Differential Travel Time Data Research Articles

Estimating three-dimensional current fields in the Yeosu Bay using coastal acoustic tomography system

Observation of current speeds in coastal seas is crucial because it can provide useful information for ship operations, fishing activities, and rapid responses to marine disasters. Coastal acoustic tomography (CAT) is a technology that can continuously monitor environmental changes such as current velocity and water temperature using reciprocal acoustic signals between CAT stations in coastal seas. This technology is different from traditional pointwise or intermittent sectional observations in that it can produce time-varying two- or three-dimensional current fields. The results of previous studies using CAT systems have been limited to reproducing horizontal maps of depth-averaged two-dimensional current fields. Utilizing results from a high-resolution coastal ocean model, this study developed a novel technique for estimating three-dimensional (3-D) current fields by combining the inverse method with an artificial intelligence (AI) model. Following three steps are the procedure for the test of estimating the 3-D current fields. First, utilizing the ray tracing model ‘Bellhop,’ reciprocal travel times among five CAT stations using the coastal ocean model outputs are computed. These five stations correspond to the locations where in-situ CAT systems were established for continuous monitoring of current changes in Yeosu Bay, Korea. Subsequently, the range-averaged currents at the five layers were estimated by incorporating this travel time difference data into an AI model trained using the same coastal ocean model outputs. Finally, the inverse method is applied to each layer to estimate the 3-D current fields. The validation results revealed that the newly developed method performed well in both summer and winter. Time-varying two-layer-like current fields were reasonably produced, occasionally revealing an out-of-phase relationship between the upper and lower layers depending on the tidal phases. This method yielded average root-mean-squared errors of less than 4 cm/s on six simulation paths for acoustic signal propagation. Furthermore, when the same method was applied to in-situ CAT observations, the average correlation coefficient (R) of the along-channel current of each layer was found to be approximately 0.9 or higher. These results suggest that this novel method can be effectively applied to the continuous monitoring of 3-D current fields in coastal seas using a CAT system.

Three travel time inverse problems on simple Riemannian manifolds

We provide new proofs based on the Myers–Steenrod theorem to confirm that travel time data, travel time difference data and the broken scattering relations determine a simple Riemannian metric on a disc up to the natural gauge of a boundary fixing diffeomorphism. Our method of the proof leads to a Lipschitz-type stability estimate for the first two data sets in the class of simple metrics.

Pn velocity and anisotropy at the lithospheric mantle wedge beneath the middle-eastern Gangdese Metallogenic Belt, southern Tibet, China

The lithospheric mantle wedge beneath the middle-eastern Gangdese Metallogenic Belt (GMB), located at the forefront of the India-Eurasia collision-subduction zone retains many traces of structure evolution, which can be revealed by the velocity and anisotropy structure of the uppermost mantle of this region. In this study, high-resolution structures of the Pn velocity and anisotropy beneath the middle-eastern GMB are presented using interstation Pn differential travel-time data recorded by a dense broadband seismic array. The average Pn-wave velocity of 8.13 km/s beneath the GMB is close to that of the peridotite. The Pn velocity and anisotropy show prominent differences across the Jiali dextral strike-slip fault (JLF, ∼31° N) on the east of Shenza-Xietongmen rift, as indicates that the JLF (31°N) is an uppermost mantle trace of the Shiquan River-Nam Tso Mélange Zone. Low Pn velocity (<8.0 km/s) and a nearly EW-oriented Pn fast-wave direction to the north of the JLF, suggesting the EW extension of the Eurasian plate. NE-oriented Pn fast-wave direction with strange intensity (>0.03) to the south of the JLF indicates the direct influence of the Indian plate northward subduction. The rifts located at the boundary of high and low-velocity anomalies imply that the rift system formed under the influence of lithospheric mantle activity. Based on the NE compression caused by the Indian plate’s northeastward subduction, some of the forced upwellings of thermal material generated within the mantle wedge tip caused an EW extension of the crust. Subsequently, the rifts system was locally formed.

Long‐Wavelength Topography and Multiscale Velocity Heterogeneity at the Core‐Mantle Boundary

AbstractThe structure of the lowermost mantle and the core‐mantle boundary (CMB) has profound implications for Earth's evolution and current‐day dynamics. Whilst tomographic studies of VS show good agreement in the lowermost mantle, consensus as to VP and especially CMB radius has not yet been reached. We perform a hierarchical Bayesian inversion for VP in the lowermost 300 km of the mantle and the radius of the CMB using differential travel time data. Concurrent with finding VP perturbations of 0.56% RMS amplitude that spatially agree with previous studies in areas of low posterior variance, we find 4.5 km RMS amplitude CMB radius perturbations with a broadly north‐south hemispherical character, with spherical harmonic power evenly distributed between degrees 1–3. These results suggest that CMB radial processes are set by a longer scale process than the VP perturbations.

Seismology on Mars: An analysis of direct, reflected, and converted seismic body waves with implications for interior structure

The InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission has been collecting high-quality seismic data on Mars since early 2019 that provide the first direct observations of its interior structure. Here we report on a complete analysis of the part of the marsquakes known as the low-frequency seismic events (main energy below 1 Hz) that are sensitive to the deep interior. To identify body-wave arrivals in the highly-scattered martian seismograms, we employ four complementary approaches: 1) time-domain envelopes; 2) polarised waveforms and their time-domain envelopes; 3) polarisation analysis; and 4) waveform matching. Through careful application of this processing scheme to each marsquake, we are able to significantly increase the number of phase picks relative to earlier analyses (from 41 to 76), including body-wave arrivals from direct (P and S), reflected (PP, SS, PPP, SSS, and ScS), and converted (Ps and Sp) phases. To constrain the depth of the marsquakes, we also identify depth phases (pP and sS). Following this, we invert an initial set of phase picks for models of interior structure, event distance, and depth, while predicting travel times for seismic phases not identified at the outset. Based on the predictions, we repick (every pick is subject to our processing scheme), thereby enlarging our differential travel-time data set (all picks are relative to the main P-wave arrival), and subsequently re-invert for an updated set of interior structure models, distances, and depths. Proceeding thus, we present updated radial seismic velocity models of the crust, mantle, and core. We observe crustal interfaces at average depths of 10, 25, and 45 km, respectively, of which the former two are interpreted as intra-crustal interfaces and the latter as the crust-mantle boundary. We find an upper mantle structure consistent with a low-velocity zone associated with a thermal lithosphere and a thermal gradient in the range 2.4–2.9 K/km that extends to a depth of ~450 km. The thermal structure of the Martian mantle indicates potential and core-mantle-boundary temperatures in the ranges 1650–1750 K and 1900–2100 K, respectively, implying an entirely liquid core at present. Based on the identification of ScS phases, we obtain an improved estimate of the Martian core radius (1820–1870 km) and mean core density (6–6.2 g/cm3).

Dynamics of Tidal and Residual Currents Based on Coastal Acoustic Tomography Assimilated Data Obtained in Jiaozhou Bay, China

AbstractTo investigate the dynamics of tidal and residual currents in a shallow bay with narrow bay mouth and large tidal flat area, this study conducted three sets of coastal acoustic tomography (CAT) observations at 13 stations in deep area of Jiaozhou Bay (JZB), China, from December 6–18, 2015. Thereafter, travel time difference data observed via CAT were assimilated into a barotropic tidal model to realize an extrapolated study in tidal flats. The root mean square difference was 6.5 cm/s between assimilation and acoustic Doppler current profile data. Harmonic analyses indicated that the semidiurnal tide (M2) was predominant; however, relatively large nonlinear tidal currents (M4 and M6) were also found. Moreover, the spatial mean amplitudes of M2, M4, and M6 were 0.35 (excluding the tidal flat), 0.09, and 0.04 m/s, respectively. Dynamic analyses showed that continuity and advection terms were the main contributors to M4 in the bay mouth, whereas M4 in the tidal flats was generated by continuity and friction terms. Furthermore, bottom friction was the primary contributor to M6. In addition, residual currents with multi‐vortex structures appeared near the bay mouth, the maximum speed reached 0.60 m/s, and advection, continuity, and bottom friction contributed more to the generation of residual current eddies. Horizontal pressure gradient of residual elevation and advection of tidal and residual currents were the dominant factors maintaining the residual current near the bay mouth. The generation mechanisms of M4 and M6 were first discussed based on the assimilation of the current field data in JZB.

Physics-based coastal current tomographic tracking using a Kalman filter.

Ocean acoustic tomography can be used based on measurements of two-way travel-time differences between the nodes deployed on the perimeter of the surveying area to invert/map the ocean current inside the area. Data at different times can be related using a Kalman filter, and given an ocean circulation model, one can in principle now cast and even forecast current distribution given an initial distribution and/or the travel-time difference data on the boundary. However, an ocean circulation model requires many inputs (many of them often not available) and is unpractical for estimation of the current field. A simplified form of the discretized Navier-Stokes equation is used to show that the future velocity state is just a weighted spatial average of the current state. These weights could be obtained from an ocean circulation model, but here in a data driven approach, auto-regressive methods are used to obtain the time and space dependent weights from the data. It is shown, based on simulated data, that the current field tracked using a Kalman filter (with an arbitrary initial condition) is more accurate than that estimated by the standard methods where data at different times are treated independently. Real data are also examined.

Coastal ocean current tomography using a spatio-temporal Kalman filter

The method of ocean acoustic tomography (OAT) can be used to invert/map the ocean current in a coastal area based on measurements of two-way travel time differences between the nodes deployed on the perimeter of the surveying area. Previous work has attempted to relate the different measurements in time using the Kalman filter. Now, if the ocean dynamics or model is known, one can also determine the current field given an initial distribution or the travel-time difference data on the boundary, and can even forecast the current changes. Based on the ocean dynamics, the current field is shown to be spatially and temporally correlated. We derive their relation and use that as the state model for the Kalman filter; the coefficients are estimated from data using an auto-regressive analysis. Armed with this model, it is shown based on simulated data that the current field can be tracked as a function of time using the Kalman filter (with an arbitrary initial condition) with a higher accuracy than that estimated by OAT. The reason of the improvement, the use of spatial-temporal state model (versus using only the temporal evolution), is studied. The method has also been applied to real data.

Current field tracking based on spatiotemporal evolution model with distributed networked underwater sensor system

Distributed networked underwater sensor (DNUS) system can provide ocean measurements over a wide area with a large number of sensors. This paper studies the estimation of the two-dimensional ocean current field with acoustic travel time difference data observed by a DNUS system. Considering that the current field is correlated in time and space, we present a statistical-based tomography method based on Kalman filter to reconstruct and track the ocean current field. A spatiotemporal autoregressive (AR) model is used to describe the evolution of the current field. In the spatiotemporal AR model, the observation region is divided into sub-triangle grids. The sub-triangles are partitioned into clusters by distance and each cluster is assigned with one AR coefficient. The AR coefficients are updated adaptively with the past estimated current velocities. The proposed method is verified with the synthetic observational data generated by a barotropic ocean model and the tomography experiment conducted near Zhoush...

Imaging the inner core under Africa and Europe

The inner core under Africa is thought to be a region where the nature of inner core texture changes: from the strongly anisotropic ‘western’ part of the inner core to the weakly anisotropic, or isotropic ‘eastern’ part of the inner core. Additionally, observations of a difference in isotropic velocity between the two hemispheres have been made. A very large new dataset of simultaneous PKPdf and PKPbc observations, on which differential travel times have been measured, is used to examine the upper 360km of the inner core under Europe, Africa and the surrounding oceans. Inversion of the differential travel time data for laterally varying inner core anisotropy reveals that inner core anisotropy is stronger under central Africa and the Atlantic Ocean than under the western Indian Ocean. No hemispherical pattern is present in Voigt isotropic velocities, indicating that the variation in anisotropy is due to differing degrees of crystal alignment in the inner core, not material differences. When anisotropy is permitted to change with depth, the upper east-most part of the study region shows weaker anisotropy than the central and western regions. When depth dependence in the inner core is neglected the hemisphere boundary is better represented as a line at 40°E than one at 10°E, however, it is apparent that the variation of anisotropy as a function of depth means that one line of longitude cannot truly separate the more and less anisotropic regions of the inner core. The anisotropy observed in the part of the inner core under Africa which lies in the ‘western’ hemisphere is much weaker than that under central America, showing that the western hemisphere is not uniformly anisotropic. As the region of low anisotropy spans a significant depth extent, it is likely that heterogeneous heat fluxes in the core, which may cause variations in inner core anisotropy, have persisted for several hundred million years.

Tomography of the Chukou Fault Zone, Southwest Taiwan: Insights from Microearthquake Data

The vigorous collision between the Eurasian plate and Philippine Sea plate in Taiwan causes a series of imbricate fold and thrust belts to develop at the deformation front. The Chukou Fault (CKF), characterized by a thrust type fault, located in Chiayi County, southwest (SW) Taiwan, is a prominent boundary between the fold-thrust belts and the Western Coastal Plain. Most of the seismicity in SW Taiwan is associated with this fault and its neighboring fault systems. The seismotectonic structures in the CKF zone, especially in the east, are complex due to the interactions among fault systems with distinct slip motions. To gain better insights into the seismogenic characteristics in the CKF zone, we used 1661 microearthquakes recorded by a temporary dense broadband seismic network and the Central Weather Bureau Seismic Network (CWBSN) between 2003 and 2004 to investigate the physical properties of the crust in the CKF zone. A waveform cross-correlation technique was applied to 143086 pairs of waveform data to determine the relative differential travel time between the P- and S-waves. By combining both the absolute and relative differential travel time data, we were able to obtain a new 3-D crustal P-wave velocity structure and Vp/Vs ratios. This study suggests that by using both absolute and relative differential travel time data in tomographic inversion can obtain precise 3-D velocity images and also gain better correlation between seismic events and fault structures, which is crucial for understanding the seismogenic process in our study area.

The 2013 earthquake swarm in Helike, Greece: seismic activity at the root of old normal faults

The Corinth Rift in Central Greece has been studied extensively during the past decades, as it is one of the most seismically active regions in Europe. It is characterized by normal faulting and extension rates between 6 and 15 mm yr−1 in an approximately N10E° direction. On 2013 May 21, an earthquake swarm was initiated with a series of small events 4 km southeast of Aigion city. In the next days, the seismic activity became more intense, with outbursts of several stronger events of magnitude between 3.3 and 3.7. The seismicity migrated towards the east during June, followed by a sudden activation of the western part of the swarm on July 15th. More than 1500 events have been detected and manually analysed during the period between 2013 May 21 and August 31, using over 15 local stations in epicentral distances up to 30 km and a local velocity model determined by an error minimization method. Waveform similarity-based analysis was performed, revealing several distinct multiplets within the earthquake swarm. High-resolution relocation was applied using the double-difference algorithm HypoDD, incorporating both catalogue and cross-correlation differential traveltime data, which managed to separate the initial seismic cloud into several smaller, densely concentrated spatial clusters of strongly correlated events. Focal mechanism solutions for over 170 events were determined using P-wave first motion polarities, while regional waveform modelling was applied for the calculation of moment tensors for the 18 largest events of the sequence. Selected events belonging to common spatial groups were considered for the calculation of composite mechanisms to characterize different parts of the swarm. The solutions are mainly in agreement with the regional NNE–SSW extension, representing typical normal faulting on 30–50° north-dipping planes, while a few exhibit slip in an NNE–SSW direction, on a roughly subhorizontal plane. Moment magnitudes were calculated by spectral analysis of S waves, yielding b-values between 1.1 and 1.2 in their frequency–magnitude distribution. The seismic moment release history indicates swarm-like activity during the first phase, which could have acted as a preparatory stage for the second phase (after 12 July) that presented a more typical main-shock–aftershock behaviour. The spatiotemporal analysis reveals that the swarm has occurred in a volume that is likely related with the extension at depth of the NNE-dipping Pirgaki normal fault, outcropping ∼8 km to the south. The slow velocity of eastward migration of the epicentres during June implies triggering by fluids. The situation appears different in the second phase of the sequence, which was probably triggered by a build-up of stress during the first one. The relatively deep hypocentres of the 2013 swarm, compared to the shallower seismic layer within the rift, and their coincidence with the steeply dipping Pirgaki fault, favour an immature rift detachment model. Previous results from instrumental data indicate that approximately the same region had been activated during July–August 1991. The availability of the dense permanent seismological network data thus allowed for a detailed analysis of this crisis, a better understanding of its mechanical context and of the earlier events.

Subducted slabs stagnant above, penetrating through, and trapped below the 660 km discontinuity

AbstractA newPwave tomographic model of the mantle was constructed using more than 10 million travel times. The finite‐frequency effect of seismic rays was taken into account by calculating banana‐donut kernels at 2 Hz for all first arrival time data, and at 0.1 Hz for broadband differential travel time data. Based on this model, a systematic survey for subducted slab images was developed for the circum‐Pacific; including the Kurile, Honshu, Izu‐Bonin, Mariana, Java, Tonga‐Kermadec, southern and northern South America, and Central America, arcs. This survey revealed a progressive lateral variation of the configuration of slabs along arc(s), which we interpret as an indication for successive stages of slab subduction through the Bullen's transition region with the 660 km discontinuity at the middle. We identified the four distinct stages: I ‐ slab stagnant above the 660 km discontinuity; II ‐ slab penetrating the 660 km discontinuity; III ‐ slab trapped in the uppermost lower mantle (at a depth of 660–1000 km); and IV ‐ slab descending well into the deep lower mantle. The majority of slab images are found to be either at Stage I or III, suggesting that Stages I and III are relatively stable or neutral and II and IV are relatively unstable or transient. There is a remarkable distinction for the deepest hypocentral distribution between slabs at Stage I and slabs at Stages II or III.

The April–June 2007 Trichonis Lake earthquake swarm (W. Greece): New implications toward the causative fault zone

On 10 April 2007, three moderate earthquakes with Mw=4.9–5.1 occurred in the vicinity of Trichonis Lake (W. Greece). A local network composed of 12 three-component digital seismographs was installed in the epicentral area and recorded more than 1600 events. The double-difference algorithm HYPODD, incorporating both catalog and waveform cross-correlation differential travel-time data, was applied for the successful relocation of 1490 earthquakes. The latter led to the distinction of a main NW-SE trending and NE-dipping zone, as well as of three neighboring faults; a conjugate NW-SE striking and SW-dipping marginal fault mapped along the northeastern flanks of the lake; a E-W trending and south-dipping low-angle normal fault, possibly related to the major Agrinio Fault Zone (AFZ), parallel to the northern bank of the lake; a NE-SW striking and NW-dipping normal fault, likely related to a segment of the active Evinos fault, located south of the lake. Calculation of the Coulomb stress induced by the combination of the 1975 Mw=6.0 event and the three largest events of 10 April 2007 on the inferred structures, reveals that most of the seismicity lies within the “stress-loaded” region, except for the westernmost activity, which probably belongs to the deep part of the AFZ. A total of 178 reliable focal mechanisms were determined by regional and local body-wave modeling (5 largest events) and P-wave first motion polarity data. The types of the obtained focal mechanisms are predominantly normal and strike-slip, however, numerous earthquakes were found to exhibit reverse faulting. Inversion of focal mechanism data showed that the prevailing principal horizontal component σ3 is quite homogeneous throughout the activated area with a roughly NW-SE trend, parallel to the strike of the Hellenides. On the contrary, the compressional field σ1 appears in two patterns: NE-SW trending onshore and NW-SE trending beneath the lake. This apparent rotation of σ1 by 90° reveals a complex system enclosed by the suggested NW-SE trending antithetic faults in depths between 7 and 9km. The calculated stress ratios beneath the lake imply that vertical forces are close to the overburden pressure. The overall inferred stress pattern is rather linked to topographic variations, locally imposing increase or decrease of the vertical forces. The presence of the water in the lake possibly plays an additional important role, penetrating through the bedrock, reducing the friction coefficient, while the pore pressure and, consequently, the effective stress increase. Thus, shearing along mature fractures is enhanced, likely yielding the observed diversity.

Uppermost mantle structure of the eastern margin of the Tibetan plateau from interstation Pn traveltime difference tomography

Tomographic inversion with interstation Pn differential traveltime data provides a reliable velocity image of the uppermost mantle across the Longmenshan Fault Belt, which borders the eastern Tibetan plateau and Sichuan Basin. Low Pn velocities (<7.8km/s) are revealed beneath the eastern Tibetan plateau to the west of the Longmenshan Fault Belt, while high Pn velocities (>8.1km/s) appear beneath the Sichuan Basin to the east, suggesting a relatively weak upper mantle beneath eastern Tibetan plateau and a strong upper mantle beneath the Sichuan Basin. The Pn velocities are consistent with the proposition that the rigid and stable lithosphere of the Sichuan Basin may act as a barrier to the eastward escape flow of the ductile crust of the eastern Tibetan plateau. Consistency of Pn velocities with the smoothed topography, crustal thickness and Bouguer gravity implies that the dynamic processes of the lithospheric mantle should be responsible for the crustal thickening and relief of uplift in the eastern Tibetan plateau. However, the Longmeshan Fault Belt appears to be separated into two segments by the Huya and Leidong faults as manifested by significant discrepancies in topography gradient, seismic activity, rupture process and aftershock distribution of Wenchuan earthquake. This segmentation is also supported by difference in Pn velocities along the strike of the fault belt, suggesting different tectonic environments in the crust and lithospheric mantle. Understanding of the mechanism for the segmentation requires further investigation.

Uppermost mantle structure of the North China Craton: Constraints from interstation Pn travel time difference tomography

The uppermost mantle is the key area for exchange of heat flux and material convection between the crust and lithospheric mantle. Spatial variations of lithospheric thinning and dynamic processes in the North China Craton could inevitably induce the velocity heterogeneity in the uppermost mantle. In this study, we used Pn arrivals from permanent seismic stations in North China and surrounding regions to construct a tomographic image of the North China Craton. The tomographic method with Pn travel time difference data were used to study the velocity variations in the uppermost mantle. Pn velocities in the uppermost mantle varied significantly in the Eastern, Central and Western blocks of the North China Craton. This suggests that the lithosphere beneath different blocks of the North China Craton have experienced distinct tectonic evolutions and dynamic processes since the Paleozoic. The current uppermost mantle has been imprinted by these tectonic and dynamic processes. Fast Pn velocities are prominent beneath the Bohai Bay Basin in the Eastern Block of the North China Craton, suggesting residuals of the Archean lithospheric mantle. Beneath the Tanlu Fault Zone and Bohai Sea, slow Pn velocities are present in the uppermost mantle, which can be attributed to significant lithospheric thinning and asthenospheric upwelling. The newly formed lithospheric mantle beneath Yanshan Mountain may be the dominant reason for the existence of slow Pn velocities in this region. Conversely, the ancient lower crust and lithospheric mantle already have been delaminated. In the Central Block, significant slow Pn velocities are present in Taihangshan Mountain, which also extends northward to the Yinchuan-Hetao Rift on the northern margin of the Ordos Block and Yinshan Orogen. This characteristic probably is a result of hot asthenospheric upwelling along the active tectonic boundary on the margin of the Western Block. The protracted thermal erosion and underplating of hot asthenospheric upwelling may induce lithospheric thinning and significant slow velocities in the uppermost mantle. Fast velocities beneath the Western Block suggest that the thick, cold and refractory Archean lithospheric keel of craton still is retained without apparent destruction.

Reciprocal sound transmission measurement of mean current and temperature variations in the central part (Aki-nada) of the Seto Inland Sea, Japan

A 30 km-range reciprocal sound transmission experiment was carried out on the line connecting Honshu and Shikoku (the first and fourth biggest main Japanese islands, respectively) in the central part (Aki-nada) of the Seto Inland Sea, Japan, during March–May 2010 to measure the mean current and temperature variations over the sea. The range-averaged current along the sound transmission line was estimated to have a mean and standard deviation of (3.8–4.4) ± (1.7–1.8) cm/s after converting the travel time difference data into currents, including a fortnightly tidal variation in the range of ±30 cm/s. The positive mean current implies slow water movement from the west to east through Aki-nada. The range-averaged speed of sound was estimated by converting from the mean travel time or one-way travel time into the speed of sound, and further converted into temperature for fixed values of salinity and depth, according to the standard speed of sound formula. Besides the precise measurement (to an accuracy of 0.01°C) of semidiurnal and diurnal tidal variations and seasonal warming, the temperature data showed periodic variations with periods of 7.0 and 21.1 days that had never been observed in Aki-nada before. This study suggests that reciprocal sound transmission is a powerful technique for the long-term accurate measurement of mean current and temperature variations in coastal and inland seas.

Long-term measurement of stream flow and salinity in a tidal river by the use of the fluvial acoustic tomography system

Summary Long-term variation of stream flow of a tidal river was measured by an innovative technology, called the fluvial acoustic tomography (FAT). The reciprocal sound transmission was performed between two acoustic stations, located on both sides of the river. Even in the tidal river with the periodic intrusion of salt wedges, the cross-sectional average velocities along the river stream axis, estimated from the travel time difference data, were consistent with the average velocities, observed by an array of moored downward-looking ADCPs. The cross-sectional average salinity was also estimated by using the mean travel time data collected from the reciprocal sound transmission, the mean values of temperature measured by the conductivity–temperature (C–T) sensors, and the ray simulation result. The derived salinity data from the FAT are comparable with that obtained by the C–T sensors. It is concluded that the fluvial acoustic tomography (FAT) is a prospective method for the continuous monitoring of tidal river discharge and temperature/salinity variations.

PP-Pdifferential traveltime measurement with crustal correction

SUMMARY PP-P differential traveltime data are useful for constraining the seismic structure of the upper mantle. The cross-correlation method has been used for these measurements, whereby a synthetic PP waveform is constructed from the observed P and is cross-correlated with the observed PP. PP waveforms are, however, distorted by interference with the precursors and post-cursors produced by reflections and conversions at crustal discontinuities. Accordingly, the measurement of PP-P differential traveltimes can suffer a bias. Synthetic experiments have shown that this bias becomes significant when seismograms are lowpass filtered. To measure PP-P differential times without such bias, we developed a method to take crustal effects into consideration in the cross-correlation method. In this new method, the response of incident P to the crustal structure beneath the PP bounce point is calculated and convolved with the observed P in constructing the synthetic PP waveform. As an experiment we applied this to real seismograms lowpass filtered with corners at 0.5, 0.1 and 0.05 Hz. We took crustal models from the global database CRUST2.0. This experiment shows that the method is effective in reducing the bias in traveltime measurements, even for measurements with long period waveforms below 0.05 Hz. With the assumption that the crustal structure is described by CRUST2.0, the application of our method to the observed seismograms provides PP-P values mutually consistent among the measurements using three different lowpass filters with corners at 0.5, 0.1 and 0.05 Hz.

SS‐wave sensitivity to upper mantle structure: Implications for the mapping of transition zone discontinuity topographies

Studies of the depths of the upper mantle discontinuities have relied on the differential travel times between the SS wave and its precursors. We examine the effect of velocity heterogeneities on these differential travel time data. Ray‐theory, which is used to correct these data for the effect of velocity heterogeneities, is shown to be inaccurate if the characteristic heterogeneity length scales are similar to those seen in regional models or if the station is near the SS wave radiation nodal plane.