Conventional seismic sources emit acoustic waves that contain frequencies that are higher than what we can use. Seismic data sampled at 500 Hz (2 milliseconds sampling rate) has a Nyquist frequency of 250 Hz and the anti-aliasing high cut of the Analog to Digital Converter (ADC) high-limits the band at 220Hz. We can sample at 1 millisecond instead of at 2 millisecond. If our sampling rate were 1KHz, the Nyquist would be 500Hz, and the ADC high-cut would be at 440Hz. Would that provide higher resolution of oil and gas reservoirs? No, because waves at high frequencies are anyway attenuated and scattered in the overburden. The high frequency limit depends on the attenuation and scattering in the overburden. Depending on the depth of the target and the complexity and constitution of the overburden, the high-frequency limit of seismic data is at most 150 Hz, and usually much lower than that. Deep targets, especially sub-salt or sub-basalt require very long shot-receiver offsets and very large source are usually high-limited by about 30 Hz.
Airguns emit high frequencies that are not useful. Yet, the high-frequency acoustic waves propagate with almost no attenuation in water and have the potential to impact marine life.
In addition to natural marine life, oilfied operators employ human divers. With more environmentally friendly seismic sources, it may be possible to reduce seismic exclusion range from divers.
We want to reduce the high frequency content of seismic sources, especially that of large sources that are used to image sub-salt and sub-basalt targets.
The low frequencies are useful because they attenuate and scatter less in the overburden, because they are very useful in velocity model building with methods such as Full Waveform Inversion, and because building blocky reservoir models depends on low frequency content. The missing low frequencies of the seismic data must often to be substituted by data from well logs which may be misleading far away from the wells where they were acquired. Broadband is good for resolution. The width of the main peak of the wavelet depends on the high-frequency limit, but the depth of the side lobes depends on the low-frequency limit.
On the low side, the band is not limited by the over-burden but by the frequency response of the receivers, the notch frequency of the receiver and source ghosts, and the frequency content of the sources. In the last two decades, great improvement has been made to broaden the band of seismic data. The most important improvement was the upgrade from 16 bit to 24 bit recording. 24 bit recording reduces instrument noise. The other improvement, which is associated with deghosting is increasing the receiver depth. Deep deployment of the receiver, whether in ocean bottom nodes, or in streamers that are towed 15m or more deep, reduces ambient noise. Noise reduction enables deghosting in data processing. Because the amplitude at the ghost notch is very small, the process of deghosting is a division by a small number, almost by zero. We can divide by a small number if we do not boost noise too much.
The improvements in receiver instrumentation and in deghosting data processing are important, but we are still using essentially the same airguns that we used when we had 16 bit recording, when we deployed streamers 7 meter deep, and when our hydrophones had an instrument RC analog low-cut of 7Hz. Now we have 24 bit recording, we deploy streamers 20 meters deep, our hydrophones have 2Hz RC low-cut. Isn't it time to improve the sources?
The LPS is an evolution of the airgun. It is charged with air at a pressure that is much lower than that of conventional airguns but has volumes that are much larger. Instead of hundreds of cubic inch at thousands of PSI, the LPS has volumes of thousands of cubic inches at pressures of hundreds of PSI. The difference in Volume-Pressure aspect, in addition to other design features, makes the LPS a low-frequency source.
The upgrade from conventional airguns to LPS is relatively easy. The compressors, the deck handling, the deployment and retrieval, and the towing and handling are similar to conventional airguns. Data processing is also not very different. The peak to bubble ratio is lower, but with careful designature we can use the low frequency signal in the LPS-bubble.
Onshore, Vibroseis is an alternative to dynamite. Offshore, it has not happened. At least not yet. Great challenges in Marine Vibroseis are associated with low frequency content and with the Doppler effect of the source motion. But these challenges can be overcome. Although Marine Vibroseis is a longer term solution than LPS, we still believe that it is worth investing R&D effort in Marine Vibroseis.
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