An introduction to this special section: Seismic acquisition

This section on seismic acquisition is based on the papers that were presented under the Data Acquisition Session at the CSEG GeoConvention 2012: Vision in May, 2012. Of the five papers that were presented, the authors of three agreed to publish their material in a more complete format in The RECORDER. Unlike many "special sections" we do not have a particular theme on which the authors were requested to write, such as land acquisition, marine acquisition or simultaneous sources, etc., and therefore we have quite a diverse range of subjects.

In recent years in the geophysical industry we have seen a considerable increase in interest in the acquisition and use of frequencies that are lower than what has typically been recorded over the last several decades. Some of this interest has been a result of an increase in exploration for and development of reservoirs below salt or basalt. The other primary reason for the interest has been for seismic inversion where, if the low frequency content of the velocity model could be evaluated directly from the seismic data, the inversion results would be significantly better than when using velocities interpolated from wells. Although much of the published work has been in offshore environments such as the Gulf of Mexico and offshore Brasil, there has also been an interest for land acquisition.

The first paper The Hussar low-frequency experiment (Margrave, et al) gives a preliminary report on an interesting onshore field test to evaluate the low-frequency signal content that can be generated by both vibroseis and explosives. Different vibroseis types are compared as well as the use of specially designed non-linear sweeps that were specifically targeted to enhance the low frequencies. The test also compares the use of different types of recording sensor (MEMS digital sensors, together with different types of geophones with differing resonant frequencies.) Although, as stated, this is a preliminary report some conclusions can already be deduced and I am looking forward to seeing future publications on the subsequent data processing and analysis of the data and the final conclusions.

In the pursuit of low frequencies in the offshore environment there have been a number of recent developments. The major issue that affects the frequency bandwidth that can be recorded in marine acquisition is the ghosting effect that impacts both the downgoing energy from the source array(s) and that which arrives back at the streamers. The ghost reflections attenuate the low frequencies in addition to the high frequencies and result in severely diminished bandwidth. These innovations have included the use of motion sensors in the streamers, such as geophones in PGS's Geostreamer that measure the vertical motion component of the wavefield and WesternGeco's IsoMetrix system that utilizes 3C MEMS sensors to measure and record the vertical and crossline acceleration. On the source side there have also been a number of improvements in source deployment technology to improve the frequency bandwidth.

Sablon et al, in their paper Broadseis: Enhancing interpretation and inversion with broadband marine seismic, give a very brief overview of the Broadseis system, while most of the paper discusses the benefits that can be seen after the data acquisition has been completed. CGGVeritas's Broadseis method uses an innovative variable depth streamer deployment to provide "ghost notch diversity" in the spectral content of the data. This technology, combined with very specifically matched data processing algorithms result in greatly enhanced bandwidth at both the low and high frequency ends of the spectrum. This bandwidth expansion results in benefits that can be realized in various stages of data processing, seismic inversion and the final interpretation.

In the last decade the industry has seen the introduction of more than a dozen different "cable-less" systems, and there have been many papers on the relative benefits of cabled vs. cable-less recording systems presented at numerous workshops and conventions. Most of these papers have been related to the differences in operational methodologies, power consumption of the electronics and battery maintenance, HSE with regards to the weight of cables and batteries, and whether we need to have good QC of the data quality in real time during the data acquisition or whether we can relegate this until acquisition is complete.

Keith Millis and Andrea Crook in their paper The Benefits of Receiver Infill Stations, A Technical Case Study look at the "cabled vs. cable-free" question from a different perspective. They show the benefits of a cable-free acquisition system from the geophysical standpoint with their educational analysis of cmp fold, offset and azimuth distributions for different layout geometries.

As Special Coordinator for this special section I would like to thank all of the authors for their contributions and the time they have spent preparing the articles.