Amplitude Anomalies Research Articles

Direct hydrocarbon detection (DHI) techniques for prospective exploration targets in Chaoshan Depression

Located in northern South China Sea, Pearl River Mouth Basin is a large Meso-Cenozoic basin. And in southeasters part of the basin lies Chaoshan Depression, which is mainly a residual Mesozoic depression, with a construction of Meso-Cenozoic strata over 7000m thick and good hydrocarbon accumulation conditions. The selected DHI technique, amplitude attribute of -90 degree phase component derived by phase decomposition is employed to the zone of interest (ZOI) of B structure, Chaoshan Depression. And it is found that there are evident amplitude anomalies occurring around ZOI of structure B. Phase decomposition is applied to forward modelling results of the ZOI, and high amplitudes occur on the -90 degree phase component more or less when ZOI is charged with hydrocarbon, which shows that the amplitude abnormality in ZOIs of structure B is probably caused by oil and gas accumulation. Meanwhile, another DHI attribute of spectral decomposition peak amplitude is applied also to ZOIs of B structure, and there is also amplitude anomalies coincides with those of -90 degree phase component. The two DHI attributes’ coincidence strengthens the confidence of hydrocarbon accumulation prediction results.

Contrasting interannual impacts of European and Greenland blockings on the winter North Atlantic storm track

Based on the daily reanalysis, the present study examines the interannual influences of the European blocking (EB) and Greenland blocking (GB) frequencies on the winter North Atlantic storm track (WNAST) intensity. There are contrasting relationships between the two types of atmospheric blockings and the WNAST. The EB and GB frequencies are significantly positively and negatively related to the WNAST intensity, respectively. Composite analysis shows a meridional dipole of the WNAST anomalies associated with both frequent EB and GB. The area of EB-related positive WNAST anomalies is obviously larger than that of the negative anomalies, which directly results in their in-phase relationship. However, the amplitudes of the GB-related negative WNAST anomalies that are larger than those of the positive anomalies directly lead to their out-of-phase relationship. The EB-related intensified WNAST is dependent on the enhanced baroclinic energy conversion and the strengthened atmospheric baroclinicity induced by the anomalous westerly winds on the northern side of the EB-related anticyclone. In contrast, the GB-related weakened WNAST is determined by the attenuated atmospheric baroclinicity and baroclinic energy conversion on the southern flank of the WNAST climatology.

Full-Waveform Inversion of High-Frequency Teleseismic Body Waves Based on Multiple Plane-Wave Incidence: Methods and Practical Applications

ABSTRACTTeleseismic full-waveform inversion has recently been applied to image subwavelength-scale lithospheric structures (typically a few tens of kilometers) by utilizing hybrid methods in which an efficient solver for the 1D background model is coupled with a full numerical solver for a small 3D target region. Among these hybrid methods, the coupling of the frequency–wavenumber technique with the spectral element method is one of the most computationally efficient ones. However, it is normally based on a single plane-wave incidence, and thus cannot synthesize secondary global phases generated at interfaces outside the target area. To remedy the situation, we propose to use a multiple plane-wave injection method to include secondary global phases in the hybrid modeling. We investigate the performance of the teleseismic full-waveform inversion based on single and multiple plane-wave incidence through an application in the western Pyrenees and compare it with previously published images and the inversion based on a global hybrid method. In addition, we also test the influence of Earth’s spherical curvature on the tomographic results. Our results demonstrate that the teleseismic full-waveform inversion based on a single plane-wave incidence can reveal complex lithospheric structures similar to those imaged using a global hybrid method and is reliable for practical tomography for small regions with an aperture of a few hundred kilometers. However, neglecting the Earth’s spherical curvature and secondary phases leads to errors on the recovered amplitudes of velocity anomalies (e.g., about 2.8% difference for density and VS, and 4.2% for VP on average). These errors can be reduced by adopting a spherical mesh and injecting multiple plane waves in the frequency–wavenumber-based hybrid method. The proposed plane-wave teleseismic full-waveform inversion is promising for mapping subwavelength-scale seismic structures using high-frequency teleseismic body waves (>1 Hz) including coda waves recorded at large N seismic arrays.

“Tiny Wiggles” in the Late Miocene Red Clay Deposits in the North‐East of the Tibetan Plateau

AbstractSmall amplitude or short period geomagnetic anomalies known as “tiny wiggles” (TWs) are often hard to identify because of magnetic signal smoothing in the marine record of geomagnetic reversals. We report here the late Miocene record of geomagnetic reversals in the aeolian red clay sediments of Linxia Basin in China that enables us to identify two TWs. We performed magnetostratigraphy dating and used spectral analysis to distinguish orbital cycles in the records of magnetic susceptibility (MS) and sedimentary grain size (GS) and develop an orbitally tuned age model. The presence of two TWs in the study section, that correspond to C5n.2n‐3 and C5r.2r‐1, is confirmed by orbital calibration of our age model through recognition of eccentricity, obliquity and precession in MS and GS records.

Investigation of the relationship between Euro-Atlantic and West Asia blockings and major stratospheric sudden warmings in the period of 1959–2018

The main aim of this paper is to investigate and compare the possible connection between tropospheric blocking events and major stratospheric sudden warmings (SSWs) in the two periods of 1959−1988 and 1989−2018 to present a dynamical insight into the blocking formation and behaviors. After identifying and characterizing two types of SSWs including wavenumber-1 (W1) and wavenumber-2 (W2) in both the periods, the behaviors of blocking events coincided with major SSWs are examined and compared in the two periods. Then, the relationship between blockings and major SSWs is discussed applying the Empirical Orthogonal Function (EOF) method.In general, 18 and 16 major SSW events were identified in the first and second periods, respectively. The investigation of planetary wave activities indicates that the maximum anomalies of geopotential height amplitude and meridional eddy heat flux in the PRE-SSW phase of both types of SSWs, particularly major W2 warmings, in the second period occurred earlier than those in the first period. The peaks of blocking activities in the second period have also been moved eastward compared with the first period in both prior to and after the onset of SSWs. Moreover, the frequency of blocking event during the PRE-SSW phase of major W1 warmings in Euro-Atlantic and a large part of West Asia in the second period is less than that of the first period, while the occurrence of blocking during the PRE-SSW phase of major W2 warmings in Eastern Europe and West Asia has been increased in the second period. In the POST-SSW phase, blocking activity associated with major W2 warmings is enhanced in West Asia during the second period. In addition, the maximum blocking activities preceding major SSWs in the second period was occurred 5-days prior to the first period. The results of cross-correlation coefficients between blockings and SSWs show significant relationship between them with time lag of about 10-days prior to the onset of warmings in both the periods, especially in the recent years.

Improved imaging of gas hydrate reservoirs and their plumbing system using 2D elastic full-waveform inversion

Gas hydrates are ice-like crystalline materials that form under submarine environments of moderate pressure and low temperature. Another key factor to their formation is the abundance in gas supply from depth in addition to local biogenic gas. Detailed imaging and velocity analysis of the plumbing system of gas hydrates can provide confidence that amplitude anomalies in seismic data are related to gas hydrate accumulations. We have conducted 2D elastic full-waveform inversion (FWI) along a 14 km long segment of a 2D multichannel seismic profile to obtain a high-resolution velocity model of a hydrate system on the southern Hikurangi margin. We compare the FWI velocity model to previously published semblance- and tomography-based velocity models from the same data to explore how much more can be gained from the FWI. The FWI yielded a structurally more accurate velocity model that better delineated the low-velocity zone associated with free gas beneath the bottom simulating reflector (BSR) compared to the semblance- and tomography-based velocity models. Our results also find a lateral velocity inversion, that is, a narrow low-velocity zone surrounded by bands of higher velocities at a seaward-verging protothrust fault, which the two other methodologies failed to resolve. The FWI provides an improved lateral resolution making it an important tool when imaging the “plumbing” systems of gas hydrate reservoirs. In the southeastern limb of the anticline, our results find that the closely spaced landward-vergent protothrusts provide gas-charged fluids for hydrate formation above the BSR. Moreover, at the center of the anticline, our results find that a seaward-vergent protothrust fault appears to be acting as a conduit for gas-rich fluids into strata, although there is no accumulation of any significant hydrate above the BSR at the apex of the anticline. Our finding emphasizes the significance of densely spaced faults and fractures for providing gas for hydrate formation in the hydrate stability zone.

ГЕОДИНАМІКА

Interest in research on the detection of earthquake (EQ) precursors is growing year by year. In this direction, the paper analysed the results of earlier studies, as well as positive results of some studies conducted in the last 5 years. In particular, during the study of EQs, ultra-low frequency (ULF) precursors attract special attention. The study compared the results of electromagnetic (EM) monitoring studies conducted in the ULF range in earlier years and the results of EM monitoring studies conducted in the last 5 years have been compared. The positive results of the researchers investigating the changes in the EM field before the EQ in the ULF range were reviewed. Thus, ULF anomalies from relatively weak (with 4<Mw<5) and shallow (with a depth of less than 50 km) EQs were repeatedly observed in 2017 in Indonesia. Before strong EQs, ULF promising EQ precursors were revealed. High ULF amplitude anomalies were recorded before the 2011 Tohoku megaEQ. Anomalous changes of the Earth's induction vector were identified in 6 observatories in Japan. Similar anomalies were also recorded in the ULF range (0.001-0.083 Hz) by the Teoloyucan (Mexico) and Tucson (the United States) geomagnetic observatories from August 1 to September 16, 2017, before the Chiapas EQ in Mexico with a magnitude 8.1. On the whole, the research discovered several dozen EM precursors of EQs with different amplitude, spectral and time parameters. The study was based on the analysis of numerous data for the periods 1976-2010 and 2007-2016 conducted by various researchers. In addition, an original approach is proposed. It consists in the study of geoelectric field changes (ULF precursors of EQs) as they are more sensitive. Processing and interpreting these changes can lead to precise detection of EQ precursors. Thus, this makes it possible to identify geodynamic active zones in which an EQ may occur.

Investigation of Scale-Dependent Groundwater/Surface-water Exchange in Rivers by Gradient Self-Potential Logging: Numerical Modeling and Field Experiments

Exchanges of groundwater and surface-water are fundamental to a wide range of water-supply and water-quality management issues but challenging to map beyond the reach scale. Waterborne gradient self-potential (SP) measurements are directly sensitive to water flow through riverbed sediments and can be used to infer exchange locations, direction (gain versus loss), scale, and relative changes, but to date applications to river corridor hydrology are limited. Numerical modeling and field experiments were therefore performed herein, each emphasizing waterborne gradient SP logging for identifying and locating focused vertical groundwater discharge (surface-water gain) and recharge (surface-water loss) in a river. Two and three-dimensional numerical models were constructed to simulate the polarities, appearances, and peak amplitudes of streaming-potential and electric-field anomalies on a riverbed and in the surface-water that were attributable to steady-state vertical fluxes of groundwater through high-permeability conduits in the riverbed. Effects of varied hydraulic length-scale of exchange and surface-water depth were tested through numerical modeling. Modeling results aided in data acquisition and interpretation for three repeated field experiments performed along a 1.5–2.0 km reach of the Quashnet River in Cape Cod, Massachusetts, where focused, meter-scale groundwater discharges occur at discrete locations within otherwise ubiquitous and more diffuse groundwater upwelling conditions. Strong gradient SP anomalies were repeatedly measured in the Quashnet River at previously confirmed locations of focused groundwater discharge, showing the efficacy of waterborne gradient SP logging in identifying and characterizing groundwater/surface water exchange dynamics at multiple river network scales.

On the drape and level flying aeromagnetic survey modes with terrain effects, and data reduction between arbitrary surfaces

In addition to the physical parameters such as magnetization intensity distribution, the volume and the shape of the magnetized material, directions of the magnetization and the ambient field, the distance between the observation surface and the causative sources significantly affects the shape and the amplitudes of the magnetic anomalies. Aeromagnetic surveys are performed using either a draped surface or a constant elevation plane above sea level. These surveys can easily reconnoiter large territories in a short time. However, the magnetic anomalies may be attenuated resulting in some losses in the data resolution based on the flight height of the survey. In this paper, these effects were investigated in a detailed manner through some synthetic anomalies generated from 2D and 3D hypothetical subsurface models. Besides, magnetic terrain effects were also examined in the synthetic simulations which were produced for different scenarios. Real aeromagnetic anomalies obtained using a drape flying survey mode over the rugged high topographic relief of the Mount Nemrut stratovolcano (Bitlis, eastern Turkey) and its close vicinity were also investigated. Numerical simulations show that although both data acquisition modes have some weak sides, the level flying mode is more advantageous than the drape flying mode in general. Better anomaly interpretation can be achieved by reducing the draped data set into the one observed over a horizontal plane or vice versa and comparing these two data sets. Lastly, a simple computational process which can be performed in the Fourier wavenumber domain is proposed for data reduction procedure.

Anomaly Detection in Seismic Data–Metadata Using Simple Machine-Learning Models

Abstract In modern seismological analysis, it is not unusual to process huge amounts of data, as illustrated by two case studies exemplified in this work, both assessing the quality of several millions of segments selected for computing local and energy magnitudes. In this scenario, quality control tools to filter, discard, or rank data are of extreme importance and should ideally be simple, fast, and generalizable. Using machine-learning tools, we present here a simple and efficient model based on the isolation forest algorithm for detecting amplitude anomalies on any seismic waveform segment, with no restriction on the segment record content (earthquake vs. noise) and no additional requirements than the segment metadata. By considering a simple feature space composed of amplitudes of each segment’s power spectral density (PSD) evaluated at selected periods suitable for both local and teleseismic applications, feature selection revealed that one single feature, the PSD at 5 s, is sufficient to achieve the best predicting performances. The evaluation results report average precision scores around 0.97, and maximum F1 scores above 0.9, both remarkable results with respect to the simplicity of the approach used and the generality of the problem tackled. The trained model producing the best evaluation results is the backbone of a publicly available software, which computes an amplitude anomaly score in [0, 1] for any given seismic waveform, and can be beneficial in several applications such as discarding anomalies from data sets, ideally in a preprocessing stage, and detecting potential metadata problems on data center side. When applied to our two case studies, the software was revealed to be fast and effective, and the computed anomaly scores allow additional flexibility in addition to the proven wide-range applicability.

Unveiling the mysteries of SST evolutions in the equatorial Pacific at the onset of El Niño events

AbstractIt is generally accepted that the El Niño-Southern Oscillation (ENSO) dominates interannual climate variability. Yet its genesis and maintenance mechanisms are still under intense debate with no scientific consensus. Some authors argued that the westerly winds originating over the equatorial Indian Ocean significantly enhanced and extended eastward in the western and central equatorial Pacific during El Niño events, thus advecting the warm pool eastward along the equator and causing SST anomalies. However, this assertion is unlikely to be quantitatively supported by observational data. Here we present detailed observational data and modeling evidence to demonstrate that the westerly winds remained little changein intensity in the western equatorial Pacific, with a wider zonal extent only during most El Niño events, and with a slight increase even if in the most pronounced 1997 El Niño. Instead, an eastward equatorial current near the equator has been observed and considered to play a significant role in shifting the eastern edge of the warm pool eastward, elevating SSTs in the central and eastern equatorial Pacific and giving rise to El Niño, with the interactions between the eastward warm pool and the upwelling in the eastern cold tongue ascertaining the amplitudes of SST anomalies.

Spiciness Anomalies of Subantarctic Mode Water in the South Indian Ocean

AbstractThis study investigates spreading and generation of spiciness anomalies of the Subantarctic Mode Water (SAMW) located on 26.6 to 26.8 σθ in the south Indian Ocean, using in situ hydrographic observations, satellite measurements, reanalysis datasets, and numerical model output. The amplitude of spiciness anomalies is about 0.03 psu or 0.13°C and tends to be large along the streamline of the subtropical gyre, whose upstream end is the outcrop region south of Australia. The speed of spreading is comparable to that of the mean current, and it takes about a decade for a spiciness anomaly in the outcrop region to spread into the interior up to Madagascar. In the outcrop region, interannual variability in mixed layer temperature and salinity tends to be density compensating, which indicates that Eulerian temperature or salinity changes account for the generation of isopycnal spiciness anomalies. It is known that wintertime temperature and salinity in the surface mixed layer determine the temperature and salinity relationship of a subducted water mass. Considering this, the mixed layer heat budget in the outcrop region is estimated based on the concept of effective mixed layer depth, the result of which shows the primary contribution from horizontal advection. The contributions from Ekman and geostrophic currents are comparable. Ekman flow advection is caused by zonal wind stress anomalies and the resulting meridional Ekman current anomalies, as is pointed out by a previous study. Geostrophic velocity is decomposed into large-scale and mesoscale variability, both of which significantly contribute to horizontal advection.

Hydrocarbon detection based on Phase decomposition in Chaoshan Depression, northern South China Sea

Located in northern South China Sea, Chaoshan Depression is mainly a residual Mesozoic depression, with a construction of Meso-Cenozoic strata over 7000m thick and good hydrocarbon accumulation conditions. Amplitude attribute of -90°phase component derived by phase decomposition is employed to detect Hydrocarbon in the zone of interest (ZOI) in Chaoshan Depression. And it is found that there are evident amplitude anomalies occurring around ZOI. Phase decomposition is applied to forward modeling results of the ZOI, and high amplitudes occur on the -90°phase component more or less when ZOI is charged with hydrocarbon, which shows that the amplitude abnormality in ZOI is probably caused by oil and gas accumulation.

Thickness of sediments in the Congo basin based on the analysis of decompensative gravity anomalies

The Congo basin is one of the largest intracratonic basins in the World, locating within a cold lithospheric plate. The structure of the thick sedimentary layer is investigated by seismic studies only in limited places. Here, we present a map of sedimentary thickness for the whole Congo basin, based on the inversion of the decompensative gravity anomalies. Contrary to the conventional Bouguer or isostatic gravity anomalies, the effect of the isostatic compensation of sediments is reduced in the decompensative anomalies, which provides a possibility to recover the full effect of low-density sediments. The calculated decompensative correction reaches ±70 mGal and exceeds the amplitude of the isostatic anomalies, especially in the long wavelengths. The final decompensative anomalies are negative over the whole basin and their patterns well correspond to its tectonic fragmentation. By inverting these anomalies with the predefined density-depth relationship we have obtained the sedimentary thickness map for the whole Congo basin. The maximum basement depth exceeding 10 km is found in the Lokoro basin and basins in the South. In the Lomami basin, thickness of sediments reaches about 6.5 km. It is important to note, that these deep depressions, are not covered by seismic studies. Furthermore, we found a new deep basin adjacent to the Lokonia High (on the SW side) that we propose to name as the Salonga basin.

Characterization of gas accumulation at a venting gas hydrate system in the Shenhu area, South China Sea

Bottom-simulating reflections (BSRs) in seismic data have been widely accepted to indicate the base of the methane gas hydrate stability zone (MGHSZ), and free gas was thought to exist only below it. However, real geologic systems are far more complex. We have evaluated the results of 3D seismic, logging while drilling, in situ, and coring measurements at a venting gas hydrate system in the Shenhu area of the South China Sea. Our studies reveal that free gas has migrated upward through the thermogenic gas hydrate stability zone into the MGHSZ and become a part of the gas hydrate system. Seismic amplitude anomalies and core results suggest the presence of free gas above the base of MGHSZ at 165 mbsf and the presence of thermogenic gas hydrates below it in well SC-W01. Analyses of P-wave velocity, S-wave velocity, density, and porosity logs reveal that free gas occurs above and below the MGHSZ as well. Integrating log and core analysis with seismic interpretation suggests that the variation in seismic amplitude within the chaotic zone is associated with variable gas saturations, and a large amount of methane and thermogenic gases accumulate near the complex BSRs. We suggest that relative permeability likely plays a significant role in the free-gas distribution and the formation of gas hydrates within a venting gas hydrate system, whereas the effect of dissolved gas short migration is not ignored. Our results have important implications for understanding the accumulation and distribution of gas hydrates and free gas in the venting gas hydrate system and seeps at the seafloor.

The interplay of stratal and vertical migration pathways in shallow hydrocarbon plumbing systems

AbstractHydrocarbon plumbing systems have been extensively documented in the past two decades using high‐resolution 3D seismic data, exploiting the ability of seismic imaging techniques to reveal the subsurface geometry of gas charged sediments. In this paper, we present a detailed study of a hydrocarbon plumbing system from the South China Sea, that involves both vertical and lateral (stratal) hydrocarbon migration in Miocene to Recent clastic sediments that comprise multilayer stacking of thinly layered clays, silts and sands. We show that a transtensive fault system that provides lateral seal for fault‐dip traps of deep Miocene reservoirs, and offers a vertical pathway for migration to shallower silty units. These silty units in turn form a ‘spillway’ in a regional, northward migration path. This path involves filling each shallow fault‐dip trap to spill point towards the fault tips, with stratal migration forced around the outer flanks of the fault‐related folds. Successive fill‐to‐spill leads to a continuous trail of amplitude anomalies that merge into a continuous, larger, gas‐charged anomaly pattern. The migrating gas finally accumulates within a zone bounded by a large boundary fault with full juxtaposition seal. The pattern of anomaly distribution suggests that the hydrocarbon migration has been active in the Late Pleistocene and is probably continuing at the present day. Hence this plumbing system may be one of very few examples described to date in which dynamic hydrocarbon migration pathways have been directly imaged by seismic data.

Conceptual interpretation seismic 3d using rms amplitude and dip-azimuth attribute analysis for identification structure and facies model in physical geographic

Interpretation of seismic 3D data using attributes analysis to determine the condition of the subsurface geology and distribution facies as physical geography study. The results of interpretation formed surface horizon analysis of structures located in the southwest (SW) is the high of the continuous anticline to the northeast (NE) and the structure of the valley are in the south (S) continuously until the northeast (NE). From the results of the RMS amplitude attribute map shows that high amplitude anomalies uneven section southwest (SW). Show areas indicated as coal facies deployment area and assuming a low impedance possibility for the existence of coal facies. The map dip, azimuth, dip-azimuth can be analyzed map depicts stance or azimuth direction and dip (slope) time domain structure with angle pattern shown seismic trace with section 4501 and the arbitrary line a - b and line c - d clarify seen a pattern of fractures (faults) on a small scale where the direction nearly perpendicular to the strike of major fault.

Temperature Variations in Multiple Air Layers before the Mw 6.2 2014 Ludian Earthquake, Yunnan, China

On 3 August 2014, an Mw 6.2 earthquake occurred in Ludian, Yunnan Province, China (27.245° N 103.427° E). This damaging earthquake caused approximately 400 fatalities, 1800 injuries, and the destruction of at least 12,000 houses. Using air temperature data of the National Center for Environmental Prediction (NCEP) and the tidal force fluctuant analysis (TFFA) method, we derive the temperature variations in multiple air layers between before and after the Ludian earthquake. In the spatial range of 30° × 30° (12°–42° N, 88°–118° E) of China, a thermal anomaly appeared only on or near the epicenter before earthquake, and air was heated from the land, then uplifted by a heat flux, and then cooled and dissipated upon rising. With the approaching earthquake, the duration and range of the thermal anomaly during each tidal cycle was found to increase, and the amplitude of the thermal anomaly varied with the tidal force potential: air temperature was found to rise during the negative phase of the tidal force potential, to reach peak at its trough, and to attenuate when the tidal force potential was rising again. A significance test supports the hypothesis that the thermal anomalies are physically related to Ludian earthquakes rather than being coincidences. Based on these results, we argue that the change of air temperature could reflect the stress changes modulated under the tidal force. Moreover, unlike the thermal infrared remote sensing data, the air temperature data provided by NCEP are not affected by clouds, so it has a clear advantage for monitoring the pre-earthquake temperature variation in cloudy areas.

Determining 3D seismic characteristics of the conduit system of the Changchang sag, Qiongdongnan Basin

Although some giant gas fields found in the deepwater area of the Qiongdongnan Basin (QDNB), China, are often associated with mud diapirs and/or gas chimneys, no comprehensive 3D work has been undertaken to characterize them. We have conducted a 3D seismic investigation using the root-mean square, coherence, and instantaneous frequency attributes to provide a better understanding of the conduit systems in the QDNB. The results show that the conduit system that we investigated can be separated vertically into four zones in the following order: (1) a structurally diapiric weak zone at the base, followed by (2) an injected or reinjected sandstones zone, (3) a gas chimney zone, and (4) a mud volcanic zone at the top. The morphology of the structurally weak zone is elliptical, formed by the intersection of northwest–southeast- and nearly east–west-trending tectonic faults. We infer that this zone provides pathways for the ascent of the diapiric mud that was probably sourced by the underlying overpressured mudstones. The injected or reinjected sandstones zone is characterized by high amplitude anomalies, and it was probably fed by the lobes of underlying submarine fans. The gas chimney zone, which is characterized by low frequencies and weak amplitudes, is probably composed of a mixture of uprising mud and free gas formed from the underlying overpressured mudstones, whereas the mud volcano with a Christmas-tree pattern and is composed of a central crater, the southern flank of which is a mudflow, formed when the uprising mud migrating upward through faults got to the paleo seafloor. Finally, we have developed schematic illustrations that would aid in understanding the different stages of the formation and internal architecture of this conduit system.

Time-lapse detection using raypath interferometry

The objective of most seismic time-lapse studies is to detect rock-property changes in a subsurface formation caused by fluid withdrawal or injection, often by comparing seismic reflection images of the subsurface before and after the operation. Because rock-property changes can affect the amplitudes of seismic reflection events associated with the boundaries of the formation, amplitude anomalies are the usual target of time-lapse experiments. Sometimes, however, particularly in harder, less porous rocks, a seismic amplitude anomaly can be relatively small and difficult to detect. There is a secondary time-lapse effect, however, which may be detectable even in the absence of a significant reflectivity anomaly: the time delay of reflections from layers beneath a formation whose wave propagation velocity has been altered by the pore-fluid change. We have developed a near-surface correction technique for land data, which we call joint raypath interferometry, to specifically enhance and detect small time delays between corresponding events on two or more comparable time-lapse seismic images. We demonstrate the technique first on a numerical model, and then on an actual time-lapse field survey in which a reflection amplitude anomaly is difficult to detect.