Introduction

Measurements of direct current (DC) resistivity are useful for detecting frozen ground, because the resistivity values significantly increase in accordance with water's phase change from liquid to solid. Conventional vertical electrical soundings (VES) involve the placement of four-electrode arrays and have been the most commonly used geophysical technique for identifying the occurrence of frozen ground (see Chapter 5). Problems have arisen from the necessary assumption of a horizontally bedded subsurface structure for standard VES interpretation. This situation is further complicated by concave and convex surfaces in topography and significant lateral variation in subsurface water conditions even over small areas, especially in mid-latitude mountainous areas and discontinuous permafrost zones. Therefore, interpretation of data sets obtained by VES alone often remains ambiguous.

As described in detail in Chapter 1, ERT is the hybrid of vertical and horizontal resistivity soundings in which a large number of four-electrode combinations (so-called quadripoles) are measured. This technique two-dimensionally delineates subsurface electrical structures and has recently been recognised as an effective method for investigating frozen ground under complicated topography and/or subsurface structures. In the following case studies ERT was applied in discontinuous and sporadic permafrost areas in order to delineate and characterise frozen ground in the Asian mountain permafrost regions of the Daisetsu Mountains, northern Japan, the Kanchenjunga Himal, eastern Nepal, and the Shijir valley of northeastern Mongolia. On the basis of these research experiences, this note describes practical issues for applying ERT imaging in mountain permafrost regions.