scholarly journals On low frequencies emitted by air guns at very shallow depths — An experimental study

Geophysics ◽  
2019 ◽  
Vol 84 (5) ◽  
pp. P61-P71 ◽  
Author(s):  
Daniel Wehner ◽  
Martin Landrø ◽  
Lasse Amundsen
Keyword(s):  
Large Volume ◽  
Field Experiments ◽  
Field Measurements ◽  
Seismic Source ◽  
Sea Surface ◽  
Frequency Signal ◽  
Low Frequencies ◽  
Air Gun ◽  
Seismic Acquisition ◽  
Marine Seismic

In marine seismic acquisition, the enhancement of frequency amplitudes below 5 Hz is of special interest because it improves imaging of the subsurface. The frequency content of the air gun, the most commonly used marine seismic source, is mainly controlled by its depth and the volume. Although the depth dependency on frequencies greater than 5 Hz has been thoroughly investigated, for frequencies less than 5 Hz it is less understood. However, recent results suggest that sources fired very close to the sea surface might enhance these very low frequencies. Therefore, we conduct dedicated tank experiments to investigate the changes of the source signal for very shallow sources in more detail. A small-volume air gun is fired at different distances from the water-air interface, including depths for which the air bubble bursts directly into the surrounding air. The variations of the oscillating bubble and surface disturbances, which can cause changes of the reflected signal from the sea surface, are explored to determine whether an increased frequency signal below 5 Hz can be achieved from very shallow air guns. The results are compared with field measurements of a large-volume air gun fired close to the sea surface. The results reveal an increased signal for frequencies below 5 Hz of up to 10 and 20 dB for the tank and field experiments, respectively, for the source depth at which the air gun bubble bursts directly into the surrounding air. For large-volume air guns, an increased low-frequency signal might also be achieved for sources that are slightly deeper than this bursting depth. From these observations, new design considerations in the geometry of air-gun arrays in marine seismic acquisition are suggested.

scholarly journals Low-frequency acoustic signal created by rising air-gun bubble

Geophysics ◽  
2017 ◽  
Vol 82 (6) ◽  
pp. P119-P128
Author(s):  
Daniel Wehner ◽  
Martin Landrø
Keyword(s):  
Acoustic Signal ◽  
Field Experiments ◽  
Waveform Inversion ◽  
Low Frequency ◽  
Frequency Signal ◽  
Low Frequencies ◽  
Air Bubble ◽  
Rising Bubble ◽  
Air Gun ◽  
Different Depths

In the seismic industry, there is increasing interest in generating and recording low frequencies, which leads to better data quality and can be important for full-waveform inversion. The air gun is a seismic source with a signal that consists of the (1) main impulse, (2) oscillating bubble, and (3) rising of this air bubble. However, there has been little investigation of the third characteristic. We have studied a low-frequency signal that could be created by the rising air bubble and find the contribution to the low-frequency content in seismic acquisition. We use a simple theory and modeling of rising spheres in water and compute the acoustic signal created by this effect. We conduct tank and field experiments with a submerged buoy that is released from different depths and record the acoustic signal with hydrophones along the rising path. The experiments simulate the signal from the rising bubble separated from the other two effects (1 and 2). Furthermore, we use data recorded below a single air gun fired at different depths to investigate if we can observe the proposed signal. We find that the rising bubble creates a low-frequency signal. Compared with the main impulse and the oscillating bubble effect of an air-gun signal, the contribution of the rising bubble is weak, on the order of 1/900 depending on the bubble size. By using large air-gun arrays tuned to create one big bubble, the contribution of the signal can be increased. The enhanced signal can be important for deep targets or basin exploration because the low-frequency signal is less attenuated.

Repeatability issues of high-frequency signals emitted by air-gun arrays

Geophysics ◽  
2013 ◽  
Vol 78 (6) ◽  
pp. P19-P27 ◽  
Author(s):  
Martin Landrø ◽  
Lasse Amundsen ◽  
Jan Langhammer
Keyword(s):  
High Frequency ◽  
Field Measurements ◽  
Typical Feature ◽  
Main Peak ◽  
Frequency Signal ◽  
High Frequency Signal ◽  
Surface Reflection ◽  
Air Gun ◽  
Source Signal ◽  
Marine Seismic

Recent field measurements of the acoustic signals generated by marine seismic air-gun arrays showed that the amount of high-frequency signals (above 10 kHz) increased with the size and total volume of the gun array. We found that for frequencies between 10 and 20 kHz, a strong signal is observed 7–14 ms after the main peak of the source signal. We believe that this signal was generated by ghost cavitation. We observed that this signal was significantly stronger than the high-frequency signal generated at the same time as the peak signal occurs within the bandwidth between 10 and 20 kHz. We found that this high-frequency signal was fairly repeatable from one shot to another. By “fairly,” we mean that individual high-frequency events were not repeatable; however, the envelope energy of this cascade of events was repeatable from one shot to another. The typical feature of the envelope of the high-frequency signal was that it lasted for approximately 6–7 ms and showed a monotonic increase in amplitude for the first 5–6 ms, followed by a sudden drop. The sea surface reflection coefficient for these high-frequency events seemed to decrease in magnitude as the frequency increased.

scholarly journals Field Measurements of Surface and Near-Surface Turbulence in the Presence of Breaking Waves

2015 ◽  
Vol 45 (4) ◽  
pp. 943-965 ◽  
Author(s):  
Peter Sutherland ◽  
W. Kendall Melville
Keyword(s):  
Field Experiments ◽  
North Pacific Ocean ◽  
Large Fraction ◽  
Field Measurements ◽  
Breaking Waves ◽  
Sea Surface ◽  
Near Surface ◽  
Wave Energy Dissipation ◽  
Eddy Simulation ◽  
The North

AbstractWave breaking removes energy from the surface wave field and injects it into the upper ocean, where it is dissipated by viscosity. This paper presents an investigation of turbulent kinetic energy (TKE) dissipation beneath breaking waves. Wind, wave, and turbulence data were collected in the North Pacific Ocean aboard R/P FLIP, during the ONR-sponsored High Resolution Air-Sea Interaction (HiRes) and Radiance in a Dynamic Ocean (RaDyO) experiments. A new method for measuring TKE dissipation at the sea surface was combined with subsurface measurements to allow estimation of TKE dissipation over the entire wave-affected surface layer. Near the surface, dissipation decayed with depth as z−1, and below approximately one significant wave height, it decayed more quickly, approaching z−2. High levels of TKE dissipation very near the sea surface were consistent with the large fraction of wave energy dissipation attributed to non-air-entraining microbreakers. Comparison of measured profiles with large-eddy simulation results in the literature suggests that dissipation is concentrated closer to the surface than previously expected, largely because the simulations did not resolve microbreaking. Total integrated dissipation in the water column agreed well with dissipation by breaking for young waves, (where cm is the mean wave frequency and is the atmospheric friction velocity), implying that breaking was the dominant source of turbulence in those conditions. The results of these extensive measurements of near-surface dissipation over three field experiments are discussed in the context of observations and ocean boundary layer modeling efforts by other groups.

Some notes on air-gun development as a marine seismic source

2009 ◽  
Vol 28 (11) ◽  
pp. 1334-1335 ◽  
Author(s):  
Ben F. Giles
Keyword(s):  
Seismic Source ◽  
Air Gun ◽  
Marine Seismic

Design of Near-orthogonal Air-gun Sequences for Marine Seismic Source Encoding

2016 ◽  
Author(s):  
M.B. Mueller ◽  
D.F. Halliday ◽  
D.J. van Manen ◽  
J.O.A. Robertsson
Keyword(s):  
Seismic Source ◽  
Air Gun ◽  
Marine Seismic ◽  
Source Encoding

Single water gun far‐field pressure signatures estimated from near‐field measurements

Geophysics ◽  
1985 ◽  
Vol 50 (2) ◽  
pp. 257-261 ◽  
Author(s):  
M. H. Safar
Keyword(s):  
High Resolution ◽  
Data Acquisition ◽  
Seismic Data ◽  
Near Field ◽  
Field Measurements ◽  
Reference Source ◽  
Far Field ◽  
Air Gun ◽  
Seismic Data Acquisition ◽  
Marine Seismic

An important recent development in marine seismic data acquisition is the introduction of the Gemini technique (Newman, 1983, Haskey et al., 1983). The technique involves the use of a single Sodera water gun as a reference source together with the conventional air gun or water gun array which is fired a second or two after firing the reference source. The near‐field pressure signature radiated by the reference source is monitored continuously. The main advantage of the Gemini technique is that a shallow high;resolution section is recorded simultaneously with that obtained from the main array.

A simple method for determining the S80 water gun depth

Geophysics ◽  
1986 ◽  
Vol 51 (2) ◽  
pp. 424-426 ◽  
Author(s):  
M. H. Safar
Keyword(s):  
High Resolution ◽  
High Frequency ◽  
Seismic Source ◽  
Frequency Signal ◽  
High Frequency Signal ◽  
Simple Method ◽  
Marine Oil ◽  
Principal Reason ◽  
Air Gun ◽  
High Level

The water gun, which is becoming a popular seismic source, has proven to be an important development in marine oil prospecting. The principal reason is that, unlike the air gun, the pressure signature radiated by the water gun consists of a single bubble pulse and contains a high level of high‐frequency signal. These important features make the water gun a suitable seismic source for high‐resolution surveys. Water guns currently used are the S80, which has been used by Horizon since 1977, and the P400, introduced in 1983. The S80 and P400 water guns were developed by Sodera.™

Wavefield decomposition based on acoustic reciprocity: Theory and applications to marine acquisition

Geophysics ◽  
2013 ◽  
Vol 78 (2) ◽  
pp. WA41-WA54 ◽  
Author(s):  
Roald Gunnar van Borselen ◽  
Jacob Fokkema ◽  
Peter van den Berg
Keyword(s):  
Free Surface ◽  
Seismic Source ◽  
Sensor Data ◽  
Constant Depth ◽  
Seismic Acquisition ◽  
Single Sensor ◽  
Recorded Data ◽  
Marine Seismic ◽  
Wavefield Decomposition ◽  
Multisensor Data

In marine seismic acquisition, the free surface generates seismic events in our recorded data that are often categorized as noise because these events do not contain independent information about the subsurface geology. Ghost events are considered as such noise because these events are generated when the energy generated by the seismic source, as well as any upgoing wavefield propagating upward from the subsurface, is reflected downward by the free surface. As a result, complex interference patterns between up- and downgoing wavefields are present in the recorded data, affecting the spectral bandwidth of the recorded data negatively. The interpretability of the data is then compromised, and hence it is desirable to remove the ghost events from the data. Rayleigh’s reciprocity theorem is used to derive the relevant equations for wavefield decomposition for multisensor and single-sensor data, for depth-varying and depth-independent recordings from marine seismic experiments using a single-source or dual-source configuration. A comparison is made between the results obtained for a 2D synthetic example designed to highlight the strengths and weaknesses of the various acquisition configurations. It is demonstrated that, using the proposed wavefield decomposition method, multisensor data (measurements of pressure and particle velocity components, or multidepth pressure measurements) allow for optimal wavefield decomposition as independent measurements are used to eliminate the interference patterns caused by the free surface. Single-sensor data using constant-depth recordings are found to be incapable of producing satisfactory results in the presence of noise. Single-sensor data using a configuration with depth-varying measurements are able to deliver better results than when constant-depth recordings are used, but the results obtained are not of the same quality when multisensor data are used.

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