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- Volume 15, Issue 4, 2017
Near Surface Geophysics - Volume 15, Issue 4, 2017
Volume 15, Issue 4, 2017
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An ultra‐high‐resolution 3D marine seismic system for detailed site investigation
Authors Olivier Monrigal, Ivan de Jong and Henrique DuarteABSTRACTCurrently, marine ultra‐high‐resolution 3D surveys tend to be characterized by signal frequency ranges of 0–600 Hz and bin sizes of the order of 3 to 6 metres. This may be acceptable in industry geohazard studies for top‐hole well drilling, but it does not have sufficient resolution for the very detailed information required by geotechnical engineers for the design and positioning of the turbine foundations in offshore wind farms.
We present a newly developed ultra‐high‐resolution 3D system, which takes the 3D‐resolution and 3D‐imaging one step further: to frequency ranges up to 2.5 kHz and 1‐m bin sizes. This system is based on an ultra‐high‐resolution sparker source utilising negative discharge technology, thus guaranteeing a stable and repeatable source signature. The high‐fidelity multi‐channel recording system is composed of four 24‐trace streamers with GPS positioning on both sources and receivers. Sophisticated navigation processing guarantees decimetre precision and the resultant 1‐m bin sizes. In addition, slant streamer technology is applied to ensure a broadband seismic response. Processing techniques, including critical corrections for wave motion and tides, have been developed.
Trial surveys were conducted in Thailand to validate the method and later in Croatia to study the shallow geology for the foundations of a 2.5‐km bridge across a sea inlet. The resultant 3D cubes gave very detailed views of the subsurface allowing the imaging of stratigraphic structures and small‐scale features, such as individual boulders in a boulder bed, that had hitherto not been identified.
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Advanced processing for UHR3D shallow marine seismic surveys
Authors Henrique Duarte, Nigel Wardell and Olivier MonrigalABSTRACTTraditionally, commercial ultra‐high‐resolution 3D multichannel seismic surveys have used sources with relatively low‐frequency content (up to 600 Hz) with corresponding relatively low resolution. The new generation of multi‐tip sparkers, however, utilising negative discharge technology, is able to produce repeatable high‐frequency signals (above 2.5 kHz) with the potential to image hundreds of metres of the shallow stratigraphy at < 50 cm vertical resolution. These sources would be especially suited to ultra‐high‐resolution 3D studies such as the foundations of wind turbines installations for offshore wind farms.
However, simple downsizing of conventional processing sequences, designed for sources of significantly lower frequency content, is not enough. To preserve the signal properties of the sparker source and produce accurately positioned seismic images, new strategies are needed. The implementation of processing solutions that accurately determine source and receiver depths for all shots is also paramount to correctly reduce the data to the desired vertical datum.
A basic methodology for deriving geometry and shot corrections for wave motion and tides from the 3D data themselves has already been demonstrated. With Differential Global Positioning System (DGPS) navigation available on both source and streamers and the correct integration of the seismic recording and positioning systems, this concept has been further developed and refined to be more precise. Being able to derive components of the streamer position and depth is of great importance for effective broadband processing from the slanting streamers that are deployed in acquisition. New procedures and processing techniques have been developed, and their efficacy is demonstrated on case studies of recently acquired 2D and 3D multichannel data.
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Tuning, interference and false shallow gas signatures in geohazard interpretations: beyond the rule
Authors Bonita J. Barrett, Dei G. Huws, Adam D. Booth, Øystein Wergeland and J.A. Mattias GreenABSTRACTShallow gas presents a significant geohazard for drilling operations, with implications for costly well deviations and inherent blowout risks. The archetypal seismic signature of shallow gas—a “bright spot”—can be falsely induced by tuning, whereby reflections from closely separated horizons stack and constructively interfere. According to established guidelines, maximum constructive interference is typically expected where horizons are separated by one‐quarter wavelength of the seismic wavelet. Here, we test the circumstances in which false gas signatures can be induced from tuning and the conditions in which the guidelines for interference become problematic. We simulate normal‐incidence seismic data for a variety of reflectivity models, incorporating different contrasts in reflectivity magnitude and polarity. We simulate acoustic impedance by supplying initial geological parameters to Gassmann’s rock physics equations, allowing bulk density and compressional (P‐) wave velocity to vary between 1200–2100 kg/m3 and 1460–1670 m/s, respectively, for non‐gassy sediments and 1160 kg/m3 and 170–200 m/s for gassy sediments. Tuning is considered for a Ricker wavelet source pulse, having both peak frequency and effective bandwidth of 60 Hz. Tuning effects are able to mask a gas pocket, corresponding to a “false negative” signature that represents a significant hazard for drilling operations. Furthermore, the widely adopted assumption for constructive interference is not always valid as the brightest seismic responses can appear for thicker and thinner beds, depending upon the stratigraphy. Similar observations are made both qualitatively and quantitatively for real seismic responses, in which reflections from a series of dipping clinoforms interfere with those from an overlying unconformity. We conclude that greater attention should be paid to the interpretation of shallow gas risk; specifically, the effect of reflector geometries should not be overlooked as a means of producing or masking seismic amplitudes that could be indicative of a hazardous gas accumulation.
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A petrophysical approach to the investigation of shallow marine geology
Authors Francis Andrew Buckley and Lewis CotteeABSTRACTInterpretations of commercial marine shallow seismic data are generally limited to a prognosis of lithologies based on sparse ground truth data and regional stratigraphic models and a probability assessment of encountering perceived geohazards. However, an approach based on analyses of available petrophysical data can provide a more robust assessment of shallow marine lithologies and a more confident interpretation of shallow gas and overpressured formations. Generation of well‐ and borehole‐log acoustic impedance curves provides the starting point for these investigations and, although not all of the requisite data may be present in shallow section wireline suites, the Faust and Gardner equations, and other empirically derived relationships, can supply acceptable mathematical approximations. Impedance inversions of reflection seismic data facilitate more confident interpretations of lithological units, especially when combined with additional datasets such as gamma‐ray, resistivity, porosity, and S‐wave sonic, as elastic and extended elastic inversions. Anomalous seismic events interpreted to represent a probability of encountering shallow gas using traditional interpretation methods may be further investigated using amplitude‐versus‐offset cross‐plotting techniques, including fluid factor calculations, and the potential for overpressured gas accumulations or flow sands may be estimated from velocity‐derived pore pressure calculations. Geological and synthetic seismic modelling exercises provide the opportunity to test petrophysical interpretations in the absence of ground truth control data.
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State‐of‐the‐art remote characterization of shallow marine sediments: the road to a fully integrated solution
ABSTRACTCurrent methods for characterizing near‐surface marine sediments rely on extensive coring/pene‐trometer testing and correlation to seismic facies. Little quantitative information is regularly derived from geophysical data beyond qualitative inferences of sediment characteristics based on seismic facies architecture. Even these fundamental seismostratigraphic interpretations can be difficult to correlate with lithostratigraphic data due to inaccuracies in the time‐to‐depth conversion of geophysical data and potential loss and/or compression of high‐porosity and under‐consolidated sea‐floor material during direct sampling. To complicate matters further, when quantitative information is derived from marine geophysical data, it often describes the sediments using terminology (e.g., acoustic impedance and seismic quality factor) that is impenetrable to geologists and engineers. In contrast, for hydrocarbon prospecting, reservoir characterization using quantitative inversion of geophysical data has developed enormously over the past 20 years or more. Impedance and amplitude‐versus‐angle inversion techniques are now commonplace, whereas computationally expensive waveform inversions are gaining traction, and there is a well‐developed interface between these geophysical and reservoir engineering fields via rock physics.
In this paper, we collate and review the different published inversion methods for high‐resolution geophysical data. Using several case study examples spanning a broad range of depositional environments, we assess the current state of the art in remote characterization of shallow sediments from a multidisciplinary viewpoint, encompassing geophysical, geological, and geotechnical angles. By identifying the key parameters used to characterize the subsurface, a framework is developed whereby geological, geotechnical, and geophysical characterizations of the subsurface can be related in a less subjective manner. As part of this, we examine the sensitivity of commonly derived acoustic properties (e.g., acoustic impedance and seismic quality factor) to more fundamentally important soil properties (e.g., lithology, pore pressure, gas saturation, and undrained shear strength), thereby facilitating better integration between geological, geotechnical, and geophysical data for improved mapping of sediment properties. Ultimately, we present a number of ideas for future research activities in this field.
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How understanding past landscapes might inform present‐day site investigations: a case study from Dogger Bank, southern central North Sea
Authors Carol Cotterill, Emrys Phillips, Leo James, Carl‐Fredrik Forsberg and Tor Inge TjeltaABSTRACTThe integration of geophysical and geotechnical datasets acquired during a site survey for the Dogger Bank wind farm has enabled a new litho‐ and seismo‐stratigraphy to be established. Although previously believed to be a relatively simple “layer‐cake”, the data reveal that the sedimentary sequence within the foundation zone includes a complex series of buried landscapes with implications for both foundation siting and design. The most significant is a Weichselian glacially derived landscape dominated by a large thrust‐block moraine complex buried beneath a thin Holocene sequence. This glacial landscape profoundly affects the structure and physical properties of sediments within the foundation zone due to locally intense glaciotectonic deformation and the occurrence of sub‐aerially desiccated horizons recording fluctuating palaeo‐climatic conditions. Understanding these landscapes, coupled with the geophysical and geotechnical data, enables the development of a predictive “geo‐model” that may be used to target areas of uncertainty, reducing the requirement for boreholes (over Cone Penetration Tests) at every potential foundation location.
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Identifying and mitigating against potential seafloor and shallow drilling hazards at a complex Gulf of Mexico Deepwater site using HR3D seismic and AUV data
Authors Kern Kassarie, Stephen Mitchell, Martin Albertin, Andrew Hill and Robert CarneyABSTRACTThis paper presents an integrated and iterative approach to the identification and understanding of seafloor and shallow geohazards at a complex geologic setting in the deepwater Gulf of Mexico. The approach, focusing on geophysical data gathering and processing, allowed continuous improvement in the understanding of individual geohazard risks ahead of any eventual selection of wildcat drilling locations.
The progressive development of increased image resolution has included the use of the following data types: exploration 3D, autonomous underwater vehicle seabed clearance, short‐offset 3D, a full‐waveform inversion velocity model, HR3D acquisition, HR3D, autonomous underwater vehicle high‐resolution sonar, and seabed photographic transects.
Data have assisted in revealing the presence and the understanding of, numerous seafloor and sub‐seafloor geologic features that could potentially affect the choice of well location, well trajectory, and final drilling practices. Features that have been defined have included active benthic communities, widespread seafloor faulting, multi‐scale active seabed expulsion features, slope‐failure scarps, mass transport complexes, shallow gas, and various effects from the presence and transmission of shallow overpressure towards the seafloor.
The final, integrated geologic model that is developed will allow selection of well surface locations and trajectories that meet local regulatory requirements for safe drilling and environmental protection, as well as the adoption of safe drilling practices to mitigate identified hazards.
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Direct monitoring of active geohazards: emerging geophysical tools for deep‐water assessments
ABSTRACTSeafloor networks of cables, pipelines, and other infrastructure underpin our daily lives, providing communication links, information, and energy supplies. Despite their global importance, these networks are vulnerable to damage by a number of natural seafloor hazards, including landslides, turbidity currents, fluid flow, and scour. Conventional geophysical techniques, such as high‐resolution reflection seismic and side‐scan sonar, are commonly employed in geohazard assessments. These conventional tools provide essential information for route planning and design; however, such surveys provide only indirect evidence of past processes and do not observe or measure the geohazard itself. As such, many numerical‐based impact models lack field‐scale calibration, and much uncertainty exists about the triggers, nature, and frequency of deep‐water geohazards. Recent advances in technology now enable a step change in their understanding through direct monitoring. We outline some emerging monitoring tools and how they can quantify key parameters for deep‐water geohazard assessment. Repeat seafloor surveys in dynamic areas show that solely relying on evidence from past deposits can lead to an under‐representation of the geohazard events. Acoustic Doppler current profiling provides new insights into the structure of turbidity currents, whereas instrumented mobile sensors record the nature of movement at the base of those flows for the first time. Existing and bespoke cabled networks enable high bandwidth, low power, and distributed measurements of parameters such as strain across large areas of seafloor. These techniques provide valuable new measurements that will improve geohazard assessments and should be deployed in a complementary manner alongside conventional geophysical tools.
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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)