山区找水的“相对衰减时法”探讨

实测资料表明,二次电场衰减时与视电阻率关系点分布区的下限存在正向密切区间、反向密切区间和不密切区间,即衰减时与视电阻率关系十分密切.忽视视电阻率的变化,有可能造成激电找水效果上的失误.针对衰减时与视电阻率关系的区间特性,我们采用“相对衰减时”的概念作为激电找水的含水因素(相对衰减时S_R＝ρS/_s).采用该参数的实际意义在于反向密切区间突出了含水异常,正向密切区间消除因极距增大引起的非水异常.因此,相对衰减时在研究山区地下水的富集情况时灵敏度较高.应用相对衰减时法在不同水文地质条件的山区找水中,取得了较好的效果,特别是在我国著名的玫瑰泉地下水工程中,在电阻率法和常用的衰减时法解释推断有困难时,能比较准确地确定了富水区,成井后获得水量6万t/d,建成大型农业地下水源地.

A TENTATIVE DISCUSSION ON "RELATIVE DECAY TIME METHOD" FOR WATER PROSPECTING IN MOUNTAIN AREAS

It is indicated from measurements that the lower limit of the area contain ing the relational points between decay time of secondary electrical field and apparent resistivity has directly intimate interval, inversely intimate interval and non-intimate interval. This suggests that the decay time has close relation ship with the apparent resistivity, and any neglect of the variation in apparent resistivity may lead to error in the search for water by IP method.In view of the interval characteristics of the relationship between decay time and apparent resistivity, we have employed a concept of "relative dacay time" as the water-bearing factor in water-search using IP method (the relative decay timeS_R＝ρS/_s).The significance of adopting this parameter lies in making prominent the water-bearing anomalies in inversely intimate interval and elimi nating anomalies without water caused by increase in polar distance in directly intimate interval. The relative decay time therefore has a comparatively high sensitivity in the investigation into concentration of underground water in mountain areas.The application of relative decay time technique to water prospecting in mountain areas with different hydrogeologic conditions has already yielded quite good results. In the groundwater engineering of the well-known Meigui Spring where the resistivity method and the commonly-used decay time method had difficulties in interpretation and inference, the relative decay time method has located water-rich area with remarkable accuracy. Since the completion of the project, a water supply of 60,000t/day can be obtained and a large-size agricul tural groundwater source area has been established.