Variable-depth Streamer Acquisition Research Articles

Gas prediction using low-frequency components of variable-depth streamer seismic data applied to the deepwater area of the South China Sea

The deepwater area of the South China Sea has become a contentious aspect of marine oil and gas exploration in China in recent years due to its abundance of gas reservoir resources. However, the significant burial depth, rugged seabed, complex geological structure and notch effect resulting from the conventional constant-depth streamer (CDS) acquisition method give rise to missing low- and high-frequency components of deep signals. In addition, CDS seismic data present a low signal-to-noise ratio and resolution, thereby complicating gas prediction. Nevertheless, the variable-depth streamer (VDS) acquisition method can not only protect low- and high-frequency data and broaden the frequency band of seismic data, but it is also highly reliable and presents significant amplitude-preserving capacities. In consideration of these outstanding features, we propose combining low-frequency components of VDS data and fluid mobility attributes to predict gas reservoir distributions. Using a broadband wavelet and a narrowband wavelet as source wavelets to simulate VDS and CDS data of a 2D gas-bearing geologic model, we adopt a matching pursuit algorithm based on the Wigner-Ville distribution to calculate individually fluid mobility attributes corresponding to these two synthetic datasets of 1–10 Hz. The results of our studies based on synthetic and real data indicate that abundant low frequency components of VDS data can more accurately and objectively predict gas reservoir distributions while preventing the generation of multiple solutions for identifying gas reservoirs using low-frequency signals of CDS data. As a result, VDS seismic data can serve as a more precise and reasonable basis for gas exploration and development in the deepwater area of the South China Sea.

An efficient 4D processing flow for variable-depth streamer data

Variable-depth streamer acquisition is now widely used, achieving a bandwidth of more than 6 octaves (2.5 to 200 Hz) and excellent lateral resolution and providing improved impedance and reservoir analysis. This acquisition and processing method is based on towing the streamer deep, using a variable-depth profile optimized to ensure receiver notch diversity at the imaging stage and a deghosting method that takes advantage of the notch diversity. For 4D processing of monitors now acquired with variable-depth cables and with a baseline consisting of a flat towed streamer, we must deal with two processing issues. First, raypaths of traces acquired at the same surface locations differ; second, receiver ghosts lead to different wavelets. A 4D coprocessing flow is introduced that codatums 4D vintages to a common datum early in the sequence. Furthermore, a new deghosting technique is applied to variable-depth and conventional data. This processing flow gives excellent repeatability with 4D noise of less than 10% on a zero-time repeatability test with North Sea data from 2013.

Simplify the variable-depth streamer data processing through pre- migration deghosting: a case study from NWS Australia Data

Variable-depth streamer acquisition, as one of the key marine broadband solutions, has shown great advantages in providing high resolution seismic imaging and better low frequency penetration than conventional data from examples around the world. By utilizing the notch diversity, the receiver ghost was fully removed through proprietary joint deconvolution method in the imaging domain which produces broadband imaging with bandwidth up to six octaves. However, this post- migration deghosting method requires the receiver ghost to be well preserved prior to final migration thus introduces complexity in key steps like multiple attenuation, velocity analysis and migration than the conventional processing flow.The objective of this work is to simplify the processing flow by applying bootstrap deghosting method right after shot domain de-noise. After this step, processing flow will be very similar to the conventional flow. This new flow has been tested on the variable-depth streamer data from NWS Australia. Compared to the post-migration deghosting flow, the new results not only provide similar benefit as to broader bandwidth and rich images, but also show improvement on multiple attenuation and primary preservation as well as seismic inversion.

Variable-depth streamer acquisition: Broadband data for imaging and inversion

Conventional marine acquisition uses a streamer towed at a constant depth. The resulting receiver ghost notch gives the maximum recoverable frequency. To push this limit, the streamer must be towed at a quite shallow depth, but this compromises the low frequencies. Variable-depth streamer (VDS) acquisition is an acquisition technique aimed at achieving the best possible signal-to-noise ratio at low frequencies by towing the streamer very deeply, but by using a depth profile varying with offset in order not to limit the high-frequency bandwidth by notches as in conventional constant-depth streamer acquisition. The idea is to use notch diversity, each receiver having a different notch, so that the final result, combining different receivers, will have no notches. The key step to process VDS acquisitions is the receiver deghosting. We found that the optimal receiver deghosting, instead of being a preprocessing step, should be done postimaging, by using a dual-input, migration and mirror migration, and a new joint deconvolution algorithm that produces a 3D real amplitude deghosted output. This method can be applied poststack, the inputs being the migration and mirror migration images and the output being the deghosted image. Using a multichannel joint deconvolution, the inputs are the migrated and mirror migrated image gathers and the outputs are the prestack deghosted image gathers. This method preserves the amplitude-versus-offset behavior, as the deghosted output can be seen on synthetic examples to be equal to a reference computed by migrating the data modeled without any reflecting water surface. A real data set was used to illustrate this method, and another one was used to check the possibility of performing prestack elastic inversion on the deghosted gathers.

Variable-depth streamer acquisition: broadband data for imaging, post and pre-stack inversion

Variable-depth streamer acquisition is a broadband streamer acquisition technique where the depth profile of the streamer is optimized in order to ensure receiver ghost diversity, which in turn allows the deconvolution of the residual ghost at the imaging stage, pre-stack or post-stack. This technique allows the streamers to be towed at an average depth of several tens of meters, which combined with the use of solid streamers, ensures the raw data has an exceptionally good signal-to-noise ratio, especially at low frequencies. This variable-depth streamer acquisition and processing has been field-tested on a variety of locations, achieving bandwidth up to 6 octaves (2.5Hz -160 Hz). This broad bandwidth translates into improved results for the acoustic impedance inversion. The lack of low frequencies in conventional seismic data means that a low frequency model must be incorporated in the inversion process, obtained by interpolating low-passed filtered impedance logs between well locations. With variable-depth streamer data, high-resolution NMO-derived seismic velocities are used to define the low frequency model in a range 0-5Hz, while the reflectivity provides information from 2.5Hz. Variable-depth streamer data thus have the potential to fill the usual gap between the high frequencies of the seismic velocities and the low frequencies of the reflectivity, the 2.5-5 Hz octave being the overlapping zone. Pre-stack elastic inversion has also been performed, providing both impedance and Vp/Vs sections, proving the feasibility of pre-stack deghosting of variable-depth streamer data.

UKCS Cornerstone: a variable-depth streamer acquisition case study

George Moise, Geoff Body, Vincent Durussel, Fabrice Mandroux and Jo Firth fill in the details of how a broadband marine acquisition survey deploying variable-depth streamers was undertaken in the North Sea.