Spatial Differences in Potential Changes of Rock Loaded to Fracturing: Characteristics and Mechanism

    Electrical signal changes in process of loading rock to fracturing is an important rock petrophysical phenomenon, which is of great significance for understanding the abnormal electric field, magnetic field and electromagnetic radiation related to rock fracturing, geohazards and tectonic earthquake. Positive holes (P-holes) activation due to the broken of peroxy defects is one of the important mechanisms causing rock current, and peroxy defects are common in most of the crust rocks. During the loading of rock specimen, the activation, transmission and accumulation of P-holes in different place inside the rock are bound to be variable, and present as the spatial difference in the changes of electrical signals. However, the spatial differences in the changes of electrical signals during loading rock to fracturing and the correlation between the electrical signal and the internal physical-and-mechanical states of rock have not been well studied so far.

    Therefore, we carried out potential monitoring experiments in different areas of the rock under local stress and synchronous acoustic emission monitoring. The rock specimen was specially designed in 3D shape of cube-frustum. The lower cube acting as the loaded part was wrapped with copper foil and connect to the negative electrode of the potentiometers; while the upper square frustum acting as the free part were pasted with copper foil at three places and connect to the positive electrode of potentiometers, respectively, as in Fig. 1a.

    The experimental results, as in Fig. 1b, showed that during the early rock loading, the characteristics of potential changes in each area were basically the same, with slight differences in amplitude, which were directly related to the size of micro-crack development area in the corresponding part. When the loaded rock reached to macroscopic fracturing and got failure, the characteristics of potential change in different areas were significantly different (Fig. 1c), which were directly related to the formation of macroscopic fracture inside. When no macroscopic fracture surface was formed at the intersection zone of the free part and the loaded part below the potential monitoring area, P-holes would transmit upward to the upper surface of frustum along the stress gradient, resulting in the rise in potential. Conversely, if a macroscopic fracture surface was formed at the intersection zone (Fig. 1d), the upward transmission of P-holes would be blocked, and more P-holes reached the surface of the loaded cube, resulting in the decrease in potential. Furthermore, we kept loading the rock fragments after macroscopic failure, and found that during the friction or the relative slip process between fragments, the combined influence of furrow effect, adhesive friction and slip shear also led to P-holes activation and dislocation sliding, resulting in potential rising again. The potential risings in different areas were related to the degree of friction and the distribution of macroscopic fracture surfaces in the corresponding parts.

Figure 1 Experimental schema diagram and results. (a) Diagram of the experimental schema; (b) The variations of potentials; (c) Potential changes when the rock got failure; (d) Failure pattern.