Travel Times Of Seismic Waves Research Articles

An Analysis of the Accuracy of Time Domain 3D Image Geology Model Resulted from PSTM and Depth Domain 3D Image Geology Model Resulted from PSDM in Oil and Gas Exploration

This study aims to obtain a geological model which is close to the truth and compare accuracy between the time domain 3D image of the PSTM results with the depth domain 3D image of PSDM results. There are 3 parameters to determine the accuracy of an interval velocity model in the production of a geology model: depth gathering that is already flat, semblance that has concurred with zero residual move-out axes, and depth image which conforms to the marker (well seismic tie). The analytical method employed is Horizon Based Tomography, which is a method to correct the seismic wave travel time error along the analyzed horizon. Reducing errors in the travel time of the seismic wave will decrease depth errors. This improvement is expected to provide correct information about subsurface geological conditions. The results showed that the depth domain image generated by the PSDM process represents the actual geological model better than time domain image produced by the PSTM process, evidenced by the sharpening of the reflector continuity, reduction of pull-up effect, and high resolution.

On Certain Conventional Misconceptions in Seismology

The history of science abounds in examples when misconceptions dominated the scientific community and did it significant harm. New ideas in science have always cut their paths with great difficulty. Inertial thinking and the temptation to adhere to stereotypes often hamper the development of knowledge and lead to erroneous interpretations of experimental data. Often a scientist has to face the skepticism of his colleagues and even the distrust of the community. Moreover, if new knowledge contradicts the established methodological schemes that are generally accepted in a particular scientific field, a creator of innovations can almost inevitably expect attack. Misconceptions inhibit, but sometimes lead to the development of our knowledge in understanding processes occurring deep in the Earth’s interior. These examples, of course, do not exhaust all misconceptions in the earth sciences, and any researcher, if desired, can easily supplement this list. Misconceptions in science culminate in researchers obtaining incorrect results or simply hitting a dead end. Thus, if the coordinates of hypocenters are incorrectly determined, a subsequent chain of incorrect conclusions arises when studying particular regions. In addition to the objective consequences of misconceptions, subjective consequences cause considerable harm. Subjective consequences entail the effect of when individual scientists are held captive by misconceptions—due to ignorance or misunderstanding of individual aspects of certain problems—on the emergence of new ideas and approaches in a particular field. Most often, this manifests itself when scientists review articles and dissertations by their peers. In this article, the author gives several examples of misconceptions, which, in his opinion, are typical of the field in which he has worked for many years. In particular, this concerns misconceptions related to determining the hypocentral coordinates of earthquakes, seismic tomography, the inverse dynamic problem, reversal of seismic wave travel-time curves, etc. Twelve misconceptions are considered in all, which in one way or another affect the development of alternative methods for interpreting seismological data and our understanding of the Earth’s structure.

Travel time tomography with local sparse modeling and dictionary learning

In this talk, we present a machine learning-based approach to 2D travel time tomography. Travel time tomography methods image slowness structures (e.g., Earth geology) from acoustic and seismic wave travel times across sensor arrays. Typically, slowness is obtained via an ill-posed linear inverse problem, which requires regularization to obtain physically plausible solutions. We propose to regularize this inversion by modeling rectangular groups of slowness pixels from the image, called patches, as sparse linear combinations of atoms from a dictionary. In this locally-sparse travel time tomography (LST) method, the dictionary, which represents elemental slowness features, is initially unknown and is learned from the travel time data using an unsupervised machine learning task called dictionary learning. This local model constrains small-scale slowness features, and is combined with a global model, which constrains larger-scale features with L2 regularization. In contrast to conventional regularization, which allows only for smoothness or discontinuous slowness, LST permits increased resolution where warranted by data. A maximum a posteriori formulation ofLST is derived, which is solved as an iterative algorithm. LST performance is evaluated on both synthetic and real data in the context of ambient noise tomography.

Prestack depth imaging in complex structures using VTI fast marching traveltimes

The presence of sedimentary layers in the Earth’s subsurface results in seismic anisotropy, which makes wave velocity dependent on the propagation angle. This phenomenon causes complexities and errors both kinematically and dynamically in seismic imaging. Among these errors are the mispositioning of migrated events and failure to retain energy during dip-moveout. A fundamental and challenging issue in seismic imaging is the computation of seismic wave traveltime from the source to the receiver via the reflection point. A powerful method for determining traveltime is the application of finite difference to solve the eikonal equation. In this study, we employ a fast marching eikonal solver in the isotropic and vertical transverse isotropy (VTI) concepts. We also test the results by using the Kirchhoff depth migration algorithm. Instead of using a linear eikonal equation, which is commonly used in the industry, we consider a nonlinear approximation because it is more realistic and accurate than the former. The Marmousi synthetic data and a real dataset are used for testing purposes. The comparison of isotropic and VTI traveltimes demonstrates a considerable lateral difference among wavefronts. The results of Kirchhoff imaging show that the VTI algorithm generates images with perfect positioning and higher resolution than the isotropic one, specifically in deep areas. Finally, we conclude that our anisotropic approach is stable, fast, and generates high-quality images with accurate details in deep structures.

Commentary on “Earthquake Depths in the Crimea–Black Sea Region” by V.E. Kulchitsky, B.G. Pustovitenko and V.A. Svidlova

It is well known that the results of determining earthquake parameters depend to a large extent on data processing algorithms and velocity models of the seismic wave propagation medium used in solving hypocenter problems. In 1992, V.Yu. Burmin developed a hypocentric algorithm that minimizes the functional of distances between the points corresponding to the theoretical and observed travel times of seismic waves from an earthquake source to recording stations. The determination of the coordinates of earthquake hypocenters in this case is much more stable than for the commonly used minimization of the functional of discrepancies in the seismic wave arrival times at a station. Using this algorithm and the refined velocity model of the medium, V.Yu. Burmin and L.A. Shumlyanskaya reinterpreted the earthquake parameters for the Crimea–Black Sea region. The most important result of this reinterpretation was the conclusion about the occurrence of deep earthquakes with a source depth of more than 60 km in the region. This result contradicts the conventional beliefs about the seismicity of the region and therefore aroused strong criticism from experts directly involved in compiling the existing catalogs of regional earthquakes. These comments and criticisms are presented by V.E. Kulchitsky with coauthors in a work published in this issue of the journal. In the present paper, we analyze the comments in detail and respond. In particular, we show that the previously used methods of seismic data processing made it highly unlikely by default that deep earthquakes would appear in the results. As an example, we refer to the use of travel-time curves for depths down to 35 km. It is clear that deep earthquakes could not have been found with this approach.

Commentary on “Once More about Deep Earthquakes in the Caucasus” by A.A. Godzikovskaya

In 2016, V.Yu. Burmin with coauthors published the results of a new interpretation of the data from seismological bulletins on the territory of Armenia. In the reinterpretation, the initial data were thoroughly verified and discarded, and an original earthquake location algorithm developed by the author was used. The algorithm minimizes the functional of distances between points (X, Y, H) and (x, y, h) corresponding to theoretical t i and observed $${\tilde t_{\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{i} }}$$ travel times of seismic waves from earthquake sources to recording stations. As a result, it was found that, in addition to crustal earthquakes, deep earthquakes with a source depth of down to 300 km also occurred. This unconventional result has raised objections from some seismologists, which are stated in the work by A.A. Godzikovskaya, published in this issue of the journal. In present paper, we respond to the objections by A.A. Godzikovskaya. Among her main arguments is the difference in the form with which deep and crustal earthquakes are recorded. However, the form of recording depends not only on the source depth, but also on the velocity structure of the medium beneath the seismic station and the source mechanism, which determines the pattern of seismic energy radiation from the source, and on the duration of the faulting process in the source itself. The question of the typical form for recording seismic waves of earthquakes with different mechanisms and depths for this region requires special study.

On the Problem of Deep Earthquakes in Crimea–Black Sea Region

It is a common opinion that only crustal earthquakes can occur in the Crimea–Black Sea region. Since the existence of deep earthquakes in the Crimea–Black Sea region is extremely important for the construction of a geodynamic model for this region, an attempt is made to verify the validity of this widespread view. To do this, the coordinates of all earthquakes recorded by the stations of the Crimean seismological network are reinterpreted with an algorithm developed by one of the authors. The data published in the seismological catalogs and bulletins of the Crimea–Black Sea region for 1970–2012 are used for the analysis. To refine the coordinates of hypocenters of earthquakes in the Crimea–Black Sea region, in addition to the data from stations of the Crimean seismological network, information from seismic stations located around the Black Sea coast are used. In total, the data from 61 seismic stations were used to determine the hypocenter coordinates. The used earthquake catalogs for 1970–2012 contain information on ~2140 events with magnitudes from–1.5 to 5.5. The bulletins provide information on the arrival times of P- and S-waves at seismic stations for 1988 events recorded by three or more stations. The principal innovation of this study is the use of the original author’s hypocenter determination algorithm, which minimizes the functional of distances between the points (X, Y, H) and (x, y, h) corresponding to the theoretical and observed seismic wave travel times from the earthquake source to the recording stations. The determination of the coordinates of earthquake hypocenters is much more stable in this case than the usual minimization of the residual functional for the arrival time of an earthquake wave at a station (the difference between the theoretical and observed values). Since determination of the hypocenter coordinates can be influenced by the chosen velocity column beneath each station, special attention is focused on collecting information on velocity profiles. To evaluate the influence of the upper mantle on the results of calculating the velocity model, two different low-velocity and high-velocity models are used; the results are compared with each other. Both velocity models are set to a depth of 640 km, which is fundamentally important in determining hypocenters for deep earthquakes. Studies of the Crimea–Black Sea region have revealed more than 70 earthquakes with a source depth of more than 60 km. The adequacy of the obtained depth values is confirmed by the results of comparing the initial experimental data from the bulletins with the theoretical travel-time curves for earthquake sources with depths of 50 and 200 km. The sources of deep earthquakes found in the Crimea–Black Sea region significantly change our understanding of the structure and geotectonics of this region.

An upwind fast sweeping scheme for calculating seismic wave first‐arrival travel times for models with an irregular free surface

ABSTRACTThe topography‐dependent eikonal equation formulated in a curvilinear coordinate system has recently been established and revealed as being effective in calculating first‐arrival travel times of seismic waves in an Earth model with an irregular free surface. The Lax–Friedrichs sweeping scheme, widely used in previous studies as for approximating the topography‐dependent eikonal equation viscosity solutions, is more dissipative and needs a much higher number of iterations to converge. Furthermore, the required number of iterations grows with the grid refinement and results in heavy computation in dense grids, which hampers the application of the Lax–Friedrichs sweeping scheme to seismic wave travel‐time calculation and high‐resolution imaging. In this paper, we introduce a new upwind fast sweeping solver by discretising the Legendre transform of the numerical Hamiltonian of the topography‐dependent eikonal equation using an explicit formula. The minimisation related to the Legendre transform in the sweeping scheme is solved analytically, which proved to be much more efficient than the Lax–Friedrichs algorithm in solving the topography‐dependent eikonal equation. Several numerical experiments demonstrate that the new upwind fast sweeping method converges and achieves much better accuracy after a finite number of iterations, independently of the mesh size, which makes it an efficient and robust tool for calculating travel times in the presence of a non‐flat free surface.

The influence of one-dimensional velocity sections on determining the key parameters of the earthquake sources in Azerbaijan

The paper addresses the construction of one-dimensional (1D) velocity models in the seismogenic regions of Azerbaijan taken individually and the analysis of implications of these models for estimating the key parameters of earthquake sources in Azerbaijan. We considered and analyzed the seismological data from the local earthquakes, the arrival times of the P-, P-g, Pn-, S-, Sg-, and Sn-waves recorded by the network of telemetry stations during the period from 2005 to 2014 with ml ≥ 2.5. For constructing the models, we used the VELEST program which calculates 1D velocity models from travel times of seismic waves. As a result, the 1D models were built for ten regions of Azerbaijan; the key parameters of the hypocenters of the earthquakes were recalculated; and the corrections to the body-wave arrival times at the observation stations were obtained, which increased the accuracy of locating the hypocenter of earthquakes.

Dynamics of axial torsional libration under the mantle‐inner core gravitational interaction

AbstractThe aims of this paper are (i) formulating the dynamics of the mantle‐inner core gravitational (MICG) interaction in terms of the spherical‐harmonic multipoles of mass density. The modeled MICG system is composed of two concentric rigid bodies (mantle and inner core) of near‐spherical but otherwise heterogeneous configuration, with a fluid outer core in between playing a passive role. We derive the general equation of motion for the vector rotation but only focus on the polar component that describes the MICG axial torsional libration. The torsion constant and hence the square of the natural frequency of the libration is proportional to the product of the equatorial ellipticities of the mantle and inner‐core geoid embodied in their multipoles (of two different types) of degree 2 and order 2 (such as the Large Low‐Shear‐Velocity Provinces above the core‐mantle boundary) and (ii) studying the geophysical implications upon equating the said MICG libration to the steady 6 year oscillation that are observed in the Earth's spin rate or the length‐of‐day variation (ΔLOD). In particular, the MICG torsion constant is found to be = CIC σz2 ≈ 6.5 × 1019 N m, while the inner core's (BIC − AIC) ≈ 1.08 × 1031 kg m2 gives the inner core triaxiality (BIC − AIC)/CIC ≈ 1.8 × 10−4, about 8 times the whole‐Earth value. It is also asserted that the required inner‐core ellipticity amounts to no more than ~140 m in geoid height, much smaller than the sensitivity required for the seismic wave travel time to resolve the variation of the inner core.

The imprint of crustal density heterogeneities on regional seismic wave propagation

Abstract. Density heterogeneities are the source of mass transport in the Earth. However, the 3-D density structure remains poorly constrained because travel times of seismic waves are only weakly sensitive to density. Inspired by recent developments in seismic waveform tomography, we investigate whether the visibility of 3-D density heterogeneities may be improved by inverting not only travel times of specific seismic phases but complete seismograms.As a first step in this direction, we perform numerical experiments to estimate the effect of 3-D crustal density heterogeneities on regional seismic wave propagation. While a finite number of numerical experiments may not capture the full range of possible scenarios, our results still indicate that realistic crustal density variations may lead to travel-time shifts of up to ∼ 1 s and amplitude variations of several tens of percent over propagation distances of ∼ 1000 km. Both amplitude and travel-time variations increase with increasing epicentral distance and increasing medium complexity, i.e. decreasing correlation length of the heterogeneities. They are practically negligible when the correlation length of the heterogeneities is much larger than the wavelength. However, when the correlation length approaches the wavelength, density-induced waveform perturbations become prominent. Recent regional-scale full-waveform inversions that resolve structure at the scale of a wavelength already reach this regime.Our numerical experiments suggest that waveform perturbations induced by realistic crustal density variations can be observed in high-quality regional seismic data. While density-induced travel-time differences will often be small, amplitude variations exceeding ±10 % are comparable to those induced by 3-D velocity structure and attenuation. While these results certainly encourage more research on the development of 3-D density tomography, they also suggest that current full-waveform inversions that use amplitude information may be biased due to the neglect of 3-D variations in density.

Thermodynamic properties of Fe-S alloys from molecular dynamics modeling: Implications for the lunar fluid core

Density and sound velocity of Fe-S liquids for the P-T parameters of the lunar core have not been constrained well. From the analysis of seismic wave travel time, Weber et al. (2011) proposed that the lunar core is composed of iron alloyed with ⩽6wt% of light elements, such as S. A controversial issue in models of planetary core composition concerns whether Fe-S liquids under high pressure – temperature conditions provide sound velocity and density data, which match the seismic model. Here we report the results of molecular dynamics (MD) simulations of iron-sulfur alloys based on Embedded Atom Model (EAM). The results of calculations include caloric, thermal and elastic properties of Fe-S alloys at concentrations of sulfur 0–18at.%, temperatures up to 2500K and pressures up to 14GPa. The effect of sulfur on the elastic properties of Fe-rich melts is most evident in the notably decreased density with added S content. In the MD simulation, the density and bulk modulus KT of liquid Fe-S decrease with increasing sulfur content, while the bulk modulus KS decreases as a whole but has some fluctuations with increasing sulfur content. The sound velocity increases with increasing pressure, but depends weakly on temperature and the concentration of sulfur. For a fluid Fe-S core of the Moon (∼5GPa/2000K) with 6–16at.% S (3.5–10wt%), the sound velocity and density may be estimated at the level of 4000ms−1 and 6.25–7.0gcm−3. Comparison of thermodynamic calculations with the results of interpretation of seismic observations shows good agreement of P-wave velocities in the liquid outer core, while the core density does not match the seismic models. At such concentrations of sulfur and a density by 20–35% higher than the model seismic density, a radius for the fluid outer core should be less than about 330km found by Weber et al. because at the specified mass and moment of inertia values of the Moon an increase of the core density leads to a decrease of the core radius. These results are important for modeling the current state and composition of the lunar core and can be applied to models of large satellites’ core properties.

Statistical performance of CO2 leakage detection using seismic travel time measurements

AbstractMonitoring for possible CO2 leakage is an important part of a safe and effective geological sequestration program. Seismic monitoring has been implemented in several pilot sequestration sites for site characterization and CO2 leakage detection. This study evaluates the detection power of seismic wave travel time measurements and statistical tests at different CO2 leakage rate levels. A simplified rock physics model is assumed for monitoring zones at sequestration sites and the effects of leakage‐induced changes in pressure and CO2 saturation on P‐wave travel times are modeled. The empirical distributions of detection power using the P‐wave travel time for four regions in the permeability‐porosity input space at four leakage levels are obtained from the Monte Carlo uncertainty analysis with a stochastic response surface method. The detection power using the P‐wave travel time measurements and test alone is generally not high enough, unless the porosity and the permeability of the monitoring zone are high, and/or a long period of time has elapsed since the leakage occurred. For monitoring layers with lower permeability and porosity, measurements from other monitoring techniques will likely be needed to increase the probability that leakage events are detected and addressed in a timely manner. © 2015 Society of Chemical Industry and John Wiley & Sons, Ltd

The structural setting of the Ischia Island Caldera (Italy): first evidence from seismic and gravity data

Ischia Island is one of the active volcanoes of the Neapolitan area (Italy). Hazard assessment of active, densely populated volcano is primarily based on knowledge of the volcano’s past behaviour and of its present state. As a contribution to the definition of the present structural setting of Ischia Island, we constructed a new model of the shallow crust using geophysical data: seismic wave travel times and Bouguer anomaly data. We analysed these data sets through seismic tomography and gravity data inversion. The main results inferable from the 3D seismic and gravity images are the definition of the caldera rim along the perimeter of the island, as hypothesized by many authors, and the presence of a high velocity and density area inside the caldera consistent with extension of the resurgent block that characterizes the recent deformation of the island.

Seismic Wave Velocities in Deep Sediments in Poland: Borehole and Refraction Data Compilation

Sedimentary cover has significant influence on seismic wave travel times and knowing its structure is of great importance for studying deeper structures of the Earth. Seismic tomography is one of the methods that require good knowledge of seismic velocities in sediments and unfortunately by itself cannot provide detailed information about distribution of seismic velocities in sedimentary cover. This paper presents results of P-wave velocity analysis in the old Paleozoic sediments in area of Polish Lowland, Folded Area, and all sediments in complicated area of the Carpathian Mountains in Poland. Due to location on conjunction of three major tectonic units — the Precambrian East European Craton, the Paleozoic Platform of Central and Western Europe, and the Alpine orogen represented by the Carpathian Mountains the maximum depth of these sediments reaches up to 25 000 m in the Carpathian Mountains. Seismic velocities based on 492 deep boreholes with vertical seismic profiling and a total of 741 vertical seismic profiles taken from 29 seismic refraction profiles are analyzed separately for 14 geologically different units. For each unit, velocity versus depth relations are approximated by second or third order polynomials.

The density and Q-factor models based on the new data on the nutations and overtones of the free oscillations of the earth: 1. The analysis of the new GSN data for the Sumatra, Tohoku, and Okhotsk earthquakes

We discuss the problem of the ambiguity of gravity inversion, i.e., finding the depth distribution of density and the depth and frequency dependences of the Q-factor from the entire set of the present-day seismic and astrometric data on the travel times of seismic waves, the periods and attenuation factors of the free oscillations of the Earth, as well as the amplitudes and phases of the forced nutations. In the first part of the paper, we present the new and more accurate determinations of the periods and Q-factors for the fundamental tones and overtones of the spheroidal and toroidal oscillations of the Earth, which have periods longer than 3 min. These determinations are based on analyzing the signals from the Sumatra, Tohoku, and Okhotsk earthquakes of magnitude 9, which were recorded by the stations of the Global Seismographic Network (GSN) in Obninsk and Kazakhstan. It is shown that, although the Okhotsk earthquake had a lower magnitude than the other quakes analyzed (since its seismic source was extremely deep (about 600 km)), the amplitudes of the overtones excited by this event are significantly higher than the amplitudes of the overtones caused by the Sumatra and Tohoku events of magnitude 9. Moreover, the amplitudes of the overtones from the Okhotsk earthquake exceed the amplitudes of the overtones of the free oscillations caused by the other seismic events of magnitude 9 that occurred in the second half of the 20th century. Due to this, the data on the Okhotsk Sea earthquake are of utmost importance for the solution of the inverse problems of reconstructing the vertical profiles of Q-factor in the ultra-low frequency (ULF) range and for reconstructing the vertical distribution of density. Based on the new empirical data, we obtained new and more accurate estimates for the periods and attenuation factors of the free oscillations of the Earth.

Applying the Horizon Based Tomography Method to Update Interval Velocity Model, Identify The Structure of Pre-Stack Depth Migration 3D and Estimate The Hydrocarbon Reserve In SBI Field of North West Java Basin

Seismic data processing is one of the three stages in the seismic method that has an important role in the exploration of oil and gas. Without good data processing, it is impossible to get seismic image cross section for good interpretation. A research using seismic data processing was done to update the velocity model by horizon based tomography method in SBI Field, North West Java Basin. This method reduces error of seismic wave travel time through the analyzed horizon because the existence velocity of high lateral variation in research area. There are three parameters used to determine the accuracy of the resulting interval velocity model, namely, flat depth gathers, semblance residual moveout that coincides with the axis zero residual moveout, and the correspondence between image depth (horizon) with wells marker (well seismic tie). Pre Stack Depth Migration (PSDM) form interval velocity model and updating using horizon-based tomography method gives better imaging of under-surfaced structure results than PSDM before using tomography. There are three faults found in the research area, two normal faults have southwest-northeast strike and the other has northwest-southeast strike. The thickness of reservoir in SBI field, North West Java Basin, is predicted between 71 to 175 meters and the hydrocarbon (oil) reserve is predicted about with 22.6% porosity and 70.7% water saturation.

Near-Surface Multi-Point Vibration Location Method Based on Seismic Depth Migration

This paper presents a method for multi-point vibration location near-surface. Unlike traditional source location technologies, it is not using travel times of seismic waves for positioning calculation, but according to the entire seismic data record, using the wave equation migration method to calculate the source location. Similar to exploration seismic, the records from a survey line within a certain period of time are data volumes with dimensions of time and ground coordinates. Based on the data, combined with surface seismic wave propagation characteristics, by using the improved seismic depth migration algorithm, the vibration energy will return to the starting position where exactly the source location is. The method can solve the problem of location calculation error by using traditional method when several vibration at the same time or continuous vibration occurs at some point.

PROPAGATION OF SEISMIC WAVES THROUGH A SPATIO-TEMPORALLY FLUCTUATING MEDIUM: HOMOGENIZATION

Measurements of seismic wave travel times at the photosphere of the Sun have enabled inferences of its interior structure and dynamics. In interpreting these measurements, the simplifying assumption that waves propagate through a temporally stationary medium is almost universally invoked. However, the Sun is in a constant state of evolution, on a broad range of spatio-temporal scales. At the zero wavelength limit, i.e., when the wavelength is much shorter than the scale over which the medium varies, the WKBJ (ray) approximation may be applied. Here, we address the other asymptotic end of the spectrum, the infinite wavelength limit, using the technique of homogenization. We apply homogenization to scenarios where waves are propagating through rapidly varying media (spatially and temporally), and derive effective models for the media. One consequence is that a scalar sound speed becomes a tensorial wavespeed in the effective model and anisotropies can be induced depending on the nature of the perturbation. The second term in this asymptotic two-scale expansion, the so-called corrector, contains contributions due to higher-order scattering, leading to the decoherence of the wavefield. This decoherence may be causally linked to the observed wave attenuation in the Sun. Although the examples we consider here consist of periodic arrays of perturbations to the background, homogenization may be extended to ergodic and stationary random media. This method may have broad implications for the manner in which we interpret seismic measurements in the Sun and for modeling the effects of granulation on the scattering of waves and distortion of normal-mode eigenfunctions.

The effect of inhomogeneous water on seismic imaging in deepwater areas of the South China Sea

This paper examines the effect of inhomogeneous water on seismic imaging in deep water areas. An appropriate partial differential equation is derived for the acoustic pressure field in inhomogeneous water, including current effects. Seismic wavefields are simulated and the results show that the traveltime of seismic waves can be affected. The maximum traveltime perturbation at zero offset is 20 ms. In particular, the structure of horizontal reflectors below the water is distorted by mesoscale eddies. Variations of water temperature are the fundamental cause of the distortion. Finally, a calibration method for the distortion of seismic imaging caused by inhomogeneous water is presented.