根据地面物理现象的观测而推测地下情况的技术叫做物理勘探。观测的物理现象有地震、声波、电、放射能等。利用电量进行测定的物理勘探称为电探。它有测定天然电位分布的自然电位法(SP法)和测定地层电阻分布的电阻率法。在有关土木、建筑的地基调查中,经常使用后者。图—1是电阻率法中使用的电极布置例,其电流电极C_1、C_2和电位电极P_1、P_2设置在地表面。现在,若于C_1、C_2之间通以电流I,量测出P_1、P_2间的电位差V的话,地层 The technique of inferring subsurface conditions based on observations of surface physical phenomena is called physical exploration. Observed physical phenomena are earthquakes, sonic waves, electricity, radioactivity and so on. The physical survey that uses electricity to measure is called electric exploration. It has a natural potential method (SP method) for determining the natural potential distribution and a resistivity method for determining the resistance distribution of the formation. The latter is often used in ground-based surveys on civil engineering and architecture. Fig. 1 shows an example of the electrode arrangement used in the resistivity method. The current electrodes C_1 and C_2 and the potential electrodes P_1 and P_2 are disposed on the ground surface. Now, if the current I between C_1 and C_2 is measured and the potential difference V between P_1 and P_2 is measured, the formation