Jan Langhammer

Source signature determination from ministreamer data

Two methods for estimating the pressure wavefield generated by a marine airp‐gun array are tested. Data have been acquired at a ministreamer located below the source array. Effective source signatures for each air gun are estimated. In the first method a nonlinear inversion algorithm is used, where the forward modeling scheme is based upon a physical modeling of the air bubble generated by each air gun. In the second method a linear inversion method is used, with the assumption that the physics in the problem can be described by the acoustic wave equation with explosive point sources as the driving term. From the estimated effective source signatures, far‐field signatures have been calculated for both methods and compared with measured far‐field signatures. The error energy between the measured and estimated far‐field signatures was approximately 8 percent for both methods.

Experimental study of viscosity effects on air-gun signatures

In computer modeling of air guns the theory is often based on the assumption of an ideal freely oscillating spherical air bubble in an ideal fluid. Theoretical and experimental air‐gun signatures do not match perfectly. Synthetic signatures are under‐damped compared to real signatures. Several physical effects have been proposed as an explanation of this mismatch, among them viscosity effects. The viscosity of the surrounding liquid may contribute to the damping of the bubble oscillation. The numerical value of the viscosity of the surrounding liquid has to be increased considerably beyond the actual value of water to obtain sufficient damping of the synthetic signatures. We therefore performed an experiment to study the effect upon the pressure signature from an air gun when changing the viscosity of the surrounding medium. The motivation for the experiment was to quantify the influence of viscous terms on the output pressure waveform from an air gun. The experiment was carried out in an 850dm3 tank. The...

Air-gun bubble damping by a screen

A method for damping unwanted bubble oscillations from a seismic air gun is presented. The method exploits the fact that the primary pressure peak generated by an air gun is produced during the first 5-10 ms after firing. The air bubble is destroyed by mounting a perforated screen with an optimal radius about the gun. Once the primary pressure peak has been generated by the bubble, the bubble is destroyed by the screen, leading to a corresponding decrease in the measured pressure amplitude of the secondary bubble oscillations. Controlled near-field measurements of 40-cubic inch and 120-cubic inch air guns with and without damping screens are used. The primary to bubble ratio improves from 1.4 without a screen to 4.4 with a screen in the near-field. The corresponding values for estimated far-field signatures are 1.8 to 9.0 when the signatures are filtered with an out-128 Hz (72 dB/Oct) DFS V filter.

Damping of secondary bubble oscillations for towed air guns with a screen

A method for damping unwanted bubble oscillations from a horizontally towed seismic air gun is presented. The air bubble is destroyed by a perforated screen mounted at an optimal radius about the gun. Once the primary pressure peak has been generated by the emerging bubble, the bubble continues to expand and is destroyed by the screen, leading to a corresponding decrease in the measured pressure amplitude of the secondary bubble oscillations. For a stationary gun fired first without, and then with, the screen fitted, the primary-to-bubble ratio improves in the near field from 1.7 to 5.2, respectively, at a firing depth of 3 m and from 1.5 to 5.5, respectively, at 5 m depth. The primary-to-bubble ratio for a towed air gun in the quasi-far-field improves from 2.0 to 11.0 at 4 m depth and from 1.5 to 8.7 at 7 m depth when the screen is fitted. The boat speed was 1.6 knots and the signatures were filtered with an out-128 Hz (72 dB/Oct) DFS V filter.

Bubkil‐airgun bubble damping by a screen

A method for damping out unwanted bubble oscillations from a seismic airgun is presented. The method exploits the fact that the primary pressure peak generated by an airgun is produced during the first 5--10 ms after the firing time. The air bubble is destroyed by mounting a perforated screen with an optimal radius to the gun. Once the primary pressure peak has been generated by the bubble, the bubble is destroyed by the screen, leading to a corresponding decrease in the measured pressure amplitude of the secondary bubble oscillation. Controlled near-field measurements of a 40 and 120 cubic inch airgun with an without damping screens are presented. The primary-to-bubble ratio improves from 1.4 to 4.1 in the nearfield, while the corresponding values for estimated far-field signatures are 1.8 to 9.0, given that the signatures are filtered with an out {minus}128 Hz (72 dB/Oct) DFS V filter.

Bubkil - Airgun bubble damping by a screen

A method for damping unwanted bubble oscillations from a seismic airgun is presented. The method exploits the fact that the primary pressure peak generated by an air gun is produced during the first 5-10 ms after firing.