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- Volume 18, Issue 1, 2020
Near Surface Geophysics - Issue 1 - Quantitative Geophysical Characterisation of Marine Near‐Surface, 2020
Issue 1 - Quantitative Geophysical Characterisation of Marine Near‐Surface, 2020
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Acoustic measurements of marine sediments with pebbles and cobbles
Authors Charles W. Holland, Jan Dettmer, Gavin Steininger, Stan E. Dosso and Allen LowrieABSTRACTThe majority of sediment acoustics research has focused to date on sediments with sand‐sized particles or smaller; measurements for sediments containing cobbles (6–26 cm) are rare. This paper presents the first measurements (to the authors’ knowledge) over a wide frequency range of compressional‐wave velocity and bulk density for a sediment with cobbles. The in situ velocity from inversion from wide‐angle reflection‐coefficient data at 0.4–2 kHz for cobbles suspended in sand is found here to be 1800 m/s with 95% credibility interval bounds of [1787–1828] m/s and for cobbles suspended in clay 1532 [1528–1536] m/s. Measured core velocities at 50 kHz and 200 kHz are lower for each sediment due to multiple scattering. The in situ bulk density for cobbles suspended in sand is 2.25 [2.21–2.28] g/cm3 and for cobbles suspended in clay 1.83 [1.78–1.87] g/cm3. Though cobbles in the upper few metres of sediment may be considered unusual on the mid‐shelf at mid‐latitudes, they appear to be present over tens of square kilometres on the Malta Plateau in a several metre thick layer starting at about 1 m below seafloor. In fact, geologic process considerations suggest that cobbles may be generally more common in mid to outer shelf environments than the paucity of measurements would suggest. Increased interest in the Arctic continental shelf environment, where pebbles and cobbles are expected to be prevalent, provides an additional motivation for this work.
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Automated static and moveout corrections of high‐resolution seismic data from the Baltic Sea
Authors Sönke Reiche, Benjamin Berkels and Benedikt WeißABSTRACTHigh‐frequency multichannel seismic systems provide detailed images of the shallow marine subsurface. In order to exploit the redundancy inherent in such data optimally, traveltime corrections need to account for normal moveout and static effects due to vertical source and receiver variations. Misalignment of reflections in common‐midpoint gathers will significantly lower the frequency content in the final stack, making this correction particularly important for very high‐frequency seismic data. Traditionally, normal moveout correction involves labour‐intensive picking of stacking velocities, while static corrections can be, by some techniques, performed automatically. In this paper, we present a high‐frequency seismic case study from the Baltic Sea, using seismic image matching as a novel, fully automated technique to perform joint moveout and static corrections. Our multichannel test profiles were acquired offshore Rügen island for wind farm development. Owing to the regular passage of up to 1.5 m high ocean waves during data acquisition, these boomer profiles suffer from strong static effects. We perform joint normal moveout and static corrections by defining the nearest common offset section as a fixed reference frame and minimizing its difference in traveltime with respect to all available common offset sections. Time shifts are computed independent of a pre‐defined traveltime curve, using the normalized cross‐correlation as a measure of data similarity while penalizing irregular displacements by a regularization term. Time shifts are converted to stacking velocities based on the traditional hyperbolic traveltime equation. Our results are compared with those derived by conventional manual velocity analysis and subsequent trim static corrections. We find that image matching produces stacks of similar quality and stacking velocity models of similar to slightly better quality compared with the conventionally derived ones, revealing the potential of this technique to automatize and significantly speed up this first part of the seismic processing chain.
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The image‐source method: a tool for geoacoustic inversion
Authors Samuel Pinson, Laurent Guillon and Charles W. HollandABSTRACTThe image source method is a fast tool to perform sound‐speed profile estimations for sediment tomography. It uses a broadband source and an array of receivers and is based on the representation of the reflected wave from sediment interfaces by image sources and on array processing techniques. Image sources are automatically detected by the array and the sound‐speed profile is deduced directly from their locations. Thus, the method has a low computational cost and can be applied to large data sets. The image source method has been under development during the last decade and has proven its efficiency on simulated data and at‐sea experiment data. The objective of this paper is to review these advances of the image source method.
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Characterization of seabed properties from Scholte waves acquired on floating ice on shallow water
Authors Tor Arne Johansen and Bent Ole RuudABSTRACTSeismic surveying of the coastal areas in the Arctic is best facilitated during wintertime when the sea ice is land‐fast. This eases the logistics of the operation and assures that there is no damage made to the vulnerable tundra. Seismic experiments on floating ice on shallow water performed in a fjord in Svalbard in the Norwegian Arctic show prominent Scholte waves. The dispersion relation of Scholte waves can provide the shear wave velocities of the seabed sediments. Scholte wave data can potentially be obtained when the seismic source and geophone receivers are both placed on top of the floating ice. However, the Scholte wave data become more distinct by using an air gun lowered some metres below the ice. A rock physics model based on a two‐step differential effective medium scheme has been tuned to predict seismic properties found for very loose sediments, among these very high P‐wave to S‐wave velocity ratios. The rock physics model enables us to convert seismic velocities obtained from Scholte wave data to quantitative estimates of the sediment composition.
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High‐resolution seismic velocity field estimation techniques and their application to geohazard, lithology and porosity prediction
Authors Vita Kalashnikova, Ivar Meisingset, Rune Øverås and Daria KrasovaABSTRACTSeismic velocity is an attractive parameter for geohazard interpretation, pore pressure analysis, play and prospect evaluations, and other geological studies, but ordinary seismic processing velocities often do not have a good enough resolution. We adapt a dynamic time warping algorithm to estimate geologically reasonable high‐resolution velocities from average‐quality seismic data that can be used for geohazard analysis based on 3D seismic data. To predict free gas and/or excess pore water pressure in thin shallow layers, we use velocity inversion. It is a method for simultaneous inversion of velocity data to geological attributes. It runs in the depth domain, uses a background velocity model for balancing of input velocities to wells and a normal compaction trend model to simultaneously estimate lithology, pore pressure and net apparent erosion attributes, while porosity is calculated from a sandstone–porosity relationship. The overall workflow is applied for geohazard analysis at two marine sites. The first example is a deep‐water one from the Norwegian Sea, where thin and possibly overpressured or gas‐filled layers are identified in the Pleistocene section. The second example is in a region with limestone‐dominated lithology, where thin overpressured shales can cause severe drilling problems. In both examples, the thicknesses of the layers prone to geohazards are estimated to be about half of the wavelength. Dedicated high‐resolution velocity estimations, such as those obtained through the proposed workflow with dynamic time warping, applied to standard 3D seismic data and followed by dedicated velocity inversion routines, are, therefore, a necessity for proper geohazard assessment.
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Near‐shore geophysical and geotechnical investigations in support of the Trieste Marine Terminal extension
ABSTRACTThe Port of Trieste is an international hub for land and sea trade with the dynamic markets of central and eastern Europe. Thanks to its deep natural draft (about 18 m), the modern high‐capacity vessels can moor to the piers. In view of the foreseen increase in maritime traffic, this harbour is undergoing modernization in order to improve the commercial traffic capability. In this expansion plan, the container Trieste Marine Terminal, Pier VII, is seeking an extension by about 200 m. In support of this feasibility study, multidisciplinary data acquisition was conducted in order to characterize the seabed, the sub‐bottom sediments and the bedrock (flysch formation) in front of the Trieste Marine Terminal. The acquisition of high‐resolution swath bathymetry, side‐scan sonar and magnetometer data allowed a detailed analysis of the seabed conditions from an environmental and safety perspective. High‐resolution seismic reflection data enabled us to characterize the Plio‐Quaternary soft sediments and the underlying bedrock. A static underwater refraction survey was performed using hydrophone array deployed on the sea bottom to obtain seismic velocities and to achieve a reliable time‐to‐depth conversion of reflection seismic data by first‐arrival tomographic inversion. In addition to geophysical investigations, 11 offshore boreholes were drilled for detailed logging. In situ standard penetration tests were performed on core samples with the use of a pocket penetrometer and pocket vane in order to obtain uniaxial compressive strength, undrained shear strength and undrained cohesion values, and assess the cohesive soils. During drilling, 17 undisturbed samples and 12 semi‐disturbed samples were extracted to perform laboratory tests for the identification of the principal geotechnical parameters. The goal was to obtain a reliable geological/geotechnical model in front of the Trieste Marine Terminal – from the seabed to the bedrock. Below the seafloor, a sequence of about 20–30 m thickness, containing Plio‐Quaternary soft sediments, overlies the flysch, which locally presents alteration with rocks of reduced quality. The geophysical–geotechnical integrated approach allowed us to identify and map the top of the bedrock and provided valuable information for planning the pier extension project.
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Detection and characterization of fracture zones in bedrock in marine environment: possibilities and limitations
Authors Georgios A. Tassis, Panos I. Tsourlos and Jan Steinar RønningABSTRACTGeoelectrical measurements have so far been tested in marine environments worldwide in order to detect subsea fracture zones. However, many of these datasets are processed without considering the extremely high electrical conductivity of seawater and its implications. This study summarizes our efforts to establish the basic rules as to whether marine electrical resistivity tomography can detect weak zones inside a resistive bedrock, a problem which the engineers in Norway usually encounter in tunnel construction sites. This study examines the theoretical response of electrical resistivity tomography in a classic Nordic environment where a highly resistive bedrock is located below the highly conductive seawater, and the capability of electrical resistivity tomography to detect fractured zones, as relevant in a geotechnical study. We performed a large number of synthetic modelling tests examining several factors that marine geoelectrical surveys are particularly sensitive to, such as the depth of the seabed, the seawater conductivity and the bedrock variation, and the survey layout and the inversion scheme. Our results indicate that electrical resistivity tomography surveys for fracture zone detection in geoelectrically demanding marine environments can be promising in case of a limited water depth, and with the use of either dipole–dipole or multiple gradient array and availability of a detailed knowledge of the conductivity distribution in water. However, results of electrical resistivity tomography surveys in such circumstances can be ambiguous since they potentially suffer from reduced resolution and due to the loss of electrical current in water and other artificial effects. Based on the results of modelling, we were able to improve interpretations of electrical resistivity tomography data from a field survey, where marine acquisition was carried at a strait in Kvitsøy, southern Norway.
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Volumes & issues
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Volume 22 (2024)
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Volume 21 (2023)
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Volume 20 (2022)
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Volume 19 (2021)
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Volume 18 (2020)
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Volume 17 (2019)
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Volume 16 (2018)
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Volume 15 (2017)
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Volume 14 (2015 - 2016)
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Volume 13 (2015)
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Volume 12 (2013 - 2014)
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Volume 11 (2013)
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Volume 10 (2012)
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Volume 9 (2011)
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Volume 8 (2010)
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Volume 7 (2009)
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Volume 6 (2008)
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Volume 5 (2007)
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Volume 4 (2006)
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Volume 3 (2005)
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Volume 2 (2004)
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Volume 1 (2003)