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Author: Andrew Long
Industry Insights - 19 August 2019
In the follow-up to the previous newsletter, I begin by considering new source designs wherein the sources are towed with larger lateral source separation. This leads to each source naturally comprising one sub-array of air guns. If we want to achieve the ambition of ‘dispersed sources’ wherein the streamer spread is surrounded by a flotilla of small sources we must also develop cost-effective and operationally-robust source vessel concepts—no easy task. I then compare various compact source concepts in terms of their three-dimensional pSPL and SEL. Some surprising results suggest that one sub-array may not be much different to several sub-arrays in terms of received sound levels. New Zealand now has one of the strictest regulatory regimes for air gun operations, and I use that as a template to suggest how ‘compact’ sources could be defined in an environmental context. I also share another surprising result wherein substantial amplitudes may be recorded in the 1-2 Hz frequency range when single air guns are continuously activated and recorded in a particular manner. Collectively, the notion of ‘compact’ sources suggests that point sources, whether impulsive or electromechanical in nature, provide several intriguing opportunities.
[PDF] Focus on Operational Efficiency and Crew Safety - Introducing Advanced ROV Technology in Marine Towed Streamer Seismic
30 May 2016
Barnacles have been hampering the seismic industry since the start of the offshore seismic era. They attach to the streamer and impact signal to noise. Particularly in the areas close to the equator, there is a constant fight against the barnacles. In-sea cleaning of streamers is typically performed from workboat, exposing the crews to risk.
A method is presented that removes the human interaction in barnacle cleaning and at the same time significantly improves cleaning window and hence operational efficiency. The method is based on a recently developed Remotely Operated Vehicle (ROV). Unlike traditional ROV’s, typically operated close to zero forward speed, this ROV is launched and operated at seismic speed. The ROV is launched off the deck of support vessels capable of operating over the streamer spread. A typical operational mode for the ROV is to carry self-propelled streamer cleaners and place them on the streamer while at operational depth. The streamer cleaners then travels along the streamer while performing the cleaning duty and are finally recovered by the ROV after having reached the streamer tail end.
[PDF] North Sea case study: Heavy oil reservoir characterization from integrated analysis of Towed Streamer EM and dual-sensor seismic data
Author: Zhijun Du, Kerry Key
ASEG - 15 February 2015
Integrated analysis of geophysical data can provide valuable information on reservoir properties, on the basis of which exploration, appraisal, and development decisions can be made. Hence, we have introduced a quantitative interpretation workflow that integrates dual-sensor seismic and Towed Streamer controlled-source electromagnetic (CSEM) data. The workflow was designed to facilitate a reliable extraction of the complementary information from the two datasets. The seismic contribution starts with a depth-converted sparse horizon model to initialize the EM inversion, but it is not placed rigidly. This makes good sense when taking into account the uncertainties in seismic data, in the time to depth conversion, and more importantly, the fact that a reservoir can be hydrocarbon-charged to an unknown degree corresponding to the spill-point or less. We show how this approach enables a robust and reliable workflow for integrating EM and 3D seismic data with data examples acquired in an area with the complex geology of the Bressay, Bentley and Kraken (BBK) fields in the North Sea. The three heavy oil reservoirs are injectites, located in close proximity to other high resistivity settings, such as the shallow gas in the overburden, regional Balder Tuff and granite intrusions, resulting in challenging imaging issues.
[PDF] Can high-resolution reprocessed data replace the traditional 2D high-resolution seismic data acquired for site surveys?
Author: Julien Oukili, Jean-Paul Gruffeille, Christian Otterbein, Benjamin Loidl
First Break - 1 August 2019
The authors describe a process that would enable operators to save both time and costs on conducting conventional site surveys by applying separated wavefield imaging to existing 3D seismic data.
Author: Hermann Lebit, Jeff Tilton, Sriram Arasanipalai, Pascal Ollagnon
GeoExpro - 2 September 2019
The timing of continental extension relative to salt deposition and the impact of pre-salt faulting is critical to reservoir formation and exploration success in the South Atlantic. Major advances in broadband seismic data processing offer clearer imaging, allowing a closer look at the elements of the prolific plays and their tectono-stratigraphic boundary conditions.
1 June 2014
Dual sensor streamer acquisition allows the data to be accurately separated into upgoing and downgoing wavefields. The two components are used as input to migration that employs primaries and multiples for separated wavefield imaging (SWIM).
SWIM turns each receiver into a “virtual” source, therefore effectively increases the surface coverage and subsurface illumination. This principle is independent of the propagation algorithm, which can be based on one-way or two-way solutions to the wave equation. We find that SWIM based on reverse time migration (RTM) better handles the steep dips of the data. On the other hand, SWIM using one-way wave equation migration (WEM) provides an efficient alternative for high-resolution high-frequency imaging.
[PDF] Broadband Velocity Model Building and Imaging Using Reflections, Refractions and Multiples from Dual-sensor Streamer Data
1 June 2014
We demonstrate a novel workflow using reflections, refractions and multiples for building highly accurate PSDM velocity models for a complex geological setting. By combining wavelet shift tomography, full waveform inversion and separated wavefield imaging, we are able to produce high-resolution velocity models that are ideally suited for imaging of broadband data. Leveraging dual sensor streamer technology and the wavefield separation that comes with it, we are using up- and down-going wavefields in imaging and tomography to improve resolution and illumination. Further, we utilize the refracted, low-frequency energy for FWI. As the streamer is towed deep, we preserve the low frequencies that are so important for the success of FWI, but without sacrificing a broadband signal that is key for producing high-resolution reflection images of the shallow overburden and deep reservoir sections.
[PDF] Determining Resistivity from Towed Streamer EM Data Using Unconstrained Inversion - Tie to Well and Discovery Examples
1 June 2014
We show that unconstrained anisotropic 2.5D inversion of Towed Streamer EM data in complex geological settings can produce resistivity models that are consistent with both interpreted log and seismic data, and known discoveries. We consider two cases from recent surveys in the Celtic and Barents Seas offshore Ireland and Norway respectively. In the Celtic Sea case we show an example where we have compared the results of unconstrained inversion to publically available log data. Not only is the overall depth trend recovered, but the main variation of the resistivity is captured as well as, in some intervals, comparable average interval resistivity.
For the Barents Sea case we show an example resistivity and anisotropy section from one of eight survey lines that traverse the Skrugard discovery across its short axis (about 2km). While the resistivity section highlights that the sub-surface resistivity is complex, the somewhat simpler anisotropy section reveals an anisotropy anomaly that is co-incident with both the lateral, and depth, extent of Skrugard.
1 June 2014
Towed streamer electromagnetic (EM) data over the Mariner oilfield in the UK sector of the North has been inverted using a fast and efficient 3D anisotropic inversion code. The electric field data were acquired with a single vessel using a horizontal bipole source and sensors housed in a towed streamer in a densely sampled grid over the subsurface volume of interest. The inversion algorithm is based on the 3D contraction integral equation method and utilizing a re-weighted regularized conjugate gradient technique to minimize an objective functional.
This inversion method is proven to be fast and efficient for large data sets and is here shown to be suitable for towed streamer EM data from complex geological environments such as the Mariner area. In this case, the final 3D resistivity cube after inversion and with a corresponding misfit of 6.4 %, agrees well with the expected structure from seismic data and well logs. In particular, the 3D cube contains a resistive anomaly of 8-10 Ωm corresponding to the Maureen and Heimdal reservoirs on top of the resistive chalk and basement.
Author: Andrew Long
Industry Insights - 23 September 2020
I discuss the important similarities and distinctions between RTM (Reverse Time Migration), LS-RTM and FWI, and also relate their fundamental reliance upon back-projection to Deep Neural Network training used in many machine learning pursuits. Hence, the title of this newsletter points to the powerful concept that is paving the ways forward for reducing uncertainty in seismic imaging and seismic characterization in coming years. This newsletter is also written to accompany the TechByte webinars on ‘Least-Squares Migration’ and ‘Illumination vs AVO’.