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- Volume 67, Issue 6, 2019
Geophysical Prospecting - 6 - Geophysical Instrumentation and Acquisition, 2019
6 - Geophysical Instrumentation and Acquisition, 2019
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Marine vibrators: the new phase of seismic exploration
Authors R.M. Laws, D. Halliday, J.‐F. Hopperstad, D. Gerez, M. Supawala, A. Özbek, T. Murray and E. KraghABSTRACTMarine seismic vibrators are generally considered to be less intrusive than airguns from an environmental perspective. This is because they emit their energy spread out in time, rather than in a single, high‐intensity pulse. There are also significant geophysical benefits associated with marine vibrators, and they stem from the ability to specify in detail the output acoustic waveform. The phase can be specified independently at each frequency. Such detailed control cannot be achieved with conventional airgun sources, where the phase can only be modified using simple overall time delays. The vibrator phase can be employed in several different ways: it can be applied to the overall source phase in a sequence so that it varies from one source point to the next; it can be applied to the individual vibrators within the source array so the source directivity is changed; it can be applied to the overall source phase of each source in a simultaneous source acquisition. Carefully designed phase sequences can attenuate the residual source noise, and this in turn allows extra source points to be interleaved between the conventional ones. For these extra source points, the relative phase of the vibrators within the array can be chosen to create a transverse gradient source, which illuminates the earth predominantly in directions out of the plane of the sail line without left/right ambiguity. If seismic vibrator data are acquired using interleaved conventional and transverse gradient sweeps, more information is collected per kilometre of vessel travel than is the case in conventional acquisition. This richer data acquisition leads to the possibility of acquiring all the necessary seismic data in a shorter time. Three‐dimensional reconstruction techniques are used to recover the same image quality that would have been obtained using the conventional, more time‐consuming acquisition. For a marine vibrator to be suitable for these techniques it must, in general terms, have ‘high fidelity’. The precise device specifications are defined through realistic end‐to‐end simulations of the physical systems and the processing. The specifications are somewhat more onerous than for a conventional vibrator, but they are achievable. A prototype vibrator that satisfies these requirements has been built. In a simulated case study of a three‐dimensional deep‐water ocean bottom node survey, the seismic data could have been acquired using marine vibrators in one third of the time that it would have taken using airguns.
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Making the transition from discrete shot records to continuous seismic records and source wavefields, and its potential impact on survey efficiency and environmental footprint
Authors Stian Hegna, Tilman Klüver, Jostein Lima and Jens F. WisløffABSTRACTA marine seismic method based on continuous source and receiver wavefields has been developed. The method requires continuous recording of the seismic data. The source that may consist of multiple source elements can emit signals continuously while moving. The ideal source wavefield to be used with this method should be as white as possible both in a temporal and a spatial sense to avoid deep notches in the spectrum enabling a stable multi‐dimensional deconvolution. White noise has such properties. However, equipment that can generate white noise does not exist. In order to generate a continuous source wavefield that is approaching the properties of white noise using existing equipment onboard marine seismic vessels, individual air‐guns can be triggered with short randomized time intervals in a near‐continuous fashion. The main potential benefits with the method are to reduce the environmental impact of marine seismic surveys and to improve acquisition efficiency. The peak sound pressure levels are significantly reduced by triggering one air‐gun at a time compared to conventional marine seismic sources. Sound exposure levels are also reduced in most directions. Since the method is based on continuous recording of seismic data and the air‐guns are triggered based on time and not based on position, there are less vessel speed limitations compared to conventional marine seismic data acquisition. Also, because the source wavefield is spread out in time, the wavefields emitted from source elements in different cross‐line positions can be designed such that the emitted wavefield is spatially white in this direction. This means that source elements in multiple cross‐line positions can be operated simultaneously, potentially improving the cross‐line sampling and/or the acquisition efficiency.
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Perfect partial reconstructions for multiple simultaneous sources
Authors Jens Wittsten, Fredrik Andersson, Johan Robertsson and Lasse AmundsenABSTRACTA major focus of research in the seismic industry of the past two decades has been the acquisition and subsequent separation of seismic data using multiple sources fired simultaneously. The recently introduced method of signal apparition provides a new take on the problem by replacing the random time‐shifts usually employed to encode the different sources by fully deterministic periodic time‐shifts. In this paper, we give a mathematical proof showing that the signal apparition method results in optimally large regions in the frequency–wavenumber space where exact separation of sources is achieved. These regions are diamond shaped and we prove that using any other method of source encoding results in strictly smaller regions of exact separation. The results are valid for arbitrary number of sources. Numerical examples for different number of sources (three, respectively, four sources) demonstrate the exact recovery of these diamond‐shaped regions. The implementation of the theoretical proofs in the field is illustrated by the results of a conducted field test.
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Blended‐acquisition design of irregular geometries towards faster, cheaper, safer and better seismic surveying
Authors Shotaro Nakayama, Gerrit Blacquière, Tomohide Ishiyama and Satoshi IshikawaABSTRACTThe application of blended acquisition has drawn considerable attention owing to its ability to improve the operational efficiency as well as the data quality and health, safety and environment performance. Furthermore, the acquisition of less data contributes to the business aspect, while the desired data density is still realizable via subsequent data reconstruction. The use of fewer detectors and sources also minimizes operational risks in the field. Therefore, a combined implementation of these technologies potentially enhances the value of a seismic survey further. One way to encourage this is to minimize any imperfection in deblending and data reconstruction during processing. In addition, one may derive survey parameters that enable a further improvement in these processes as introduced in this study. The proposed survey design workflow iteratively performs the following steps to derive the survey parameters responsible for source blending as well as the spatial sampling of detectors and sources. The first step is the application of blending and sampling operators to unblended and well‐sampled data. We then apply closed‐loop deblending and data reconstruction. The residue for a given design from this step is evaluated and subsequently used by genetic algorithms to simultaneously update the survey parameters related to both blending and spatial sampling. The updated parameters are fed into the next iteration until they satisfy the given termination criteria. We also propose a repeated encoding sequence to form a parameter sequence in genetic algorithms, making the size of problem space manageable. The results of the proposed workflow are outlined using blended dispersed source array data incorporating different scenarios that represent acquisition in marine, transition zone and land environments. Clear differences attributed solely to the parameter design are easily recognizable. Additionally, a comparison among different optimization schemes illustrates the ability of genetic algorithms along with a repeated encoding sequence to find better solutions within a computationally affordable time. The optimized parameters yield a notable enhancement in the deblending and data reconstruction quality and consequently provide optimal acquisition scenarios.
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Prediction of signatures of airgun arrays using dual near‐field hydrophones
Authors Maksym Kryvohuz and Xander CampmanABSTRACTSuccessful estimation of airgun‐array signatures from near‐field measurements depends on the accuracy of poorly controlled model parameters such as the effective sea surface reflection coefficient and source depth. We propose a method for prediction of robust source signatures, which are insensitive to fluctuations of the latter parameters. The method uses vertical pairs of near‐field hydrophones to measure near‐field pressure and its vertical gradient, combination of which eliminates sea surface reflections from the near‐field data. This excludes the uncertainty related to the fluctuating sea state and source depth from the process of inversion of the near‐field data for source signature. The method explicitly separates the recorded near‐field pressure into its up and down going components, which allows one to measure the effective frequency‐ and angle‐dependent sea surface reflection coefficient right at the source, as well as to estimate the actual source depth. Tests on synthetic and field data demonstrate robust performance of the method.
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Research Note: Low‐frequency pneumatic seismic sources
Authors Steve Chelminski, Leighton M. Watson and Shuki RonenABSTRACTPneumatic seismic sources, commonly known as airguns, have been serving us well for decades, but there is an increasing need for sources with improved low‐frequency signal and reduced environmental impact. In this paper, we present a new pneumatic source that is designed to achieve these goals by operating with lower pressures and larger volumes. The new source will release more air creating larger bubbles with longer bubble periods than airguns. The release of the air will be tuned so that the rise time will be longer and the sound pressure level and its slope will be lower. Certain engineering features will eliminate cavitation. Larger bubbles increase low‐frequency content of the signal, longer rise times decrease mid‐frequency content and the elimination of cavitation reduces high‐frequency content. We have not yet built a full‐scale version of the new source. However, we have manufactured a small‐scale low‐pressure source incorporating most of the engineering features, and tested it in a lake. Here, we present the lake data that, as expected, show a significant reduction in the sound pressure level, increase in rise time, decrease in slope and decrease in high‐frequency content while maintaining the same low‐frequency content when the source prototype is operated at low pressure compared with high pressure. Synthetic data produced by numerical modelling of the full‐scale proposed pneumatic source suggest that the new source will improve the low‐frequency content and can produce geophysically useful signal down to 1 Hz.
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Seafloor seismic acquisition using autonomous underwater vehicles
Authors Constantinos Tsingas, Thierry Brizard and Abdulaziz Al MuhaidibABSTRACTAlthough narrow‐azimuth towed‐streamer data provide good image quality for structural interpretation, it is generally accepted that for wide‐azimuth marine surveys seabed receivers deliver superior seismic reflection measurements and seismically derived reservoir attributes. However, seabed surveys are not widely used due to the higher acquisition costs when compared to streamer acquisition. In recent years, there have been significant engineering efforts to automate receiver deployment and retrieval in order to minimize the cost differential and conduct cost‐efficient seabed receiver seismic surveys. These engineering efforts include industrially engineered nodes, nodes on a rope deployment schemes and even robotic nodes, which swim to and from the deployment location. This move to automation is inevitable, leading to robotization of seismic data acquisition for exploration and development activities in the oil and gas industry. We are developing a robotic‐based technology, which utilizes autonomous underwater vehicles as seismic sensors without the need of using a remotely operated vehicle for deployment and retrieval. In this paper, we describe the autonomous underwater vehicle evolution throughout the project years from initial heavy and bulky nodes to fully autonomous light and flexible underwater receivers. Results obtained from two field pilot tests using different generations of autonomous underwater vehicles indicate that the seismic coupling, and navigation based on underwater acoustics are very reliable and robust.
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Seismic receiver coupling to the seafloor
More LessABSTRACTThe presence of geophysical receivers on the seafloor changes the local wave field due to the receiver seafloor interaction. The resulting PP‐ and PS‐wave distortion of the wave field is often referred to as receiver coupling to the seafloor and can make data processing challenging and sometimes impossible. This paper provides an overview of the mathematical approaches to describe receiver coupling, how to estimate receiver coupling and what the difficulties and possible solutions are. The first section shows how the mathematical approach developed from a simple model considering only the vertical receiver component to include all three receiver components and their interactions with the seafloor. In the second section, I show how receiver coupling can be measured and how it can be improved using mathematical and data‐driven approaches.
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Vertical–vertical controlled‐source electromagnetic instrumentation and acquisition
Authors Stefan L. Helwig, William Wood and Bernard GlouxABSTRACTVertical–vertical controlled‐source electromagnetic is an alternative to other techniques for providing three‐dimensional resistivity images of the subsurface. It utilizes a large and powerful vertical dipole transmitter and arrays of E‐field receivers with vertical and horizontal dipole sensors. The necessary instrumentation and acquisition procedures which differ strongly from other controlled‐source electromagnetic methods are described in the paper.
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On‐site calibration of air‐coil sensor for transient electromagnetic exploration
Authors Haowen Wang, Zhihong Fu, Yao Wang, Heng‐Ming Tai and Wenli ChenABSTRACTThis paper proposes a time‐domain fitting method for on‐site calibration of the air‐coil sensor. The air‐coil sensor has been widely used in transient electromagnetic exploration. Due to limited bandwidth of the coil, the output signal is distorted, causing a phenomenon known as the transition process. To accurately measure the magnetic field from the output signals, the relationship between the coil induced electromotive force and the output voltage must be confirmed by on‐site calibration, which requires high calibration accuracy and demands simple operation, portable equipment, and adaptability to the environment. Conventional frequency response methods, however, requires a uniform magnetic field with various frequencies to obtain the frequency response curve of the air‐coil sensor. The time to acquire the signal correlates with the number of test frequencies, and the equipment used to generate a uniform magnetic field must be tailored to the shape of the air‐coil sensor under test. This paper constructs a relationship between the calibration file and the zero‐input response of the air‐coil sensor and designs an optimization algorithm to suppress the soil eddy current effect. This on‐site calibration method lifts the dependence on the uniform calibration field and reduces significantly the time required for calibration. The calibration source can be generated by cutting off the voltage source in parallel to the calibration coil, which greatly reduces the cost of the signal generator and provides a better solution for realizing the embedded self‐test devices. Experimental results show that the proposed method effectively improves the calculation accuracy of the apparent resistivity.
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Satellite gravity – enhancements from new satellites and new altimeter technology
Authors C.M. Green, K.M.U. Fletcher, S. Cheyney, G.J. Dawson and S.J. CampbellABSTRACTThis paper reviews the impacts of new satellite altimeter data sets and new technology on the production of satellite gravity. It considers the contribution of the increased data volume, the application of new altimeter acquisition technology and the potential for future developments. Satellite altimeter derived gravity has provided gravity maps of the world's seas since the 1980s, but, from 1995 to 2010, virtually all improvements were in the processing as there were no new satellite data with closely spaced tracks. In recent years, new data from CryoSat‐2 (launched in 2010) and the geodetic mission of Jason‐1 (2012–2013) have provided a wealth of additional coverage and new technology allows further improvements. The synthetic aperture radar mode of CryoSat‐2 uses a scanning approach to limit the size of the altimeter sea surface footprint in the along‐track direction. Tests indicate that this allows reliable data to be acquired closer to coastlines. The synthetic aperture radar interferometric mode of CryoSat‐2 uses two altimeters to locate sea‐surface reflection points laterally away from the satellite track. In a study to generate gravity for freshwater lakes, this mode is found to be valuable in extending the available satellite coverage. The AltiKa altimeter uses higher frequency radar to provide less noisy sea‐surface signals and its new orbit mode gives potential for further improvements in satellite gravity. Future developments include the potential for swath mapping to provide further gravity improvements.
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The Gravity Module Assembly used in airborne full tensor gradiometry surveys
By G.J. BarnesABSTRACTIn airborne gravity gradiometry, the Gravity Module Assembly is an optional gravimeter unit that is mounted on the same stabilized platform as the Full Tensor Gradiometer. Direct measurements of the gravity field are needed from this device to constrain the long wavelengths when gradient data are integrated mathematically to form high‐resolution gravity fields. The Gravity Module Assembly is, however, capable of providing independent gravity data with a specification approaching that expected from a dedicated airborne gravity system. Presented here is an error analysis of data from this instrument collected alongside the Full Tensor Gradiometer during an airborne survey. By having both gradiometry and gravity datasets, comparisons of the information content in these two types of measurement are made.
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Heli‐borne gravity gradiometry in rugged terrain
Authors Mark H. Dransfield and Tianyou ChenABSTRACTFor airborne gravity gradiometry in rugged terrain, helicopters offer a significant advantage over fixed‐wing aircraft: their ability to maintain much lower ground clearances. Crucially, this provides both better signal‐to‐noise and better spatial resolution than is possible with a fixed‐wing survey in the same terrain. Comparing surveys over gentle terrain at Margaret Lake, Canada, and over rugged terrain at Mount Aso, Japan, demonstrates that there is some loss of spatial resolution in the more rugged terrain. The slightly higher altitudes forced by rugged terrain make the requirements for terrain correction easier than for gentle terrain. Transforming the curvature gradients measured by the Falcon gravity gradiometer into gravity and the complete set of tensor components is done by a Fourier method over gentle terrain and an equivalent source method for rugged terrain. The Fourier method is perfectly stable and uses iterative padding to improve the accuracy of the longer wavelengths. The equivalent source method relies on a smooth model inversion, and the source distribution must be designed to suit the survey design.
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Real‐time compensation of magnetic data acquired by a single‐rotor unmanned aircraft system
Authors L. Tuck, C. Samson, C. Polowick and J. LalibertéABSTRACTTwo methods for low‐altitude calibration of a single‐rotor unmanned aircraft system using a real‐time compensator are tested: (1) a stationary calibration where the unmanned aircraft system executes manoeuvres while hovering in order to minimize ambient field changes due to the local geology; and (2) an adapted box calibration flown in four orthogonal directions. Both methods use two compensator‐specific limits derived from established methods for manned airborne calibration: the lowest frequency used by the compensator for the calibration algorithm and the maximum variation of the ambient magnetic intensity experienced by the unmanned aircraft system during calibration. Prior to flying, the unmanned aircraft system was magnetically characterized using the heading error and fourth difference. Magnetic interference was mitigated by extending the magnetometer‐unmanned aircraft system separation distance to 1.7 m, shielding, and demagnetization. The stationary calibration yielded an improvement ratio of 8.595 and a standard deviation of the compensated total magnetic intensity of 0.075 nT (estimated Figure‐of‐Merit of 3.8 nT). The box calibration also yielded an improvement ratio of 3.989 and a standard deviation of the compensated total magnetic intensity of 0.083 nT (estimated Figure‐of‐Merit of 4.2 nT). The stationary and box calibration solutions were robust with low cross‐correlation indexes (1.090 and 1.048, respectively) when applied to a non‐native data set.
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Designing and testing a network of passive seismic surveying and monitoring in Dehdasht (South Western Iran)
ABSTRACTFrom August 2016 to July 2017, a passive seismic survey was conducted in South Western Iran as a part of a pilot project aimed to improve the imaging in geologically complex areas. Passive seismic methods have shown to be a useful tool to infer the physical properties of the underground geological structures where traditional hydrocarbon exploration methods are challenging. For this purpose, a dense passive seismic network consisting of 119 three‐component borehole seismic stations was deployed over an area of 400 km2 around the city of Dehdasht. This paper focuses on the details of the network design, which was devoted to high‐resolution seismological applications, including local earthquake tomography and seismic attenuation imaging. In this regard, we describe the instrument types and the station installation procedures used to obtain high‐quality data that were used to retrieve three‐dimensional models of P‐ and S‐wave velocity and P‐wave attenuation in the area using tomographic inversion techniques. We also assess the network performance in terms of the seismic ambient noise levels recorded at each station site, and we revise the horizontal orientation of the sensors using surface waves from teleseismic earthquakes.
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Integrated geophysical exploration in onshore frontier basins
Authors F. Martini, E. Rogers, S. Bennett, R. Davi, J.T. Doherty and J. MonganABSTRACTFaced with the challenge of rapidly screening a huge expanse of frontier exploration acreage, often characterized by sparse vintage data, it is our experience that a combination of appropriate air‐ and ground‐based geophysical techniques contributes positively to the exploration value chain. Airborne gravity gradiometry in conjunction with conventional gravity and magnetic data, as well as geological knowledge, add significant value to the screening process. This combination can subsequently assist in optimizing the location of the more time‐consuming and expensive seismic programme. In addition, analysis and inversion of passive seismic data have also proven useful in providing depth to basement estimates, and results derived from all the techniques investigated have been consistent within several study areas. Following initial tests, where the data were independently analysed and cross‐checked for consistency (including comparisons with active source seismic data and well data, when available), the company now routinely adopts the integration of these techniques in our frontier exploration acreage to support sedimentary basin delineation and mapping. This allows the optimal positioning and focussing of the higher spend and higher footprint programmes, such as active reflection seismic.
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Sedimentary basins investigation using teleseismic P‐wave time delays
Authors Nicola Piana Agostinetti and Francesca MartiniABSTRACTPassive seismic methods have been proven successful in recent years at retrieving information about the large‐scale structure of a sedimentary basin. These methods are based on ambient noise recordings, and local and distant (teleseismic) earthquake data. In particular, it has been previously observed that the arrival time of teleseismic P‐waves recorded inside a sedimentary basin shows time delays and polarization that both strongly depend on the basin properties and structure. In this paper, we present a new methodology for determining seismic P‐wave velocity in a sedimentary basin, based on the time delay of a teleseismic P‐wave travelling through the low‐velocity basin infill, with respect to a teleseismic wave recorded outside the basin. The new methodology is developed in a Bayesian framework and, thus, it includes estimates of the uncertainties of the P‐wave velocities. For this study, we exploit synchronous recordings of teleseismic P‐wave arrivals at a dense linear array of broadband seismic stations, using data from two teleseismic events coming from two different incoming angles. The results obtained by the new proposed methodology are successfully compared to classical cross‐correlation measurements, and are used to infer properties of a sedimentary basin, such as the basin bounding fault's geometry and the average P‐wave velocity of the sedimentary basin fill.
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The construction of a simple portable electromagnetic vibrator from commercially available components
Authors Timothy Dean, Hoang Nguyen, Anton Kepic and David HallidayABSTRACTSince its introduction in the late 1950s, hydraulic vibrators have become the dominant source for land seismic surveys. The hydraulic vibrators typically used for commercial land seismic acquisition, however, are large, costly to operate and expensive to purchase. This inhibits their use for small‐scale and short‐duration surveys as well as Vibroseis research. In this paper we describe, in detail, the construction of a portable vibrator from commercially available components for a cost of less than $US2,000. Data shows that the vibrator is able to successfully transmit sweeps from 15 to 180 Hz with different spectral contents. The vibrator produces a stronger signal than a sledgehammer and we estimate its output to be around 1 kN. The frequency content of the data was concentrated at lower frequencies (<100 Hz) and the ground‐roll was far more energetic than that produced using a sledgehammer.
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Volumes & issues
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Volume 72 (2023 - 2024)
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Volume 71 (2022 - 2023)
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Volume 70 (2021 - 2022)
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Volume 69 (2021)
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Volume 68 (2020)
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Volume 67 (2019)
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Volume 66 (2018)
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Volume 65 (2017)
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