Research On Simulation And Design Of Signature Produced By Marine Multi-level Air-gun Array

Maine air-gun array has been widely applied in marine seismic prospecting. Because of its strong energy, steady performance, wide application, etc., this source has become a hot theme for geophysicists to study. With the development of high-resolution (HR) exploration, the broad-bandwidth source becomes the basis for HR image of subsurface structure. The multi-level air-gun array could well suppress the notch effect caused by source ghost, broadening the effective frequency band and obtaining ideal broad-bandwidth seismic signatures.The far-field signature from the air-gun source is not only an important standard for measuring the source’s performance, but also the importing data of great use in seismic processing. Practically, the far-filed signature can be obtained definitely by a filed measurement. But because of the flexibility and diversity of air-gun source design, it is necessary to test a lot of times for the optimum. If only using the field measurement to verify the designs, the process will be time-consuming and expensive. The air-gun wavelet model can simulate the far-filed signature from the air-gun source easily, which can greatly improve the efficiency of array design. Besides it can be used to fully study the source signal, including the change of bubble radius and bubble wall velocity, the directivity of the array signal and so on. While the air-gun’s good consistence of wavelets and performance ensure that this method is feasible, its realization depends on the rationality and accuracy of the model itself. Therefore, there is great research value and practical significance building a precise air-gun wavelet model.At first, this dissertation describes in detail the operation of the air-gun and its resultant signature as well as the influence factors. Then list previous ideal models of the air-gun’s signature and later modifications. Based on the existed model in conditions of Van der Waals gas and considering the influence from the ghost on single bubble’s oscillation. The source ghost is reflection by the sea surface of the up-going pressure wave from the air-gun, so it could put pressures on the bubble with the hydrostatic pressure, leading to the change of source wavelet.Different air-guns have various (?)tures, so they produce signals with different features even under same firing parameters and conditions. Therefore it is necessary to calibrate some parameters that relies on the air-gun type according to actual data. Moreover, in the actual recording system there are filters and sampling, which can influence the waveform recorded. For this reason, in the calibration process, the computed signal must be filtered and resampled before comparing with the actual wavelet to ensure the accuracy of the result. The resultant wavelets agrees well with the measurements.Based on the single air-gun wavelet model, the signature model for air-gun array was built considering factors like interference between bubbles produced by air-guns in the array. The computed signals are close to the measured far-field signature.Study on the principles of synchronized multi-level air-gun array, establish its signature model. In addition, research the design approach for multi-level array, discuss the effects of depth combination, depth difference and the number of layers on the elimination to the ghost.According to the built methods of the multi-level source design, combining with the wavelet model and considering actual marine exploration requirements and conditions, design and compare two-layer and four-layer air-gun sources. The test obtained good results which proves advantages of multi-level arrays.