组合球形岩溶的中梯视电阻率异常特征

THE ANOMALY OF APPARENT RESISTIVITY FOR THE MID GRADIENT METHOD ABOVE COMBINED SPHEROIDAL KARST

岩溶是水工电法的主要研究目标之一。为探明地下岩溶的发育与分布，对较为复杂的组合球形岩溶地质体，建立了计算其视电阻率的物理数学模型，并采用等效视电阻率迭代法计算沿测线水平方向及垂直测线水平方向上组合球形岩溶的中梯装置的ρｓ值。重点分析了不同组合模式的中梯ρｓ曲线形态特征，并讨论了μ２值变化，测量电极距ＭＮ大小对组合球形岩溶地质体中梯ρｓ异常的影响规律。得出中梯ρｓ曲线对沿测线水平方向的组合球形岩溶地质体分辨能力最强，而对垂直测线水平方向以及铅垂方向的组合球形岩溶地质体，中梯ρｓ曲线无法分辨；μ２值的变化既影响中梯ρｓ曲线异常幅度，也影响其形态；当两个组合球形岩溶地质体之间的距离ΔＸ与ＭＮ之比ΔＸ／ＭＮ≤１／３时，ρｓ曲线会发生畸变，直接影响分辨能力，并会导致错误的解释结果。故ΔＸ／ＭＮ值必须大于１／３，在保证观测精度的前提下。

Karst is one of the aim items of hydroelectrical prospecting. In this paper, physical and mathematical models are set up for combined spheroidal karst geologic body to calculate its apparent resistivity. With the method of equivalent resistivity iteration the mid gradient apparent resistivity curves are given along the horizontal and vertical directions of measuring line. The anomaly features of the mid gradient above combined spheroidal karst ρ s curve are analysed for varied combined modes, and the variation of μ 2( ρ 2/ρ 1) and the influence of electrode separation ( MN ) on mid gradient ρ s anomaly are discussed. It is concluded that the discrimination of the mid gradient ρ s curve is highest for the combined spheroidal karst distributed in the direction of measuring line. However, the mid gradient method cannot discriminate the combined spheroidal karst located perpendicularly to measuring line. The change in μ 2 value has an affect on not only the anomaly amplitude, but also the anomaly configuration of the curve. When ΔX/MN≤1/3(ΔX being the distance between two spheroidal bodies), a distortion of ρ s curve may appear, which affects directly the discrimination and results in incorrect interpretation. In this way, the minimum of ΔX/MN must be equal to 1/3. So long as the accuracy of measurement is ensured, the smaller the MN , the better the discrimination.