High-resolution Seismic Imaging Research Articles

ChirCor: A new tool for generating synthetic chirp-sonar seismograms

The long-term successful use of synthetic seismograms in traditional seismic reflection studies suggests that their implementation for high-resolution non-impulsive seismic sources would also be beneficial. We present here ChirCor v1.0, a program for managing physical-log data (P-wave velocity and density) and chirp-sonar seismograms, also equipped with a synthetic-trace generator, that could be used for correlating real seismograms to synthetics. The software, designed with a user-friendly graphic interface, is intended mainly for oceanographers and marine geologists as a tool for correlating high-resolution seismic images and sediment cores. ChirCor v1.0 will be distributed as freeware for non-commercial purposes. We provide instructions regarding the use of the program together with a description of the algorithms implemented in the code; we also include an example of how to operate with the program based on the data collected during the MARMARA2001 marine geological expedition.

High-resolution seismic imaging of gas accumulations and seepage in the sediments of the Ria de Aveiro barrier lagoon (Portugal)

Methane is a powerful greenhouse gas and an important energy source. The global significance and impact in coastal zones of methane gas accumulation and seepage in sediments from coastal lagoon environments are still largely unknown. This paper presents results from four high-resolution seismic surveys carried out in the Ria de Aveiro barrier lagoon (Portugal) in 1999, 2002 and 2003. These comprise three chirp surveys (RIAV99, RIAV02, RIAV02A) and one boomer survey (RIAV03). Evidence of extensive gas accumulation and seepage in tidal channel sediments from the Ria de Aveiro barrier lagoon is presented here for the first time. This evidence includes: acoustic turbidity, enhanced reflections, acoustic blanking, domes, and acoustic plumes in the water column (flares). The stratigraphy and structural framework control the distribution and extent of gas accumulations and seepage in the study area. In these shallow systems, however, tidal altitude variations have a significant impact on gas detection using acoustic methods, by changing the raw amplitude of the enhanced seismic reflections, acoustic turbidity, and acoustic blanking in gas-prone areas. Direct evidence of gas escape from drill holes in the surrounding area has shown that the gas present in the Ria de Aveiro consists of biogenic methane. Most of the gas in the study area was probably generated mainly in Holocene lagoon sediments. Evidence of faults affecting the Mesozoic limestones and clays underlying some of the shallow gas occurrences, and the presence of high-amplitude reflections in these deeper units raise the possibility that some of this gas could have been generated in deeper sedimentary layers, and then migrated upward through the fractured Mesozoic strata.

Late Permian Emeishan Flood Basalts in Southwestern China

Current debates over mantle plumes are centered on three assumptions namely: (1) cylindrical thermally anomalous materials which arise from the core mantle boundary; (2) high temperature of mantle plumes; (3) mantle plumes are stationary. A full test of the validity of these assumptions requires a comprehensive evaluation using techniques including high resolution seismic tomography, petrology and paleomagnetism. It is argued in this paper that the plume hypothesis can fairly well explain first order geologic observations. Geoscientists against the plume hypothesis often tend to over emphasized some details that may be also influenced by other factors, in addition to mantle plumes. Five criteria can be used to identify ancient mantle plumes: (1) crustal uplift prior to volcanism; (2) dyke swarms; (3) volcanic chain; (4) physical characteristics of volcanism and (5) chemical characteristics of plume derived basalts. Our investigation shows that at least three, and perhaps, four of these criteria are met for the Emeishan large igneous province, suggesting that a mantle plume played a central role in the generation of the Emeishan basalts.

Descent of the ancient Farallon slab drives localized mantle flow below the New Madrid seismic zone

The great earthquake sequence that occurred in the central Mississippi River Valley during the winter of 1811–1812 is unprecedented in the historical record of seismicity within stable continental plate interiors. We show, using viscous flow models based on high resolution seismic tomography, that the descent of the ancient Farallon slab into the deep mantle beneath central North America induces a highly localized flow directly below the New Madrid seismic zone (NMSZ). This localization arises because of structural variability in the Farallon slab and the low‐viscosity of the sub‐lithospheric upper mantle, and it represents a heretofore unrecognized and possibly significant driving mechanism for the enigmatic intraplate seismicity.

Development of a flextensional transducer for high resolution seismic imaging

Piezoelectric transducers are considered as potential sources for geophysical applications such as 4D monitoring or shallow subsurface survey, which require high repeatability and precise control of the emitted signal. However, the use of piezoelectric sources is limited by their low power and narrow bandwidth. Flextensional transducers are compact, high power, low frequency, wide‐band projectors used in underwater acoustics. The design, fabrication and test of a flextensional transducer for high resolution seismic imaging are presented in this paper. Circuit modeling is used to discuss the general properties of piezoelectric sources radiating in the ground. Finite element simulations (ATILA code) are conducted to design a wide band 500–1500 Hz flextensional transducer. On‐site tests are performed for a typical configuration of shallow subsurface survey. The prototype transducer is compared to a weight drop which constitutes a well‐known seismic source. By taking advantage of the source repeatability and using specific signal processing, it is found that the flextensional transducer is equivalent to weight drop ranging from 300 to 1300 J. Bandwidth is increased by 200 to 300%, leading to a significant improvement of image resolution.

Crustal P-wave velocity distributions and metallotectonics around the North China Craton

Abstract We have collected seismic data, performed high-resolution seismic tomography in the North China Craton and analysed the relationships between crustal seismic velocity distributions and regional tectonics and metallogenesis. In the upper and middle crust velocity anomalies are distributed along east–west- and NNE–SSW-trending structures. Most belts of Cenozoic mineral deposits including gold in the North China Craton coincide with high-velocity anomalies, and the North China basin coincides with a low-velocity zone. Compared with the upper crust, the low-velocity anomalies in the lower crust are diffuse and extensive, which suggests that high-temperature material has upwelled from the mantle. High-temperature material in the lower crust provided buoyant, hot, mineralizing fluids that uplifted and formed the Cretaceous mineral deposits in the upper crust.

Mantle dynamics of Western Pacific and East Asia: Insight from seismic tomography and mineral physics

Recent results of high-resolution seismic tomography and mineral physics experiments are used to study mantle dynamics of Western Pacific and East Asia. The most important processes in subduction zones are the shallow and deep slab dehydration and the convective circulation (corner flow) processes in the mantle wedge. The combination of the two processes may have caused the back-arc spreading in the Lau basin, affected the morphology of the subducting Philippine Sea slab and its seismicity under southwest Japan, and contributed to the formation of the continental rift system and intraplate volcanism in Northeast Asia, which are clearly visible in our tomographic images. Slow anomalies are also found in the mantle under the subducting Pacific slab, which may represent (a) small mantle plumes, (b) upwellings associated with the slab collapsing down to the lower mantle, or (c) sub-slab dehydration associated with deep earthquakes caused by the reactivation of large faults preserved in the slab. Combining tomographic images and earthquake hypocenters with phase diagrams in the systems of peridotite + water, we proposed a petrologic model for arc volcanism. Arc magmas are caused by the dehydration reactions of hydrated slab peridotite that supply water-rich fluids to the mantle wedge and cause partial melting of the convecting mantle wedge. A large amount of fluids can be released from hydrated MORB at depths shallower than 55 km, which move upwards to hydrate the wedge corner under the fore-arc, and never drag down to the deeper mantle along the slab surface. Slab dehydration reactions at 120 km depth are the antigorite-related 5 reactions which supply water-rich fluids for forming the volcanic front. Phase A and Mg-surssasite breakdown reactions at 200 and 300 km depths below 700 °C cause the second and third arcs, respectively. Moreover, the dehydration reactions of super-hydrous phase B, phases D and E at 500–660 km depths cause the fluid transportation to the mantle boundary layer (MBL) (410–660 km depth). The stagnant slabs extend from Japan to Beijing, China for over 1000 km long, indicating that the arc–trench system covers the entire region from the Japan trench to East Asia. We propose a big mantle wedge (BMW) model herein, where hydrous plumes originating from 410 km depth cause a series of intra-continental hot regions. Fluids derived from MBL accumulated by the double-sided subduction zones, rather than the India–Asia collision and the subsequent indentation into Asia, are the major cause for the active tectonics and mantle dynamics in this broad region.

High resolution seismic tomography of a Strombolian volcanic cone

We determine the 3D velocity structure of the Puy des Goules, a small, 1 km wide, Strombolian volcano that erupted 10 ky ago in central France, through a high resolution seismic survey. One major goal for this experiment was to develop methods to reach a high resolution focused on the plumbing system. This has raised different problems such as: mixing active sources (explosive, vibroseis) with different signal properties; inverting traveltime residuals of the order of 10 ms which requires the corresponding accuracy on the Digital Elevation Model (DEM), source and sensor locations (300 sites) and traveltime computations. The results of the traveltime tomography reveal three main bodies of high velocity embedded within scoria layers. These bodies can be interpreted as the central chimney and two complex feeding zones that compare quite well with the Puy de Lemptégy, a neighboring cone that was quarried and removed, showing its underlying feeding conduits and dykes. These results represent a first step toward our objective that is to determine geological structures related to natural hazards with a high resolution.

Characterization of a fractured granitic pluton: P- and S-waves' seismic tomography and uncertainty analysis

A detailed characterization of the physical properties of a granitic pluton, including a three dimensional reconstruction of the fractures network and its surrounding are obtained by high resolution seismic tomography of P and S-waves. The input P- and S-wave travel times were picked from three-component offset and azimuth variable seismic profile (OVSP) acquired in a 500 m deep vertical borehole. The travel time inversion resulted in two three-dimensional P- and S-wave velocity models, and the uncertainty, robustness and sensitivity are estimated by performing a Monte Carlo-type analysis and checkerboard test. This study provides an indicator of the uniqueness of the determined model. The 500 m deep borehole was continuously cored, and extensively surveyed by geophysical techniques. Thus, the detailed surface geological mapping, fracture index logs, borehole televiewer images and P- and S-wave velocities derived from sonic logs, provide solid constraints for the interpretation of the tomographic images. The three-dimensional tomographic images reveal fracture zones mapped at surface that intersect the borehole at different depths and they can be identified in the core descriptions and in the borehole televiewer images. The fractures identified on the cores feature relatively low values for the sonic P- and S-wave velocities, and high values for Poisson's ratio ( σ). For single type lithologies, the distribution of Poisson's ratio differentiates fractured and altered granite from fresh unfractured domains within the rock. Poisson's ratios close to 0.3 correlate with fractured volumes consistent with fracture index and sonic logs, otherwise Poisson's ratio values close to 0.25 are related to unaltered and unfractured rock.

Automatic seismic phase picking and consistent observation error assessment: application to the Italian seismicity

SUMMARY Accuracy of seismic phase observation and consistency of timing error assessment define the quality of seismic waves arrival times. High-quality and large data sets are prerequisites for seismic tomography to enhance the resolution of crustal and upper mantle structures. In this paper we present the application of an automated picking system to some 600 000 seismograms of local earthquakes routinely recorded and archived by the Italian national seismic network. The system defines an observation weighting scheme calibrated with a hand-picked data subset and mimics the picking by an expert seismologist. The strength of this automatic picking is that once it is tuned for observation quality assessment, consistency of arrival times is strongly improved and errors are independent of the amount of data to be picked. The application to the Italian local seismicity documents that it is possible to automatically compile a precise, homogeneous and large data set of local earthquake Pg and Pn arrivals with related polarities. We demonstrate that such a data set is suitable for high-precision earthquake location, focal mechanism determination and high-resolution seismic tomography.

High resolution of streamer seismic data: obtaining optimal results

Andrew Long, PGS Marine Geophysical, defines the requirements for successful high resolution 3D marine seismic surveys and how these can be met. High resolution of seismic data results when high frequency geological features in close proximity can be precisely focused without any contamination from artefacts or noise (i.e. high signal-to-noise ratio). In general, recoverable frequency bandwidth is an important factor for vertical resolution, and dense 3D spatial sampling is an important factor for spatial resolution. Vertical and horizontal resolution is linked by the success of high frequency focusing and imaging. Without pursuing a theoretical treatise, the following section discusses the geophysical principles that are relevant for high-resolution seismic imaging. High resolution of 3D seismic data is achieved by the application of rigorous geophysical principles. In particular, the systematic pursuit of 1) uniform target illumination, 2) dense spatial sampling of the reflected wavefield, and 3) careful processing and imaging, will collectively deliver an optimum result. Note that the final step can never be successfully completed without the platform of proper target illumination and wavefield sampling. Several 3D data examples are used to demonstrate how flexible survey design and implementation can robustly achieve very high resolution target imaging for all target depths and challenges.

Seismic-While-Drilling Using a Swept-Impulse Source

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 92114, "Seismic-While-Drilling Using a Swept-Impulse Source," by J.J. Kolle, SPE, and K. Theimer, Tempress, prepared for the 2005 SPE/IADC Drilling Conference, Amsterdam, 23-25 February. This paper describes a prototype swept-impulse hydraulic tool capable of generating a broadband seismic signal at the bit while drilling. The source was designed to overcome the limitations of drill-bit seismic services by allowing operations with polycrystalline-diamond-compact (PDC) bits in inclined wells and in soft formations. The swept-impulse seismic source generated a broadband, high-intensity compressional-wave (P-wave) signal that can be used for high-resolution seismic profiling and look-ahead imaging while drilling. Introduction Seismic-while-drilling (SWD) services provide operators with real-time depth correction of seismic records to improve drilling accuracy and placement of liners. Pore-pressure prediction ahead of the bit would provide substantial benefits as a result of the reduced risk of blowouts. Between 1960 and 1996, there were more than 30 blowouts per year in Texas, Louisiana, and the U.S. Gulf of Mexico. Seismic services offer the best approach to characterize formation pressures because of the substantial effect of pore pressure on seismic velocity. When integrated with other information (such as surface seismic, offset-well velocity profiles, and drilling parameters), it may be possible to predict pore-pressure levels at the bit and ahead of the bit. Through the Deepstar consortium, operators have developed the following criteria for pore-pressure-prediction services. - No delay of conventional drilling operations. - Prediction range of 1,000 ft ahead of the bit. - Accuracy better than 50 ft. - Provide formation velocities within 5% of a conventional wireline vertical-seismic profile (VSP) service. - Real-time prediction—defined as less than 10 hours. The service industry has developed a range of seismic techniques for seismic profiling and pore-pressure prediction. Existing practices include predrill seismic interpretation, wireline VSP, drillstring VSP, and drill-bit SWD. Predrill seismic surveys can provide an indication of velocity anomalies. The value of seismic surveys is limited by the large depth uncertainty, particularly in exploration areas. Wireline VSPs have been used to provide check-shot surveys using air-gun sources. On deepwater offshore rigs, the rig-time costs are high, and failure of the wireline may disable the blowout preventer, causing a loss of well control. Drillstring VSP is a relatively new service that is similar to wireline VSP except that the receivers are on the bottomhole assembly. An air-gun array is deployed from the rig and fired multiple times while a connection is being made. Data from receivers are stored in a memory tool downhole for processing after the bit run.

High-resolution 3D seismic imaging in practice

Pearl Oil (Thailand) acquired a high-density, high-resolution 3D seismic survey in 2004 to establish an accurate geologic framework for development drilling of the Jasmine oil field in block B5/27, offshore Thailand (Figure 1).

In situ seismic investigations of fault zones in the Leventina Gneiss Complex of the Swiss Central Alps

Abstract Underground seismic tomography investigations have been carried out in the Faido access tunnel of the Gotthard Base Tunnel, Switzerland. Velocity measurements were made over a total length of 2651 m of the adit with the tunnel seismic prediction system, ISIS (Integrated Seismic Imaging System). ISIS provides high-resolution seismic imaging, using an array of rock anchors equipped with 3D-geophones. The first onsets of the compressional and shear waves were used for tomographic inversion. Two-dimensional seismic-velocity models reveal a disturbed zone between 2 and 3 m inward from the tunnel wall, characterized by strong variations from 3500 to 5800 m s −1 in compressional wave velocity V P , and from 2000 to 3000 m s −1 in shear-wave velocity V S . High-velocity zones correspond to quartz lenses, and low velocities mainly indicate fractured rock. Beyond the excavation disturbance zone, the variations in seismic velocities are generally smaller. The tomographic image of the rock mass also revealed two major fault zones composed of cataclastic shear planes surrounded by wider fracture zones. These structural characteristics are also useful for the prediction of cataclastic zones at other sites.

High resolution seismic imaging of faults beneath Limón Bay, northern Panama Canal, Republic of Panama

High-resolution seismic reflection profiles from Limón Bay, Republic of Panama, were acquired as part of a seismic hazard investigation of the northern Panama Canal region. The seismic profiles image gently west and northwest dipping strata of upper Miocene Gatún Formation, unconformably overlain by a thin (<20 m) sequence of Holocene muds. Numerous faults, which have northeast trends where they can be correlated between seismic profiles, break the upper Miocene strata. Some of the faults have normal displacement, but on many faults, the amount and type of displacement cannot be determined. The age of displacement is constrained to be Late Miocene or younger, and regional geologic considerations suggest Pliocene movement. The faults may be part of a more extensive set of north- to northeast-trending faults and fractures in the canal region of central Panama. Low topography and the faults in the canal area may be the result of the modern regional stress field, bending of the Isthmus of Panama, shearing in eastern Panama, or minor deformation of the Panama Block above the Caribbean subduction zone. For seismic hazard analysis of the northern canal area, these faults led us to include a source zone of shallow faults proximal to northern canal facilities.

The unique dynamics of the Pacific Hemisphere mantle and its signature on seismic anisotropy

The portion of Earth’s mantle lying below the hemispherical region occupied by the Pacific Ocean basin is characterized by a unique style of convection. The unique dynamics below the ‘Pacific Hemisphere’ are revealed by analyzing the mantle flow predicted on the basis of high-resolution seismic images of mantle structure. In the Pacific Hemisphere we find a dome-like upwelling, originating in the lower mantle, which actively transports material into the upper mantle. A unique aspect of this flow, found only in the Pacific Hemisphere, is a ‘demi-tour’ or U-turn of the circulation which occurs in the shallow mantle, west of the East Pacific Ridge. This ‘demi-tour’ connects the westward movement of the Pacific plate to the dome-like upwelling below the central Pacific Ocean and it also provides an explanation for the asymmetric distribution of seismic anisotropy on either side of the ridge axis. In a global analysis of the correlation between mantle flow and seismic anisotropy, we find the strongest agreement occurs in the Pacific Hemisphere mantle.

Shallow high-resolution seismic imaging of the Three Springs talc mine, Western Australia

Talc, the softest mineral in the world, is a hydrated magnesium sheet silicate, formed by transformation of existing rocks under hydrothermal activity. Properties of talc—including its resistance to heat, electricity, and acids, its platyness and its insolubility—make it a very important industrial mineral. The open-cut mining of talc can therefore be an attractive and profitable industry, provided exploration and production costs are not excessive.

Geophysical signatures of a basic‐body rock placed in the upper crust of the External Zones of the Betic Cordillera (Southern Spain)

The combined use of different geophysical techniques (seismic tomography, gravity and magnetic research) carried out in the External Zones of the Betic Cordillera (Southern Spain) points to the existence of a controversial set of anomalies. The high resolution seismic tomography clearly indicates a fast anomaly of up to +7% P‐wave velocity, and the surface magnetic and aeromagnetic measurements also show a positive anomaly. The negative anomaly of the dipole is very smooth. However, the estimated Bouguer anomaly values are clearly negative. This set of anomalies has been interpreted as due to a basic‐body rock (gabbros) located in the upper crust. The existence of this body suggests that the Iberian Massif constitutes the basement of the External Zones of the Betic Cordilleras, up to near the boundary with the Internal Zones.

The influence of rheological weakening and yield stress on the interaction of slabs with the 670 km discontinuity

Results of high resolution seismic tomography showing subducting slabs deflected in the transition zone and thickened in the lower mantle seem to call for slab material weaker than inferred from mineral physics deformation mechanisms. A possible mechanism suggested by several authors could be the weakening due to grain size reduction, which should occur in the cold portion of fast slabs after an exothermic phase transition at a depth of 400 km. Since the amount of weakening as well as the rate of subsequent strengthening due to the grain growth are not precisely known, we present here a parametric study of slab behavior in the transition zone and upper part of the lower mantle. We simulate a subducting slab in a two-dimensional (2-D) Cartesian box in the numerical model with composite rheology including diffusion creep, dislocation creep and a general stress limiting rheology approximating Peierl’s creep. We concentrate on two rheologic effects: the dynamic effect of slab weakening due to grain size reduction at the phase boundary and the effect of yield stress of stress limiting rheology. The effect of trench migration on slab deformation is also included in our study. Results show that the slab ability to penetrate into the lower mantle is not significantly affected by a trench retreat in the absence of grain size weakening. However, in case of a 4 cm/yr trench retreat, grain size weakening provides a viable mechanism to deflect the slab in the transition zone, provided that stress limiting deformation mechanism would limit the effective viscosity outside the areas of grain size weakening to about 1024 Pa s.

Mapping brittle fracture zones in three dimensions: high resolution traveltime seismic tomography in a granitic pluton

SUMMARY Fractured and altered zones within a granitic pluton are mapped in three dimensions by means of high resolution seismic traveltime tomography. The input traveltimes were picked from offset and azimuth variable vertical seismic profiles (OVSP) acquired in three boreholes and from seismic shot gathers of four CDP high resolution seismic reflection profiles recorded on the surface. For the OVSP data a hydrophone streamer placed in the boreholes recorded the acoustic energy generated (a signal with a frequency content between 15 to 150 Hz) by a Vibroseis truck at source points distributed every 30 m in a rectangular grid of 620 m by 150 m. The combination of borehole and surface seismic data resulted in an increase in the ray density of the shallow subsurface. The tomographic algorithm uses a variable model grid, with a finer grid spacing close to the surface were ray density is highest and the velocity variations are strongest. Therefore the resulting velocity models feature more detail at shallow levels. A simple and smooth starting velocity model was derived from P-wave velocity logs. Careful surface geological mapping, and borehole geophysical data, P- and S-wave velocity logs and Poisson’s ratio depth functions, provided key constraints for a physically reasonable 3-D interpretation of the tomograms. The low velocity anomalies constrained by the tomographic images were interpreted as unconsolidated rock, fractures and altered zones which correlate with structures mapped at the surface or velocity anomalies identified in the logs. Subsequent resolution analysis revealed that the derived velocity model is well constrained to depths of 60 m.