Imaging Of Subsurface Structures Research Articles

2D common‐offset traveltime based diffraction enhancement and imaging

ABSTRACTThe diffracted energy in a seismic recording contains key information about small‐scale inhomogeneities or discontinuities of the subsurface. Diffractions can therefore lead to high‐resolution imaging of subsurface structures associated with hydrocarbon traps. However, seismic diffracted signals are often much weaker than specular reflections and consequently require enhancement before they can be utilized. In this paper diffractions are enhanced relative to reflections based on two traveltime techniques, namely the modified common‐reflection‐surface approach, which uses the common‐reflection‐surface technique with some modification to tailor it for diffractions and the replacement‐media approach derived here for the purpose of a simplified parametrization of diffraction traveltimes. Both approaches are implemented in the common‐offset domain with the use of a finite‐offset central ray unlike the zero‐ or small‐offset diffraction enhancement techniques that use a zero‐offset central ray. The validity of the two moveout expressions is tested using velocity data taken from a smooth isotropic Marmousi model. A feasibility test was also carried out with respect to the new replacement‐media traveltime approximation addressing the various effects of signal‐to‐noise ratio, depth and lateral displacement of the diffractor location. Finally, diffraction enhancement and imaging was performed on 2D seismic data from the Jequitinhonha basin offshore Brazil. Diffractions were significantly enhanced and a high‐resolution image of the discontinuities of the subsurface was obtained.

Two-dimensional geomorphological characterization of a filled abandoned meander using geophysical methods and soil sampling

Using geophysical methods for the geomorphological characterization of subsurface features has numerous advantages over traditional exploration methods, because of their noninvasive and rapid nature. In this study, we compared the results of four geophysical methods with each other. We also discuss their possibilities and limitations in a geomorphological investigation. Electrical resistivity tomography (ERT), refraction seismic (RS), ground penetrating radar (GPR), and multichannel analysis of surface waves (MASW) methods were applied at an abandoned meander in northern Saxony to map a predefined structure. By combining these methods, we were able to characterize and delineate subsurface features of the abandoned meander, including a point bar, a channel, and a cutbank. Core samples obtained from sonic drilling were used to validate the findings of both seismic methods. However, we found that electrical resistivity tomography and ground penetrating radar lacked penetration depth and could only be used to resolve shallower subsurface layers. The ERT, GPR, RS, and MASW can be used to gather images of subsurface structures. The MASW in particular provides supplementary information about the channel's internal structure (with respect to lateral and vertical resolution). Besides fluvial–morphological features, we also detected inverse velocity structures within the channel. This allowed us to characterize the abandoned meander using information about its layer distribution and material composition. However, we were only able to characterize and delineate the subsurface features of the abandoned meander by combining all of the aforementioned methods.

Optimum spectral window for imaging of art with optical coherence tomography

Optical coherence tomography (OCT) has been shown to have potential for important applications in the field of art conservation and archaeology due to its ability to image subsurface microstructures non-invasively. However, its depth of penetration in painted objects is limited due to the strong scattering properties of artists’ paints. VIS–NIR (400–2,400 nm) reflectance spectra of a wide variety of paints made with historic artists’ pigments have been measured. The best spectral window with which to use OCT for the imaging of subsurface structure of paintings was found to be around 2.2 μm. The same spectral window would also be most suitable for direct infrared imaging of preparatory sketches under the paint layers. The reflectance spectra from a large sample of chemically verified pigments provide information on the spectral transparency of historic artists’ pigments/paints as well as a reference set of spectra for pigment identification. The results of the paper suggest that broadband sources at ~2 μm are highly desirable for OCT applications in art and potentially material science in general.

Ice‐ and Soil‐Wedge Dynamics in the Kapp Linné Area, Svalbard, Investigated by Two‐ and Three‐Dimensional GPR and Ground Thermal and Acceleration Regimes

ABSTRACTGPR is applied to image subsurface structures below non‐sorted polygons in Kapp Linné, Svalbard, where ice and active‐layer soil wedges co‐exist within a small area. Two‐dimensional GPR images ice wedges as hyperbolic reflections extending down from the frost table. However, some ice‐wedge signals are obscured or masked by similar hyperbolic reflections produced by stones or active‐layer soil wedges. Three‐dimensional GPR images ice wedges as linear amplitude anomalies, which excludes the possibility of misinterpretation and offers more reliable results. GPR investigations show that ice wedges are distributed sporadically in lower (younger) beach ridges, but not in higher (older) ones.Inter‐site monitoring of ground temperature, soil moisture, slow ground deformation and cracking during 2004–09 and the determination of near‐surface soil texture and stratigraphy suggest that snow cover and soil thermal properties determine the distribution of ice wedges. Most ice wedges are considered to be inactive due to relatively high permafrost temperatures. Shock loggers and extensometers detected shallow (soil wedge) cracking in sandy sediments, when the ground surface temperature dropped to −12°C and the thermal gradient in the upper 20 cm of ground reached −10°C m−1. Copyright © 2013 John Wiley & Sons, Ltd.

Geophysical exploration for geothermal resources: An application of MT and CSAMT in Jiangxia, Wuhan, China

We present a case study of applying MT (magnetotellurics) and CSAMT (controlled source audio-frequency magnetotellurics) for geophysical exploration in Jiangxia (江夏), which is located in new industrial developing suburb, where artificial noises are severe. In order to know deep buried structure, fracture status, and characteristics of underground geothermal development about 2 km, we acquired MT and CSAMT data to image subsurface structure through inversion and joint interpretation. The electrical terms of the 2D MT inversion can be divided into three ranges of resistivity values: (1) a highly resistive (>350 Ω·m) layer mainly characteristic of limestone, dolomitic limestone, leuttrite, silicarenite, and packsand; (2) an intermediate resistivity (250–350 Ω·m) layer mainly constituted by siliceous shale, siltstone, battie, and ampelitic limestone; and (3) a low resistivity (20–250 Ω·m) layer, from surface to −100 m, which is related to lacustrine alluvium of Quaternary period; the deep low resistivity layer is interpreted to be representative of the geothermal field. The result of the 2D CSAMT inversion reveals two layers of different electrical resistivities: (1) the first resistive layer (20–250 Ω·m), which is related to lacustrine alluvium of Quaternary period and the heat source, and (2) the second resistive layer (250–3 000 Ω·m). The heat source appears to be bounded within the middle of exploration area and shows the N-S trend. Its depth ranges from more than 1.2 to less than 0.7 km, and its resistivity values range from 20 to 250 Ω·m in the northeast part of Jiangxia. Comparing the results of MT and CSAMT method, the positive anomalies are similar and can be assumed to be generated by the same source.

Comparison of wave equation migration in logpolar and elliptic coordinates

We extend Riemannian wavefield extrapolation to pre- and post-stack migration using 2D logpolar coordinate system in order to enhance imaging of subsurface structures with imaging of dipping reflectors and turning waves. The logpolar coordinate is a coordinate system adequate for propagating both the source and receiver wavefields. The logpolar extrapolation wavenumber introduces an isotropic slowness model stretch to the single square root operator that enables the use of Cartesian finite-difference extrapolators for propagating wavefields in logpolar meshes. Wavefield extrapolation in this coordinate does not require any ray tracing at all and it can be readily extended to high-order finite-difference schemes since it is an analytic extrapolation operator. Two migration examples illustrate the advantages of the logpolar coordinate for imaging complex geological structures. We compared results of post- and pre-stack depth migration in logpolar and elliptic coordinate systems and we found that steeply dipping events can be better imaged in logpolar coordinate than in elliptic coordinate.

Extension of the spatial autocorrelation (SPAC) method to mixed-component correlations of surface waves

Using ambient seismic noise for imaging subsurface structure dates back to the development of the spatial autocorrelation (SPAC) method in the 1950s. We present a theoretical analysis of the SPAC method for multicomponent recordings of surface waves to determine the complete 3 × 3 matrix of correlations between all pairs of three-component motions, called the correlation matrix. In the case of isotropic incidence, when either Rayleigh or Love waves arrive from all directions with equal power, the only non-zero off-diagonal terms in the matrix are the vertical-radial (ZR) and radial-vertical (RZ) correlations in the presence of Rayleigh waves. Such combinations were not considered in the development of the SPAC method. The method originally addressed the vertical-vertical (ZZ), RR and TT correlations, hence the name spatial autocorrelation. The theoretical expressions we derive for the ZR and RZ correlations offer additional ways to measure Rayleigh wave dispersion within the SPAC framework. Expanding on the results for isotropic incidence, we derive the complete correlation matrix in the case of generally anisotropic incidence. We show that the ZR and RZ correlations have advantageous properties in the presence of an out-of-plane directional wavefield compared to ZZ and RR correlations. We apply the results for mixed-component correlations to a data set from Akutan Volcano, Alaska and find consistent estimates of Rayleigh wave phase velocity from ZR compared to ZZ correlations. This work together with the recently discovered connections between the SPAC method and time-domain correlations of ambient noise provide further insights into the retrieval of surface wave Green's functions from seismic noise.

Transmitting seismic station monitors fault zone at depth

Imaging subsurface structure and monitoring related temporal variations are two of the main tasks for modern seismology. With the accumulation of seismic data, clearer and clearer pictures of the Earth's interior are being achieved. However, precise monitoring of subtle subsurface changes associated with tectonic events remains tricky and is subject to high‐performance repeatable seismic sources. Although many observations have been reported on temporal seismic velocity changes in fault zones and volcanic areas, velocity change measurements based on passive sources, such as earthquakes and ambient seismic noise, are limited in resolution by the repeatability and by the spatial and temporal distribution of natural events.

Clayey Sand Soil's Behaviour Analysis and Imaging Subsurface Structure via Engineering Characterizations and Integrated Geophysicals Tomography Modeling Methods

The geoelectrical resistivity and seismic refraction surveys which were used in this study on the test site, delivered a detailed image of the near-surface conditions in generally very good. Electrical resistivity and seismic refraction analysis proved that a combination of these integrated study of the physical environmental data provided a reasonable compromise between measurement time and image resolution. Quantitative interpretation of the resistivity and seismic models based on soil's parameters determined using laboratory practices and field survey could reproduce the range of resistivity and seismic values found on the site very well. The model explains the ambiguity in between resistivity and clayey sands found on the site and predict the dominant role of water saturation. Geophysical methods are used in this research in purpose to determine the internal structure of a soil mass. Various geophysical methods and their merits for imaging subsurface structures and condition are discussed. Seismic methods are often the most suitable because the measurements depend on the mechanical properties which are also important in the mechanical calculation of soil's behaviour analysis. Other geophysical method, such as geoelectric resistivity, is useful to determine the internal structure, but require a correlation of found boundaries with mechanical properties. This research was conducted to investigate the subsurface structures and conditions through geotechnical engineering properties and its geophysical characteristics. The computation analysis is used in this research in purpose to investigate clayey sand soil's behaviour. Electrical resistivity test and engineering laboratory practices such as soil strength test, liquid limit test, plastic limit test and grain size distribution test was also carried out to investigate clayey sand soil behaviour in Batu Uban, Penang area during monitoring period.

Seismic anisotropy in microseismic event location analysis

Seismic velocity anisotropy has been known for many years to be an important factor in correctly imaging subsurface structures with reflection seismic methods (Alkhalifah et al., 1996). Fine vertical velocity layering naturally gives rise to VTI (vertical transverse isotropic) velocity in which seismic-wave velocity is faster in the horizontal direction than in the vertical direction. Similarly, vertically oriented natural fractures and stress fields influence seismic velocities in an azimuthal or compass direction which is described as HTI (horizontal transverse isotropic) velocity variation (Schoenberg and Sayers, 1995). HTI velocity analysis in seismic reflection imaging commonly has two main objectives: placing geologic features in the correct spatial location and characterizing fracture orientation and density to optimize oil and gas production strategy.

Imaging of subsurface structures using atomic force acoustic microscopy at GHz frequencies

We describe a technique to image subsurface structures using atomic force acoustic microscopy operated at 1 GHz. The devices to be imaged are insonified with 1 GHz ultrasonic waves which are amplitude-modulated at a fraction or multiple frequency of cantilever contact resonance. The transmitted signals are demodulated by the nonlinear tip–surface interaction, enabling one to image defects in the device based on their ultrasonic scattering power which is determined by the ultrasonic frequency, the acoustic mismatch between the elastic properties of the host material and the defects, by their geometry, and by diffraction effects.

Reply to "Comment on 'Near-Surface Location, Geometry, and Velocities of the Santa Monica Fault Zone, Los Angeles, California' by R. D. Catchings, G. Gandhok, M. R. Goldman, D. Okaya, M. J. Rymer, and G. W. Bawden" by T. L. Pratt and J. F. Dolan

In a comment on our 2008 paper (Catchings, Gandhok, et al. , 2008) on the Santa Monica fault in Los Angeles, California, Pratt and Dolan (2010) (herein referred to as P&D) cite numerous objections to our work, inferring that our study is flawed. However, as shown in our reply, their objections contradict their own published works, published works of others, and proven seismic methodologies. Rather than responding to each repeated invalid objection, we address their objections by topic in the subsequent sections. In Catchings, Gandhok, et al. (2008), we presented high-resolution seismic-reflection images that showed two near-surface faults in the upper 50 m beneath the grounds of the Wadsworth Veterans Administration Hospital (WVAH). Although P&D suggest we effectively duplicated their seismic acquisition, our survey was not a duplication of their efforts. Rather, we conducted a seismic-imaging survey over a similar profile as Pratt et al. (1998) but used a different data acquisition system and different data processing methods to evaluate methods of seismically imaging blind faults in the wake of the 17 January 1994 M 6.7 Northridge earthquake. We used an acquisition method that provides both tomographic seismic velocities and reflection images. Our combined-data approach allowed for shallower imaging (∼2.5 m minimum) than the ∼20-m minimum of Pratt et al. (1998), clearer images of the fault zone, and more accurate depth determinations (rather than time images). In processing the reflection images, we used prestack depth migration, which is generally accepted as the only proper imaging method for imaging subsurface structures with strong lateral velocity variations (Versteeg, 1993), a condition shown to exist at the WVAH site. We correlated our reflection images with refraction tomography images, borehole lithology, and velocity data, Interferometric Synthetic Aperture Radar images, and changes in groundwater depths. Except for some minor differences, our seismic-reflection images coincide with previously published seismic-reflection …

Contribution of gravity and magnetic data in delineating the subsurface structure of Hammam Faroun area, Gulf of Suez, Egypt

The Hammam Faroun has a particular importance due to its geothermal activity which constitutes the main geothermal resource of Egypt. The area is located on the Sinai Peninsula, a subplate bounded by two seismically active structural zones along the Gulf of Suez and Gulf of Aqaba. High-resolution ground-based gravity and magnetic data are available for the entire Hammam Faroun area, acquired as part of a national project to explore for mineral, geothermal, and hydrocarbon resources. Gravity and magnetic data were analyzed using Source Edge Detection and Source Parameter Imaging (SPI) techniques to image subsurface structures. These analyses show that the area is characterized by a set of northwest-striking faults lying parallel to the Gulf of Suez. Orthogonal patterns are also present, possibly related to rifting of the Gulf of Suez. Depth analysis using the SPI method indicates that surface faults extend to 5-km depth. Analysis of potential-field data elucidates the structurally complex subsurface structure of the Hammam Faroun area.

Benefits of polarized versus nonpolarized dermoscopy

Dermoscopy is a widely diffuse tool that allows a better noninvasive diagnosis of pigmented and nonpigmented skin lesions, visualizing magnified images of subsurface structures. The light source can be nonpolarized or polarized, the latter being a more recent technique acquisition. Papers from the last decade focused on the appreciable differences between the two techniques. In general, by polarized dermoscopy the visualization of deeper and vascular structures is more attainable, but the main issue is the quality of the image, influencing both diagnostic accuracy and confidence level, regardless of the lighting technique. A terminology based on polarized dermoscopy is still missing, requiring further consensus-based studies.

1P336 高速原子間力顕微鏡による表面下構造のイメージングに向けた超音波ホログラフィーの開発(バイオイメージング,第48回日本生物物理学会年会)

1P336 高速原子間力顕微鏡による表面下構造のイメージングに向けた超音波ホログラフィーの開発(バイオイメージング,第48回日本生物物理学会年会)

High‐resolution onshore seismic imaging of complex volcanic structures: An example from Vulcano Island, Italy

Detailed seismic images of subsurface structures of volcanic calderas are fundamental to improve the structural and volcanological knowledge of these high‐risk volcanoes. However, high‐quality seismic data are difficult to obtain in volcanic areas, especially on shore. We report the results of a high‐resolution seismic profiling of the western sector of La Fossa Caldera (Vulcano Island, Italy). Using a high‐resolution vibrating source and both alternative acquisition and processing techniques, we were able to overcome most of the inconvenience caused by volcanic lithotypes. This study provides the subsurface distribution of volcanic deposits as well as the recognition of some significant intracaldera structures. We located a parasitic vent or hyaloclastite mound buried under La Fossa Caldera, which is dissipating CO2 in an area where earthquakes have been recorded at about 1 km depth. Furthermore, the deformation pattern found at the southernmost part of the profile is consistent with a caldera collapse after a dome intrusion. The results suggest that the use of high‐resolution vibrating sources combined with alternative seismic acquisition techniques and nonconventional processing could help to recover detailed information on the shallow structures of volcanic areas.

The Nascent Coso Metamorphic Core Complex, East-Central California: Brittle Upper Plate Structure Revealed by Reflection Seismic Data

The relationships between upper crustal faults, the brittle-ductile transition zone, and underlying magmatic features imaged by multifold seismic reflection data are consistent with the hypothesis that the Coso geothermal field, which lies within an extensional step-over between dextral faults, is a young, actively developing metamorphic core complex. The reflection images were processed using a non-linear simulated annealing approach to invert P-wave first arrivals in the seismic data for 2-D velocity structure. Velocity tomograms obtained from the inversions were employed in a prestack Kirchhoff migration to produce depth-migrated images of subsurface structure. The seismic images reveal coherent reflectivity in the upper 5 km of the Mesozoic intrusive bedrock of the Coso Range. The Coso Wash fault and other late Pleistocene and Holocene normal faults in, and adjacent to, the geothermal field are expressed on the reflection images by lateral truncations of coherent reflectors, and locally as discontinuous planar reflectors down-dip of the surface fault traces. The faults appear to terminate downward against a faint, subhorizontal, discontinuous reflective horizon ("A") at about 4 km depth that lies at or near the base of seismicity beneath the geothermal field. This horizon is interpreted to be the brittle-ductile transition zone. The seismic images also reveal a prominent high-amplitude reflector at 6 km depth directly beneath the geothermal field, and a deeper reflective horizon at a depth of about 8 to 9 km. These deeper reflectors are associated with a zone of low P- and S-wave velocities in the 6-12 km depth range beneath the geothermal field, and are probably related to the presence of magma or other pressurized fluids. The combination of magmatic heat and active normal faulting in the regional transtensional setting establishes the conditions for hydrothermal convection in the Coso geothermal field.

Application of 3D Resistivity Tomography to Delineate Subsurface Structures

We have developed a three-dimensional (3D) resistivity inversion code for resistivity tomography, where resistivity data are measured using electrodes installed in several boreholes as well as at the Earth surface. The algorithm is based on finite element approximations for forward modelling, and the ACB (Active Constraint Balancing) algorithm is adopted to enhance the resolving power of the smoothness-constrained least-squares inversion. Resolution analysis with numerical examples shows that 3D resistivity tomography is a promising tool to construct high-resolution 3D images of subsurface structures. We have also shown thai topography effects should be incorporated in the inversion to gel accurate 3D images -without artefacts, in the application of the method to a field data set acquired ai a granite quarry, we could successfully delineate the 3D attitude of faults or fractures in the site.

GPR reflection characteristics and depositional models of mud volcanic sediments—Wushanting mud volcano field, southwestern Taiwan

Ground penetrating radar (GPR) survey was conducted in the Wushanting mud volcano field (Yanchao, Kaohsiung) using a 500 MHz antennae, which allowed high-resolution imaging of subsurface structures. Seven GPR reflection characteristics are recognized. Sigmoid GPR reflection patterns resulted from a recent mud lobe deposited above an underlying older mud lobe front. Contorted GPR facies resulted from recent mud flow which encountered obstacles. Subparallel reflections resulted from mud volcano deposits of limited flowability, low velocity and gentle gradient. Hummocky reflection patterns are formed by interfingering of recent mud lobes building onto low land. Disrupted GPR facies were due to lateral breaks of continuity from mud cracks, which, according to field observation, can provide channels for erosion and form deeper erosion gullies. GPR time slices of different depths are rendered as a three-dimensional model. Approximately orbicular GPR reflection characteristics can indicate arcuate stacked mud lobe fronts of different periods. Some depositional models to explain GPR reflection characteristics can be founded upon observations of recent sedimentary phenomena. The models of this study may be applied to paleoenvironments and the depositional evolution of mud volcanoes in similar geological settings.

Integration of P- and SH-wave high-resolution seismic reflection and micro-gravity techniques to improve interpretation of shallow subsurface structure: New Madrid seismic zone

Shallow high-resolution seismic reflection surveys have traditionally been restricted to either compressional (P) or horizontally polarized shear (SH) waves in order to produce 2-D images of subsurface structure. The northernmost Mississippi embayment and coincident New Madrid seismic zone (NMSZ) provide an ideal laboratory to study the experimental use of integrating P- and SH-wave seismic profiles, integrated, where practicable, with micro-gravity data. In this area, the relation between “deeper” deformation of Paleozoic bedrock associated with the formation of the Reelfoot rift and NMSZ seismicity and “shallower” deformation of overlying sediments has remained elusive, but could be revealed using integrated P- and SH-wave reflection. Surface expressions of deformation are almost non-existent in this region, which makes seismic reflection surveying the only means of detecting structures that are possibly pertinent to seismic hazard assessment. Since P- and SH-waves respond differently to the rock and fluid properties and travel at dissimilar speeds, the resulting seismic profiles provide complementary views of the subsurface based on different levels of resolution and imaging capability. P-wave profiles acquired in southwestern Illinois and western Kentucky (USA) detect faulting of deep, Paleozoic bedrock and Cretaceous reflectors while coincident SH-wave surveys show that this deformation propagates higher into overlying Tertiary and Quaternary strata. Forward modeling of micro-gravity data acquired along one of the seismic profiles further supports an interpretation of faulting of bedrock and Cretaceous strata. The integration of the two seismic and the micro-gravity methods therefore increases the scope for investigating the relation between the older and younger deformation in an area of critical seismic hazard.