# Shallow Velocity Estimation by Multiples for Monochannel Boomer Surveys
## 4 Experimental Data Example  
As regards processing real marine data with the algorithm described above, a key step is to select the travel times of seismic signals. Apart from the problem of interpreting the multiples as peg-leg or intrabed, which may be interfering with other primaries, the minimum-phase property of the Boomer (or air gun) signal also poses challenges. The travel time for such a signal is the time break of the waveform, that is, the time when the signal amplitude has a burst with respect to the background noise. Its Recognition is easy for direct arrivals and primary reflections from the seafloor but is very difficult for multiples. Normally, to bypass this problem, the maximum (or minimum) peak is selected, and then a time delay is subtracted. This delay can be estimated for a typical waveform directly from the data, e.g., in some primary reflection from the seafloor where the noise contribution is minimum. Figure 5 shows two minimum-phase wavelets with different frequency bandwidth: The first is 300-9000 Hz, similar to the nominal range of the Boomer source we used; the second is 300-4500 Hz, which can mimic a signal after some propagation in a dissipative medium, as mainly high-frequencies are absorbed. We obtained them by filtering a unit spike at the time zero with minimum-phase Butterworth filters with the frequency ranges mentioned above. The sampling rate was 1 microsecond. If we choose the largest peak of this signal, as we should in practice to overcome the background noise, we obtain two different delays \(\Delta_{1}\) and \(\Delta_{2}\), where the second is almost double the first one. Therefore, we may have to introduce a time shift in our picked travel times, which is ideally time-dependent. In practice, however, a negative time shift of the order of 0.5 ms may be needed for the arrivals of intrabed or peg-leg multiples. Figure 6 shows a Boomer profile acquired in the Gulf of Trieste (Northern Adriatic Sea). The recording system was a Boomer AAE301, with a sampling interval of 10 microseconds and a shooting interval of about 0.7 m. The frequency band ranged from 500 to 6000 Hz. As feathering and sea waves may change the nominal offset between source and receiver, we computed the actual offset at each shot by choosing the direct arrival (red line) and multiplying its travel time by the seawater velocity. This velocity was measured independently, and it was relatively high, i.e., 1532 m/s [19], due to the high salinity and temperature in the area during the survey (see also [20]). The average offset was 4.84 m, with a standard deviation of 5 cm, which is 1%. This relative error was small enough for a fair estimation and comparable to the perturbations in the synthetic example (Figure 4). Figure 5: Two minimum-phase Butterworth wavelets with different bandwidths: 300–9000 Hz (**left**) and 300–4500 Hz (**right**). The time delay \(\Delta_{1}\) of the main peak from the time origin is about half of the delay \(\Delta_{2}\) of the second trace. The other selected horizons shown in Figure 6 are the seafloor (green) reflection and a prominent primary reflection at about 37 ms (dark blue). We preferred this particular event to others because its peg-leg multiple (yellow) was very clear; indeed, it cut dipping interfaces in the central part, at about 55 ms, thus unambiguously revealing its nature of multiple. For the other possible candidate, the reflector at about 32 ms, choosing its peg-leg multiple was relatively fuzzy. From a geological point of view, the formations between the seafloor and the selected primary are composed of unconsolidated sediments, which may be partially saturated with gas. Above this area, indeed, terrigenous sediments (the Eocene flysch) are present, characterized by seismic diffractions and deformations [21]. The erosion surface at 50 ms is the top of flysch formation. Most of the visible flat events below this surface are multiple reflections.