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通常的反射地震勘探系统大多数是采用纵波(P)、地面震源以及测量地面运动垂直分量的检波器。然而,当地下传播的常规P波假设倾斜入射并从一个界面反射,将在产生P波及水平偏振横波(SH)的同时,还产生垂直偏振横波(SV)。里克(Ricker)及林恩(Lynn,1950),惠特(White,1965),以及格雷夫斯(Graves,1979)的理论和模型工作指出,可以用测量地面运动水平分量的检波器来拾取反射垂直偏振横波。他们指出实用的转换(P-SV)地震勘探系统是有发展前途的。 1980年夏季在科罗拉多州中南部的圣路易斯盆地用P波地面震源得到了纵波及横波信息。在科罗拉多矿业学校地球物理部的科州莫法特夏令营地采集了12英里多的反射地震资料。在同一条4英里长的测线上用不同方法分别观测三次、以便对理论预测作出评价。用了垂直可控震源和垂直检波器来获取常规的纵波剖面。还用同一震源,但用沿测线方向的水平检波器来记录垂直偏振反射横波的水平分量。第三次是用水平可控震源及横向方向布置的水平检波器,以便进行独立的横波勘探。按标准的资料处理方法得出纵波(P-P)的最终偏移剖面。横波(SH-SH)资料处理很费机时,并且遇到需要谨慎处理的问题:要特别强调剩余静校正和多次波问题。然而转换波(P-SV)资料处理基本上是按照纵波那样简单的流程进行的。采集与处理过程中,必须重视在转换波系统中所需的一定的临界距离和临界角。就大多数岩性条件来说,纵波到横波的最大转换能量发生在入射角大于20°的地方。此外,格雷夫斯(1979)从理论上指出,较大的炮检距能给出最佳的P-SV转换结果。在野外,转换波测线采用单边放炮,炮检距长度超过1英里并约略大于或等于最深反射面的深度。在资料处理中设计了专门的切除,以便充分利用最大转换范围内的波。结果是十分有希望的。从利用了合适的野外方法及处理技术的P-SV转换波系统得到了良好的资料。综合纵波及转换波成果算出了横波速度。这些计算速度与独立的横波(SH-EH)测线的迭加所需的速度相符合。算出了速度比V_p/V_s,并将它们与圣路易斯盆地的岩性及岩石特征地质解释结合起来。令人鼓舞的成果表明,根据常规测量方法可同时采集纵波和转换波信息(并通过水平检波器加强),并且可用来代替SH勘探系统。
The usual reflective seismic exploration systems are mostly geophones that use longitudinal waves (P), ground sources, and geophones that measure the vertical component of ground motion. However, the conventional P wave propagation locally under the assumption that it is obliquely incident and reflected from an interface will also generate vertical polarization (SV) while generating P-wave and horizontal-polarized shear-wave (SH). Theoretical and modeling efforts by Ricker and Lynn (1950), Whit (1965), and Graves (1979) suggest that a detector that measures the horizontal component of ground motion can be used to pick up Reflection of vertically polarized shear waves. They pointed out that P-SV seismic exploration systems are promising. In the summer of 1980, P- and S-wave information was obtained from P-wave ground sources in the central St. Louis basin of Colorado. More than 12 miles of reflected seismic data were collected at the Moffat Summer Camp, Colorado Department of Geophysics, Colorado School of Mines. Three different observations were made on the same 4-mile line to assess the theoretical predictions. Vertical vibrator and vertical detector were used to obtain the conventional P-wave profile. The same source was also used, but a horizontal detector along the line of sight was used to record the horizontal component of the vertically polarized reflected transverse wave. The third is a horizontal detector with a horizontal vibrator and a horizontal orientation for independent shear wave exploration. The final migration profile of the longitudinal wave (P-P) is obtained according to standard data processing methods. SH-SH data processing is time-consuming and encounters issues that need to be handled with caution: special emphasis should be placed on residual statics and multiples. However, converted-wave (P-SV) data processing basically follows the simple procedure of longitudinal wave. During acquisition and processing, certain critical distances and critical angles required in the converted wave system must be emphasized. For most lithologic conditions, the maximum converted energy from P-wave to S-wave occurs at angles of incidence greater than 20 °. In addition, Graves (1979) theoretically pointed out that a larger offset can give the best P-SV conversion results. In the wild, the converted wave line fired unilaterally with the offset longer than 1 mile and slightly greater than or equal to the depth of the deepest reflector. Specialized resections have been designed in data processing to take full advantage of the wave in the maximum conversion range. The result is very promising. Good data have been obtained from the P-SV converted wave system using the appropriate field methods and processing techniques. Based on the results of longitudinal wave and converted wave, the shear wave velocity was calculated. These calculated velocities correspond to the velocities required for the superposition of independent shear-wave (SH-EH) lines. The velocity ratio V_p / V_s was calculated and combined with lithology and petrophysical interpretation of the St. Louis basin. Encouraging results indicate that both longitudinal and converted wave information can be acquired simultaneously (and enhanced with horizontal geophones) according to conventional measurement methods and can be used instead of the SH exploration system.