基于范德瓦尔斯气体方程的气枪子波时频域模拟验证

Simulation and verifi cation of an air-gun array wavelet in time-frequency domain based on van der waals gas equation

气枪通过释放高压气体产生声波信号,用于海洋地震勘探。理想气体模型中,当枪室内压力超过100atm时,会引入较大的误差。而范德瓦尔斯非理想气体理论中,气枪中的气体可视为实际气体,引入误差小于2%。结合准静态开放式热力学系统和气泡运动方程,考虑气泡上升、气泡相互作用和节流效应,建立了范德瓦尔斯气枪子波模型。范德瓦尔斯模型与理想气体模型模拟结果的差异与压力有关。理论上,在高压条件下,范德瓦尔斯气枪模型更接近测量结果。海洋垂直缆通过连接水泥块的钢缆延伸到海底,水听器悬浮在海水中,从而避免了船舶噪声、海浪噪声和耦合问题,提高了海洋地震资料的信噪比和分辨率。该采集方法满足记录气枪远场子波的条件,基于实际垂直缆观测系统,采用范德瓦尔斯气枪模型对不同方位角和垂向角的子波进行模拟。实验数据与模拟数据时频域吻合较好,表明范德瓦尔斯模型是准确可靠的,气枪子波的模拟精度直接关系地层的分辨率,进而影响地层的分辨率。

An air gun generates acoustic signals for seismic exploration by releasing a high-pressure gas.A large error is always gradually introduced into the ideal-gas model when the pressure in the air-gun chamber exceeds 100 atm.In the van der Waals non-ideal-gas theory,the gas in the air gun can be regarded as an actual gas,and the error is less than 2%.The van der Waals model is established in combination with the quasi-static open thermodynamic system and bubble-motion equation by considering the bubble rise,bubble interaction,and throttling eff ect.The mismatch between the van der Waals and ideal-gas models is related to the pressure.Theoretically,under high-pressure conditions,the van der Waals air-gun model yields results that are closer to the measured results.Marine vertical cables are extended to the seafl oor using steel cables that connect the cement blocks,but the corresponding hydrophones are suspended in the seawater.Thus,noise associated with ships,ocean surges,and coupling problems is avoided,and the signal-to-noise ratio and resolution of marine seismic data are improved.This acquisition method satisfies the conditions of recording air-gun far-fi eld wavelets.According to an actual vertical-cable observation system,the van der Waals air-gun model is used to model the wavelet of different azimuth and take-off angles.The characteristics of the experimental and simulated data demonstrate good agreement,which indicates that the van der Waals method is accurate and reliable.The accuracy of the model is directly related to the resolution,thus aff ecting the resolution ability of the stratum.