For the past 80 years the seismic technique has had the most significant impact on improving exploration success. However, marine seismic acquisition has essentially remained the same since the development of the towed streamer more than 60 years ago. There have been many improvements, and the quality of seismic data and the quantitative information derived from it provide a critical component of the exploration process. Yet the fundamental effects of the marine method have always impacted the integrity of the resultant seismic data.
Extracting quantitative rock property information is dependent on bandwidth, and the lack of low-frequency content in conventional marine seismic leaves a gap that causes geoscientists to depend more heavily on sparse well data. Important high frequencies also are limited by the inherent features of the method.
By acting like a mirror, the sea surface reflects both the seismic source energy at its generation and the upcoming seismic wavefield at the receivers during recording. The impact of these “ghosts” is to limit the bandwidth that can be recorded and also the data integrity due to the effect of notches at particular frequencies. The reflection coefficient at the sea surface is very close to -1, which produces a close-to-perfect reflection as well as a phase change. The impact of the resultant dipole function is to create notches in the spectrum at frequencies that depend on the respective depths of the source and receiver. For example, a depth of 6 m (20 ft) produces a notch at 125 Hz, while a 15-m (50-ft) depth produces notches at 50, 100, and 150 Hz. The combination of these source and receiver ghost functions creates an elongated wavelet, the spectrum of which contains two sets of notches. Some control can be exercised in acquisition by varying the depth of the source and streamers, but the full bandwidth remains compromised. The ideal response would be to remove the two ghosts and reduce the elongated wavelet to a single spike, with a notch-free, flat spectrum. The resultant impact on the resolution of seismic data is extremely large.
PGS introduced the dual-sensor GeoStreamer to address the receiver ghost and provided commercially practical broadband seismic. This technology uses velocity sensors combined with pressure sensors to separate the up- and down-going wavefields and remove the receiver ghost effects. Development of an equivalent ghost-free technology for the source side, the GeoSource, has now been accomplished and operates in partnership with the GeoStreamer to provide a complete ghost-free solution, GeoStreamer GS.
Removing the source ghost
The new source is time- and depth-distributed, using sub-sources deployed at specific depths and fired with specific firing time delays. The depths of the sub-sources are chosen so that the ghost functions are complementary, avoiding deep notches in the spectrum. The firing time delays of the sub-sources within a GeoSource would generally be less than one second. This means the geology illuminated by each sub-source is essentially identical, and the receivers are in the same locations when the sub-sources fire. In addition, the GeoSource preserves the same shot efficiency and density as a conventional source.
These features of the geometry are important in data processing. Techniques for separating the wavefields from sub-sources have been developed that use the known firing time delays. The methodology has many advantages, not the least of which is the robustness of the ghost removal. As a direct result of this, there are no source-related ghost notches in the spectrum of the resultant seismic data. This means that the source can be deployed at a wider range of depths within practical limits. In particular, sub-sources can be deployed deeper than conventional sources without compromising the higher frequency spectrum with deep ghost notches.
Revealing the true earth response
Seismic data acquired using the new system show excellent results. Lines were acquired in the Norwegian Sea with a variety of streamer depths and source array parameters. Conventional hydrophone-only data were acquired with a streamer depth of 8 m (26 ft) and a conventional source towed at a depth of 5 m (16.4 ft). The GeoStreamer GS line towed the dual-sensor streamer at 25 m (82 ft), while the sub-sources in the GeoSource were towed at 10 m (33 ft) and 14 m (46 ft). The conventionally acquired seismic data are significantly defocused, whereas GeoStreamer GS data clearly show detailed structure revealing the true earth response. Frequency analysis of the two datasets shows the two sets of notches caused by the source and receiver ghosts and a decaying spectrum caused by the earth filtering effect. By contrast, GeoStreamer GS demonstrates the effect of removing the various responses imposed by the acquisition system and earth filtering effects and shows a flat spectrum and a good signal-to-noise ratio in the data all the way up to ~200 Hz. The results provide a step change in data resolution and interpretability. Subtle stratigraphic and structural features are easily interpreted on the data, whereas many of the same geological features cannot be resolved on conventional data.
The acquisition-based solution enables robust removal of both the source and receiver ghosts at an early stage in the pre-processing sequence. This delivers advantages for subsequent processing steps such as demultiple, velocity analysis, and imaging, and it produces high-quality data both pre- and poststack.
The implications for future developments are significant. The resultant seismic data has high resolution and a wide broadband response covering the entire spectrum of interest. Improved reservoir delineation, reservoir characterization, and the monitoring of changes during production are all areas that will directly exploit the unprecedented bandwidth of ghost-free acquisition, reducing reliance on well data to control rock properties prediction and thereby improving exploration success.
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