深水海底节点二次定位方法

Secondary positioning of deep ocean bottom nodes

在海底节点(OBN)数据采集过程中,由于受到海底地形、海流等因素的影响,OBN的实际位置有时会偏离投放位置,需要进行二次定位,以保证地震数据的成像质量。在进行二次定位计算时,海水速度是影响OBN定位精度的重要因素。在常用的二次定位方法中,海水速度通常给定为一个常数,或者根据炮检距拟合出一条速度曲线。实际海水速度通常在垂向上存在变化,在一定范围内沿炮检距方向变化极小,速度的变化情况与上述常用的两种速度选取方式都不相同。为此,通过模型试算研究了不同速度选取方式下的二次定位精度,提出了通过反演一个合适的等效速度代替垂向变化的速度模型的方法,并在反演检波点坐标时加入海水速度这一个未知量,进一步提高了二次定位精度。模型试算结果表明:相对于使用常速度和拟合速度场这两种常用的速度给定方式,所提方法能取得更高的定位精度,进而取得较好的动校正和成像效果。

Due to the influence of bottom topography,ocean current,et al.,OBNs may deviate from their original positions in the process of data acquisition.To guarantee imaging quality,it is necessary to perform secondary positioning.Current velocity,which is an important parameter in secondary positioning,is usually given as a constant or fitted as the function of offset.But actually,current velocity often changes with depth and almost remains unchanged within a specific range of offset.This means that above velocity definitions are both incorrect.We use model tests to investigate the accuracy of secondary positioning for different velocity definitions,and then we propose to substitute depth-varying velocity in the vertical direction with equivalent velocity,which is derived from the inversion of receiver coordinates.Model tests show that this equivalent velocity is better than constant velocity and fitted velocity in higher accuracy of secondary positioning;such improvement facilitates subsequent NMO correction and imaging.