Detection of cavities using pole-dipole resistivity technique

Using pole-dipole array, electrical resistivity survey was conducted to investigate the subsurface under a subsiding building located in Kita Kyushu area, Japan. The resistivity measurements were acquired along two traverse lines and interpreted using a graphical procedure known as Bristow's method. Moreover, a vertical electrical sounding (VES) was acquired and interpreted using 1-D inversion technique. Integrated interpretation of the resistivity data reveals a presence of some weak zones (shows high resistivity anomalies) in the surveyed area. The shallower weak zone (cavity) is located between 11 to 16 meters depth under the building. This cavity elongated in N-S direction with maximum diameter of about 10 meters. This interpretation agrees well with the boring results that ascertain a presence of very loose silt formation between 8 and 19 meters depth. It is concluded that, the presence of this cavity (very loose zone) is the main reason of the subsidence of the building.