A Whole Lotta Shakin’ Going On
Seismic contractors are continually searching for methods that will expedite seismic data acquisition – which is why several efforts have been made over the past three decades to develop procedures that will allow vibrators to shake simultaneously at different source stations, with the data being recorded by a common receiver grid.
The attraction of simultaneous-source shaking is that the clock time required for data acquisition across a prospect is reduced by a factor N, with N being the number of source stations where vibrators shake simultaneously.
The data that are acquired tend to be a complicated mixture of wavefields that have traveled from different source stations to each receiver station. In this original recorded state, the data are too confusing to be used to interpret Earth properties. In order to use simultaneous-source data for geologic interpretation, this complicated composite wavefield has to be segregated into the individual wavefields that were generated at each respective source station.
If the wavefield-separation procedure is successful, the result is a set of data that is equivalent to data that would be acquired if a vibrator at each of the N source stations generated single-source data at different clock times.
In early applications, simultaneous-source techniques involved only two vibrator stations. The operational procedures usually were that the vibrator at station A did an upsweep while the vibrator at station B did a downsweep; or the vibrator at station A worked with a phase shift that differed by 180 degrees relative to the vibrator at station B.
Although the segregated wavefields generated by these early methods were often usable for subsurface imaging, the data contained more noise than desired, and these initial simultaneous-source concepts never became widely used.
A relatively recent technology development known as the High Fidelity Vibroseis System (HFVS) is an important advance in the quest to acquire vibrator data simultaneously at several source stations. The technology was developed and patented by Mobil and is now offered by most seismic contractors. Several competing simultaneous-vibrator techniques have subsequently appeared on the scene through research by other oil companies and by seismic contractors.
There are two principal attractions of all of these simultaneous-source procedures – data quality is acceptable, and the number of simultaneous sources can be expanded to as many as six or eight distance-separated vibrators.
An example of the HFVS concept being tested in a vertical seismic profile (VSP) experiment is displayed as figure 1: Data from vibrators occupying five different offset source stations were acquired with the vibrators at all stations shaking simultaneously and then shaking individually.
The responses of the vertical and inline horizontal geophones at two of these stations are illustrated on the display after the patented HFVS methodology was applied to separate each individual wavefield from the composite wavefield. When these wavefields were compared against wavefields generated by vibrators shaking individually at each source station, only minor differences between simultaneous-source data and single-source data were observed.
Figure 2 shows an example comparing single-source data and simultaneous-source data acquired in this VSP experiment. The concept exhibited in these two figures shows that VSP data can be acquired from five source stations in the same clock time needed to acquire data from only one source station.
In many situations, this increased imaging capability provides critical data at attractive cost savings.
Although VSP data are used in this example, HFVS technology and its several competing equivalents were developed to reduce the cost of 3-D seismic data acquisition. Numerous examples demonstrating how each of the currently available simultaneous-source technologies applies to 3-D data acquisition are in the literature or can be provided by seismic contractors.
Simultaneous-source technology seems to be good enough to warrant discussions with seismic contractors about its use and the potential cost savings that may result.
There may be a small add-on fee for some simultaneous-source services if a seismic contractor has to pay a royalty to use the technology. Additional data processing also is required to break the composite wavefield into its individual source-station components such as the examples shown on figure 1 – but these data-processing costs are not significant.
Under some operating conditions, several of the simultaneous-source techniques that are now available are attractive both technically and economically.