Test for comparison of array layout in natural source surface wave exploration
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摘要: 为了对比天然源面波勘探不同台阵布局的探测效果， 筛选出探测成果可靠、 效率高和便于野外施工的天然源面波勘探台阵阵形， 在天水市黄土覆盖区的同一场地分别用4种常见的阵形进行数据采集试验， 并对各种阵形数据使用空间自相关法或扩展空间自相关法提取相应的频散曲线， 通过反演得到了试验点地下的浅层速度结构模型. 分析对比试验结果表明： 4种台阵提取的频散曲线数值很相近； 频散谱能量集中度较高的是嵌套式等边三角形和圆形台阵， L形和直线形台阵相对分散； L形台阵低频段(4—8 Hz)比直线形台阵差， 其高频段(8—40 Hz)比直线形台阵好. 针对直线形台阵在高频段信噪比较低的情况， 在确保探测成果可靠性的前提下， 为了提高探测效率， 提出了在同一直线形台阵开展天然源与人工源面波联合勘探的数据采集方法. 实验结果证实， 这种联合勘探方法不仅可弥补直线形台阵高频段的不足， 确保探测精度和结果的可靠性， 而且还能实现“高低”频兼顾， 即“深浅”兼顾.Abstract: In order to select the array layout with high efficiency and convenience for surface wave exploration using natural sources, four tests for data acquisition with different layout are carried out in the same field. Then we perform comparative analysis of the dispersion curves extracted by using SPAC and ESPAC for different array layout. And then the velocity structure of the shallow layer is obtained by inversion. Test results show that the dispersion curves extracted from the four arrays are very similar, and spectrum energy in the frequency domain is more concentrated for the nested equilateral triangle and circular arrays than that for the linear and L-shape arrays. The dispersion curve extracted from the L-shape array is better than the linear array in the high frequency band (8—40 Hz), while worse in the low frequency band (4—8 Hz). Since worse dispersion appears at high frequencies for the linear array, a joint exploration with both artificial and natural sources is proposed. The test result proves that the joint method can not only improve the quality of dispersion curve in the high frequency band to ensure the reliability of the detection precision, but also achieve dispersion data in a much broader frequency range. Consequently both shallow and deeper structure can be obtained in the inversion.
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图 1 微动台阵测点布局(a)嵌套式等边三角形台阵；(b)圆形台阵；(c)L形台阵；(d)直线形台阵
Figure 1. The layout of micro-tremor arrays(a)Nested equilateral triangles array;(b)Circle array;(c)L-shape array;(d)Linear array
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图 2 4种台阵的频散谱(a)嵌套式等边三角形台阵；(b)圆形台阵；(c)L形台阵；(d)直线形台阵
Figure 2. Dispersive spectra of four arrays(a)Nested equilateral triangles array;(b)Circle array;(c)L-shape array;(d)Linear array
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图 3 4种台阵的频散曲线
Figure 3. Dispersion curves for four arrays Red line st and s for nested equilateral trianglearray，blue line for circle array，purple line for L-shape array， and light blue linefor linear array
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图 4 天然源与人工源面波联合勘探的观测系统
Figure 4. Surface wave observation system of joint exploration with artificial and natural sources
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图 5 人工源(a)和天然源(b)的频散谱以及二者的线性叠加谱(c)
Figure 5. Dispersive spectra of artificial(a) and natural sources(b)as well as the linear superposition of both spectra(c)
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