EDA介质快慢横波时差和振幅-方位曲线的实验分析

Experimental analysis on time difference and amplitude - azimuth curve of shear wave in EDA media.

在含垂直定向裂隙的各向异性(EDA)介质中,地震快、慢横波传播的时差及振幅随裂隙(缝)方位变化曲线能反映裂缝的密度、走向和介质的性质,这对多波地震资料的处理、解释有着重要意义。但受观测条件限制以及噪声的影响,在横波分裂记录上,同相轴往往会出现扭曲、倾斜和不规则抖动等现象,给快、慢横波时差的求取和振幅一方位曲线的绘制带来误差。设计了EDA介质物理模型,并利用岩石超声波测量系统获得了快、慢横波记录;分析记录可知,在相同传播方向的分裂横波具有走时和传播速度不随裂缝方位变化的恒定性特征;以此为基础,提出了利用平均法和瞬时振幅法来计算快、慢横波的走时时差和绘制振幅一方位曲线,以提高精度。利用物理模型实验得到的2个正交横波(S1波与S2波)的快、慢横波记录,分别用平均法和瞬时振幅法计算了快、慢横波走时的平均值、时差和均方差,并绘制了相应的振幅-方位曲线。结果表明,采用上述2种方法,消除了同相轴扭曲、倾斜和不规则抖动等对计算结果的影响,时差的计算是可靠的,振幅-方位曲线的绘制是合理的。

The time difference (TD) between slow and fast shear waves as well as their amplitude-azimuth curves on seismic shear wave splitting records in EDA media can be used to determine the vertically a-ligned fracture's density, stripe and properties of the media. Therefore, the right TD and amplitude-azimuth curve is important in seismic data processing and interpretation. However, it is difficult to calculate TD and map amplitude-azimuth curve for the splitting shear wave's irregular events, which usually appears in twisting, tilting and trembling mode due to noises and poor observing conditions. Since the travel time and velocity of splitting shear waves is invariable with the fracture set's stripe at the same travel path, 'constant travel time and velocity principle' is proposed. Based on, the physical modeling data including four kinds of splitting shear waves from two orthogonal shear waves S1 and S2, the average method (AM) and instantaneous amplitude method(IAM) are proposed for calculating TD and mapping amplitude-azimuth curve. The results show that both methods are capable of eliminating the influences of noise on TD and amplitude-azimuth curves. The algorithm is reliable in processing noise- rich data.