二层水平介质球面波正反演联用与震源定位

Positioning of seismic sources combined with forward and inversion of spherical wave in double-layer horizontal media

二层水平介质中原几何平均震源定位方法,无法直接确定震源的水平坐标,推广至任意水平层状介质时,包含震源深度的定位参数难以确定和修正,且难以确定震源所在层。根据将水平和铅垂坐标分离、并去根号的走时方程,反演出发震时刻,并合理修正,根据水平层状体系中球面波波阵面的正演方法,得到含震源空间坐标的非线性方程组,求解可得震源的水平和铅垂坐标。各种定位条件下的定位计算表明,正反演联合运用法的水平和铅垂坐标误差的较大值,均不大于几何平均法的震源深度计算误差,且正反演联用法比几何平均法更适合于深震源定位。对参数敏感性的分析表明,发震时刻对定位参数不敏感,震源位置误差对波速、波速比的敏感性介于几何平均法和经典线性法之间。正反演联用法不能改进几何平均法不适应较大水平竖直距离比和较大、较小埋深比的缺点,仍建议在地(矿)震易发区布置台站,并与经典线性法结合使用。正反演联用法不需要确定和修正含震源深度的定位参数,且便于逐层排除和确定震源所在层,因此将比几何平均法更适合于任意水平层状介质中的震源定位。

The positioning method of geometric average in double-layer horizontal media is unable to determine the horizontal coordinates of seismic sources directly. When it is extended to multilayer medium, the positioning parameters containing depth are difficult to be determined and corrected, and it is difficult to determine the layer where the sources are located. Using the travel time equation by which the depth and horizontal coordinates are separated and the square root is removed, the time when the earthquake happens is obtained through inversion and corrected reasonably. According to the forward modeling method for wave front of spherical wave in double-layer horizontal media, nonlinear equations containing space coordinates of the sources are got, and the depth and horizontal coordinates are yielded by solving the nonlinear equations. The calculated results of positioning under a variety of conditions show that the larger values of positioning error of depth and horizontal coordinates derived by the positioning model combined with forward and inversion are all not greater than those by the geometric average method, and the new positioning method is more suitable than the geometric average method for positioning of deep seismic sources. The sensitivity analysis of parameters shows that the origin time of earthquakes is not sensitive to the positioning parameters, and the sensitivity of the focal position error to wave velocity and wave velocity ratio is between the geometric average method and classical linear method. But the proposed model cannot improve the defect of the geometric average method. When the ratio of horizontal to vertical distance between stations and sources is large, the positioning accuracy is poor. There is also no advantages when the ratio of distances from the interface of sources and stations is too large or too small. So stations in the area where earthquakes are easily prone to happen are suggested to be arranged and the classic linear positioning methods are employed. There is no need to determine or correct the positioning parameters containing depth for the positioning model combined with forward and inversion. It is easy to eliminate or identify which layer the seismic sources are located in. Accordingly, the proposed model is more suitable than the geometric average method for positioning of seismic sources in multilayer horizontal media.