velocity model which is usually a major source of uncertainty in microseismic event locations the simultaneous inversion eliminates the requirement for an accurate predetermined velocity model in microseismic event location estimation we estimate the posterior probability density of the velocity model and microseismic event locations with the maximum a posteriori estimation and the posterior covariance approximation under the gaussian assumption this provides an efficient and effective way to quantify the uncertainty of the microseismic location estimation and capture the correlation between the velo

initial observations of mesospheric winds using idi radar measurements at the bear lake observatory the adaptation of imaging doppler interferometry idi to the dynasonde deployed at the bear lake observatory 41 9 n 111 4 w in northeastern utah enables the routine measurement of mesospheric dynamics influenced by the direction of the prevailing wind a clear seasonal variation is seen in the mesospheric echo numbers the technique also provides a monitor for the amplitude of the main tidal modes as well as for studying planetary waves with periods of several days the observations are found to be consistent with those from different instruments based at the same site thus confirming the idi characterisation of the mesospheric wind field

p wave detection thresholds pn velocity estimates and t wave location uncertainty from oceanic hydrophones p wave arrivals recorded by the u s navy s sound surveillance system sosus hydrophone arrays were used to estimate earthquake detection thresholds and pn velocities in the northeast pacific ocean the navy hydrophones have been used successfully to detect and locate oceanic earthquakes using their waterborne acoustic tertiary t waves however use of these hydrophones for seismic body wave detection allows regional seismic analyses to be extended to the oceanic environment the p wave detection threshold of the sosus hydrophones was quantified using the epicentral distance and magnitude of 250 northeast pacific ocean earthquakes earthquakes with body wave magnitudes as low as 2 have detectable p wave arrivals at epicentral distances of 500 km earthquakes with mb between 3 5 and 5 were detected 50 of the time at distances of 100 1500 km while events with mb 5 were all detected even out to distances of 1000 1500 km both p and t wave hydrophone arrival times were used to estimate the epicenters of 100 earthquakes the peak amplitude of the t wave coda and the onset of the p wave were used as the earthquake arrival times to estimate event locations t wave arrival time residuals have a gaussian distribution with zero mean which implies that using t wave peak amplitude is consistent with using the p wave onset as the arrival time there are typically 6 stations used to derive a t wave based location hence location error ellipses are not well constrained a monte carlo technique was employed to estimate t wave event location uncertainty t wave locations have error bars of 1 km in latitude and longitude when 3 hydrophones are used for a location estimate the detected p wave arrivals and earthquake locations were used to measure pn velocities pn velocity values of 7 9 0 1 and 8 0 0 1 km s were found for the pacific and juan de fuca plates respectively a pn velocity of 7 5 0 1 km s was measured for rays traveling northward from the mendocino triple junction along the cascadia subduction zone a pn velocity of 7 7 0 3 km s was estimated for ray paths originating onshore western north america and traveling to the offshore hydrophones

hydraulic stimulation of natural fractures as revealed by induced microearthquakes carthage cotton valley gas field east texas we produced a high resolution microseismic image of a hydraulic fracture stimulation in the carthage cotton valley gas field of east texas we improved the precision of microseismic event locations four fold over initial locations by manually repicking the traveltimes in a spatial sequence allowing us to visually correlate waveforms of adjacent sources the new locations show vertical containment within individual targeted sands suggesting little or no hydraulic communication between the discrete perforation intervals simultaneously treated within an 80 m section treatment i e fracture zone lengths inferred from event locations are about 200 m greater at the shallow perforation intervals than at the deeper intervals the highest quality locations indicate fracture zone widths as narrow as 6 m similarity of adjacent source waveforms along with systematic changes of phase amplitude ratios and polarities indicate fairly uniform source mechanisms fracture plane orientation and sense of slip over

normal mode composition of earthquake t phases understanding the nature of the coupling between the underwater acoustic field and the land seismic field is important for evaluating the performance of the t phase stations in the international monitoring system for the comprehensive nuclear test ban treaty for upslope propagation in an ocean environment the places where underwater acoustic field energy couples into the land seismic field are determined to first approximation by the local water depth and the normal mode composition of the acoustic energy therefore the use of earthquake generated t phases as natural probes of water to land coupling characteristics is aided by knowledge of their modal composition data collected by a 200 element 3000 m aperture vertical hydrophone array during a 1989 experiment in the deep northeast pacific ocean are used to determine the mode composition of t phase arrivals from two mb 4 1 earthquakes near the w est coast of the u s one occurring offshore and the other on land results from an eigenanalysis approach and conventional mode decomposition for the two events are consistent and show that at 5 hz the offshore event s arrivals have higher order mode content compared to those from the event on land single hydrophone recordings at pt sur of two mb 4 4 hawaiian events in 1996 and 1997 one occurring offshore and the second on land display time frequency arrival structures that are explainable by the dispersion characteristics over the oceanic path although other effects due to complex source time functions and shear wave and dispersive propagation effects along the initial land path cannot be separated with these single element data differences in these two events arrival structures suggest differences in normal mode content consistent with those seen in the pair of 1989 events ocean path dispersion also appears to play a significant role in determining the i n water arrival structure from a 1995 french nuclear test at mururoa recordings of two hawaiian events in 1997 by the t phase station vib and the seismic station at berkeley illustrate that the water land coupling confuses the relative timing between normal modes resulting in apparent loss of information about the source

velocity calibration for microseismic monitoring a very fast simulated annealing vfsa approach for joint objective optimization to accurately locate microearthquakes that are genetically related to hydraulic fracture stimulation a thorough knowledge of the velocity structure between monitoring and fracturing treatment wells is essential very fast simulated annealing vfsa is implemented to invert for a flat layered velocity model between wells using perforation or string shot data a two point ray tracing method is used to find the ray parameter p for a ray traveling from a source to a receiver the original traveltime calculation formula is modified to account for the borehole source receiver geometry vfsa is used as a tool to optimize p and s wave velocities simultaneously unlike previous applications of vfsa two improvements result from a new study 1 both p and s wave arrival time misfits are considered in a joint objective function and 2 p and s wave velocities are perturbed simultaneously during annealing the inverted velocities follow the true values closely with a very small root mean square error indicating the inverted model is close to the global minimum solution whose rms error should be zero for synthetic examples data noise contaminates inverted models but not substantially in synthetic test results a comparison of models inverted using vfsa and occam s inversion technique indicates that inverted models using vfsa are superior to those using occam s method in terms of velocity accuracy

simultaneous inversion for microseismic event location and velocity model in vaca muerta formation velocity models play a key role in locating microseismic events however it is usually challenging to construct them reliably traditional model building strategies depend on availability of well logs or perforation shots we simultaneously invert for microseismic event locations and a velocity model under the bayesian inference framework and apply it in a field dataset acquired in the vaca muerta formation at neuquen argentina this methodology enables uncertainty and posterior covariance analysis by matching the moveouts of the p and s wave arrival times we were able to estimate a 1d velocity model to achieve improved event locations various analyses indicate the superiority of this model over a model built with the traditional strategy with this algorithm we can perform microseismic monitoring to fracturing treatments where no perforation data is available in addition we can also apply it for long term passive seismicity reservoir monitoring where changes of reservoir properties are expected

adaptive mesh seismic tomography based on tetrahedral and voronoi diagrams application to parkfield california 1 we have developed an adaptive mesh seismic tomography method based on tetrahedral and voronoi diagrams to automatically match the inversion mesh to the data distribution two different irregular mesh interpolation methods linear and natural neighbor are evaluated a synthetic test shows that the adaptive mesh seismic tomography method recovers the true synthetic model well in both the linear and natural neighbor interpolation cases use of more accurate differential data is helpful in removing artifacts in the model that result from the noisier absolute data application of the adaptive mesh scheme to parkfield california results in much more uniform sampling of the model as measured by the derivative weight sum on the optimized irregular inversion mesh compared to a regular grid the across strike cross sections resulting from using either just the absolute data or both the absolute and differential data show a clear velocity contrast at the san andreas fault in the linear and natural neighbor interpolation cases the velocity model obtained using natural neighbor interpolation is smoother than that from linear interpolation including the differential data in the inversion yields more clustered event locations and removes a velocity reversal to the southwest of the fault which is likely caused by noise in the absolute data