Saturday, August 14, 2010

Something about the air guns source

We are using air guns to implement OBS and MCS. Let's talk about something about the airguns source, mainly about its signature.

Source signature corresponds to the seismic wavelet in seismic survey. Seismic wavelet is the far-field response of energy of particle motion velocity (land seismic survey) or pressure (marine seismic survey) which propagates from the seismic source. A seismic wavelet should be carefully chosen in exploration seismology since the bandwidth, the length, and the shape of the seismic wavelet will affect the resolution of seismic survey. A good estimation of seismic wavelet is absolutely critical for seismic inversion.

A seismic wavelet can be defined with its amplitude spectrum, which shows its amplitude characteristics, and phase spectrum, which shows its phase characteristics and includes zero-phase, constant phase, minimum phase and mixed phase etc.. In exploration seismology, we can extract seismic wavelet from data by mainly three ways, pure deterministic way, pure statistical way, and through well-logging curve.

In our marine exploration seismology, air guns source has its deterministic signature, and we can extract the amplitude and phase information directly from the control terminal in the main lab after each shot. For each shot, we think they are almost consistent in amplitude and phase.

We then want to know which type of wavelet the air guns source signature belongs to, or it is unique and belongs to none of ideal wavelet. There are four types of seismic wavelet that are commonly used in seismic data processing software: Ricker wavelet, Ormsby wavelet, Klauder wavelet and Butterworth wavelet. See the following figures for their main characteristics.

(From up to bottom: Ricker, Ormsby, Klauder and Butterworth wavelets, a different choose of dominant frequency or bandpass frequencies and cutoff frequencies may give different bandwidths in frequency domain, and thus the temporal domain shapes are slightly different. )

And the following is the source signature for each air gun.

Although total source signature is not available, we can see from the shape of the single signature and conjecture from the time delay characteristics that the air gun source can be approximated by the minimum-phase Butterworth wavelet with certain parameters: both of them have vibrating evanescent tail, and the most important, minimum-phase Butterworth wavelet itself is physically realisable. Ricker wavelet is so ideal that although it is often used in wave-equation-based numerical modeling or inversion test, it is not realistic in practical cases. Another reason that we consider our air gun source signature is close to Butterworth wavelet is that the frequency range of our air guns source signature expands from low to high frequencies. We can see from MMO’s monitoring screen that after each shot, there will be a bright line in the frequency domain whose range is from very low (~ 0 Hz) to very high (~ 48kHz). Ormsby and Klauder wavelets are bandpass-type, and have nearly vertical cutoff edges at the boundary of frequency band. If we suppose them to be full-ranged, then their waveform will not be far away from practical cases. Butterworth wavelet however, will have soft edges at the boundary of its band. In fact it has a long tail in frequency domain.

To further explore the characteristics of our air guns source signature, a more careful mathematical treatment will be needed. Perhaps we can arrive at some different conclusions and find something new if we can extract the amplitude and phase information in some way, rather than merely use shape-based information, which can be inaccurate in some situations.

Another thing that should be noticed is that OBS and streamers receive different kinds of signals. As OBSs are located on the sea floor, the seismic wavelet they received are in the form of particle motions, while the streamers receive pressure as the signals, although the signals are both from air guns source. The reason is that when air guns source are triggered, energy is propagated to the sea floor and subsurface rocks through sea water in the form of pressure, since fluid can not transfer shear stress, and then only P-wave can propagate in the seawater. When energy arrives at the sea floor, it will convert to the kinematic energy of particle motions of subsurface rocks, and now there will be shear wave converted from P-wave when the P-wave meets subsurface reflectors. After some time, there will be reflections that go upwards to the sea floor from subsurface and at the contact of sea floor and seawater, all S-wave will vanish quickly, only P-wave propagates upwards to the streamers and received. As OBSs are in contact with sea floor, they will receive both S- and P-wave. And thus the wave signals received at the OBSs and streamers are different in nature.

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