66,971 research outputs found
Real–Time Imaging of Decimetre–Resolution 3D Seismic Volumes
No full textThe 3D Chirp sub–bottom profiler acquires a true 3D seismic volume with decimetric horizontal and centimetric vertical resolutions, providing an ideal platform for shallow–water engineering, archaeology, military, and geological studies. In this thesis, I show how simple processing flows built around a combination of standard Chirp/Vibroseis techniques and well known industry methods produce effective and impressive results by considering an object identification case study in a shallow–water, harbour setting (Vardy et al., 2008). Both stacked and migrated volumes are used to identify 89 individual buried targets that are correlated with coincident objects. Through subsequent dredging, a 100 % detection success is demonstrated, along with the strong similarity between the migrated reflector morphology and co–incident object shape. However, this processing approach requires extensive manual input and very long processing times (? 1 month).For this reason, a new method for pre–stack 3D Kirchhoff imaging is developed. Correlation with a series of bandwidth limited theoretical source sweeps is used to frequency decompose the raw traces for pre–stack time migration using a constant velocity. By accommodating dispersion through imaging a series of band limited traces, rather than through Fourier Transform, processing times are reduced from ? 1 month to c. two days for the object detection volume (i.e., approaching real–time application). The effectiveness of this new algorithm is examined using several synthetic volumes, allowing the degenerative effects of gaps in the fold to be explored. Finally, the application of the 3D Chirp system to geological cases is demonstrated through the geomorphological mapping of a sequence of mass movement events in Windermere, UK Lake District. Three mass movement deposits are identified in a 100 m by 400 m survey area. Through mapping of the package distributions and their interaction with the pre–existing sediments stratigraphy, they are identified as Younger Dryas climate amelioration deposits, resulting from the rapid deposition of gravitationally unstable, unconsolidated sediments. A metre–scale structural interpretation allows the depositional regimes (two being debris flow and the third a mass flow deposit) and dominant transport directions to be inferred
Deriving shallow-water sediment properties using acoustic impedance inversion
No full textIn contrast to the application of marine seismic reflection techniques to reservoir-scale applications, where seismic inversion for quantitative sediment analysis is common, shallow-water, very-high-resolution seismic reflection data are seldom used for more than qualitatively reflection interpretation. Here, a quantitative analysis of very-high-resolution marine seismic reflection profiles from a shallow-water (<50 m water depth) fjord in northern Norway is presented. Acquired using Sparker, Boomer, and Chirp sources, the failure plane of multiple local landslides correlates with a composite reflection that reverses polarity to the south. Using a genetic algorithm, a 1D post-stack acoustic impedance inversion of all three profiles is performed, calibrating against multi-sensor core logger (MSCL) data from cores. Using empirical relationships the resulting impedance profiles are related to remote sediment properties, including: P-wave velocity; density; mean grain size; and porosity. The composite reflector is consistently identified by all three data sources as a finer-grained (by one phi), lower density (c. 0.2 g/cm3 less than background) thin bed, with an anomalous low velocity zone (at least 100 m/s lower than background) associated with the polarity reversal to the south. Such a velocity contrast is consistent with an accumulation of shallow free gas trapped within the finer-grain, less permeable layer. This study represents the first application of acoustic impedance inversion to very-high-resolution seismic reflection data and demonstrates the potential for directly relating seismic reflection data with sediment properties using a variety of commonly used shallow seismic profiling sources
Decimeter-resolution 3D seismic volume in shallow water: a case study in small-object detection
No full textThe 3D Chirp subbottom profiler provides high resolution imaging of coastal and inshore seabed and subseabed structure by combining the known, highly repeatable source waveform of Chirp profilers with the coherent processing and interpretation afforded by true 3D seismic volumes. Comprising sixty hydrophone groups arranged around a Maltese cross of four Chirp transducers, 3D Chirp permits the acquisition of a true 3D volume with a horizontal resolution of 12.5 cm, providing an excellent base for shallow water engineering, archaeological, military, and geological studies. Here we present the results of surveying an atidal basin on the south coast of England aimed at mapping any bedrock protrusions along with the size and distribution of any buried objects. The 150 m by 250 m study area provided a series of unique challenges, including a large number of discrete objects ranging from tens of centimeters to several meters in size, buried in a thin, 0.5 m to 1.5 m, veneer of unconsolidated silt, overlaying a flat bedrock surface showing a high acoustic contrast and short wavelength roughness. Through the comparison of comprehensive post--survey dredging of the entire site with a prestack time migrated 3D volume, it has been possible to confirm a 100 % detection rate for all discrete buried objects larger than 0.30 m by 0.30 m in illuminated area, although one acoustic anomaly could not be accounted for in the dredging results
Pepsis nanoides Vardy 2005
No full textPepsis nanoides Vardy, 2005: 268 Paratypes: 1♀ (# MZSP 56488) ‘[Brazil], São Paulo, Rio Claro, Horto Florestal [Edmundo] Navarro de Andrade, 30.vi.[19]87, coletado no barranco, R.P. Martins, col.’ ‘ Paratype, ♀, Pepsis nanoides n. sp., Det. Vardy, 2001’; 1♀ (# MZSP 56489) ‘[Brazil], Pará, Belém, June, 1924’ ‘ F.X. Williams collector’ ‘ Paratype, ♀, Pepsis nanoides n. sp., Det.Vardy, 2001’; 1♀ (# MZSP 56490)‘[Brazil], E.[stado] de S.[ão] Paulo, 12.430 [Brazil, São Paulo, Jundiaí, 21.iv.1899, M. Beron col.], Pepsis dimidiata ’ ‘ Head Figured Specimen’ ‘ Paratype, ♀, Pepsis nanoides n. sp., Det. Vardy 2001’ ‘MZUSP’; 1♀ (# MZSP 56491)‘[Brazil], E[stado] S[ão] Paulo, Jundiahy [Jundiaí], ♀, 12.430[Brazil, São Paulo, Jundiaí, 21.iv.1899, M. Beron col.], Pepsis dimidiata ’ ‘ Coll antiga’ ‘ Pepsis dimidiata Fabr.’‘ Paratype, ♀, Pepsis nanoides n. sp., Det.Vardy,2001’‘MZUSP’; 1♀ (# MZSP 56492)‘ Brasil, S.[ão] P.[aulo], Itu, Faz.[enda] Pau D’Alho, ii.1963, F. Werner & U. Martins col.’‘ Pepsis dimidiata, Fabr., ♀, det. H. Evans 1967’ ‘ Pepsis dimidiata, Fabr.’‘ Paratype, ♀, Pepsis nanoides n. sp., det. Vardy 2001’. Comments: The original publication listed the specimen from Belém (Brazil) (# MZSP 56489) as collected on May, but the correct date in the label is June.Published as part of Andrade, Tamires de O., Ramos, Kelli S., Onody, Helena C., dos Santos, Alvaro D. & Brandão, Carlos Roberto F., 2018, Type specimens of Pompiloidea, Thynnoidea and Vespoidea (Hymenoptera) deposited in the Museu de Zoologia da Universidade de São Paulo, Brazil, pp. 1-21 in Papéis Avulsos de Zoologia 58 on page 7, DOI: 10.11606/1807-0205/2018.58.39, http://zenodo.org/record/461350
Decimetric-resolution stochastic inversion of shallow marine seismic reflection data: dedicated strategy and application to a geohazard case study
No full textCharacterization of the top 10–50 m of the subseabed is key for landslide hazard assessment, offshore structure engineering design and underground gas-storage monitoring. In this paper, we present a methodology for the stochastic inversion of ultra-high-frequency (UHF, 0.2–4.0 kHz) pre-stack seismic reflection waveforms, designed to obtain a decimetric-resolution remote elastic characterization of the shallow sediments with minimal pre-processing and little a priori information. We use a genetic algorithm in which the space of possible solutions is sampled by explicitly decoupling the short and long wavelengths of the P-wave velocity model. This approach, combined with an objective function robust to cycle skipping, outperforms a conventional model parametrization when the ground-truth is offset from the centre of the search domain. The robust P-wave velocity model is used to precondition the width of the search range of the multiparameter elastic inversion, thereby improving the efficiency in high-dimensional parametrizations. Multiple independent runs provide a set of independent results from which the reproducibility of the solution can be estimated. In a real data set acquired in Finneidfjord, Norway, we also demonstrate the sensitivity of UHF seismic inversion to shallow subseabed anomalies that play a role in submarine slope stability. Thus, the methodology has the potential to become an important practical tool for marine ground model building in spatially heterogeneous areas, reducing the reliance on expensive and time-consuming coring campaigns for geohazard mitigation in marine areas
A demonstration of time-lapse imaging using ultra-high-frequency seismic reflection data
No full textTime-lapse seismic imaging has improved our capability to measure and understand dynamic processes in the subsurface. However, there are very few examples using ultra-high-frequency (kHz-range) seismic data. Exacting requirements for navigation can be prohibitive for acquiring coherent, true-3D volumes and residual errors manifest as noise in time-lapse differences making it challenging to interpret real subsurface changes. By updating the acquisition and processing workflows for the 3D Chirp, an ultra-high-frequency sub-bottom profiler, initial results illustrate high amplitude and navigation repeatability. Post-processing was used to improve the capability and performance of real-time kinematic GPS, coupled with high-accuracy inertial measurements to yield centimetre-level absolute positioning in a range of operating conditions. A comparison of seismic reflection volumes acquired over the same area at high and low tide reveals a normalized RMS difference of 16.1% and demonstrates the capability for direct quantitative monitoring of the shallow subsurface at decimeter-resolution
High resolution imaging and quantitative analysis of HV cable and pipeline trenching in the marine environment
No full textThe life time performance of both HV cables (ORE inter-array and export cables and cross-continental shelf interconnectors) and oil and gas pipelines are limited by the physical properties of the sediment in which the cable/pipeline is buried. In the case of HV cables the burial material and burial depth have implications for heat dissipation from the cable, which in turn plays a primary role in cable rating and its lifetime operation and maintenance. For a pipeline changes in the density and strength of the overburden material can impact on buckling potential once in operation. Our current understanding of the key physical parameters of the sediment (e.g. grain size, porosity, permeability, thermal conductivity, relative density and strength) are based on in situ measurements of the ambient condition and rarely take account of physical property changes during the trenching process. We provide initial acoustic inversion results from high resolution 3D Chirp volumes from both a prototype scale, CPT calibrated, tank experiment and in situ trenched cables in a range of substrates. We shall demonstrate the potential of acoustic inversion to non-destructively quantify trench disturbance in this critical engineering scenarios.</p
Pre-stack full waveform inversion of ultra-high-frequency marine seismic reflection data
No full textThe full waveform inversion (FWI) of seismic reflection data aims to reconstruct a detailed physical properties model of the subsurface, fitting both the amplitude and the traveltime of the reflections generated at physical discontinuities in the propagation medium. Unlike reservoir-scale seismic exploration, where seismic inversion is a widely adopted remote characterization tool, ultrahigh-frequency (UHF, 0.2–4.0 kHz) multichannel marine reflection seismology is still most often limited to a qualitative interpretation of the reflections’ architecture. Here we propose an elastic FWI methodology, custom-tailored for pre-stack UHF marine data in vertically heterogeneous media to obtain a decimetric-scale distribution of P-impedance, density and Poisson’s ratio within the shallow subseabed sediments. We address the deterministic multiparameter inversion in a sequential fashion. The complex trace instantaneous phase is first inverted for the P-wave velocity to make up for the lack of low frequency in the data and reduce the nonlinearity of the problem. This is followed by a short-offset P-impedance optimization and a further step of full offset range Poisson’s ratio inversion. Provided that the seismogram contains wide reflection angles (>40°), we show that it is possible to invert for density and decompose a posteriori the relative contribution of P-wave velocity and density to the P-impedance. A broad range of synthetic tests is used to prove the potential of the methodology and highlights sensitivity issues specific to UHF seismic. An example application to real data is also presented. In the real case, trace normalization is applied to minimize the systematic error deriving from an inaccurate source wavelet estimation. The inverted model for the top 15 m of the subseabed agrees with the local lithological information and core-log data. Thus, we can obtain a detailed remote characterization of the shallow sediments using a multichannel sub-bottom profiler within a reasonable computing cost and with minimal pre-processing. This has the potential to reduce the need of extensive geotechnical coring campaigns
Multi-disciplinary investigation into the role of regional event beds in near-shore landslides
No full textShallow-water submarine landslides are a widely observed landform, seen in fjords, lakes and rivers at all latitudes. With run outs of several km and the ability to both retrogress inshore as well as generate potentially hazardous tsunamis, they represent a significant hazard to offshore and near-shore infrastructure. However, their complex architecture and sensitivity to subtle changes in the mechanical properties of soils makes them extremely difficult to study, requiring a comprehensive combined assessment of the structural and soil mechanical architecture of the slope and subsurface. Here we present a highly detailed, integrated geophysical, geological and geotechnical study of the parameters controlling submarine slope stability in a fjord in northern Norway
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