Geoscience Reference
In-Depth Information
the rock surface generating a positive surface charge relative to ground. This surface
charge is recorded by means of a capacitive sensor visible in the photograph in Figure
3 (left) as a fl at metal plate 0.8 mm above the rock surface.
The stressed rock volume becomes the “source” in which electrons and pholes
are co-activated (Freund et al., 2006). There are two points to be stressed: (i) inside
the stressed rock volume the number of charge carriers that is available to transport
electric current has gone up, allowing the rock to transport more current than before,
in the unstressed state; (ii) There are two kinds of charge carriers, both electronic but
one kind (the electrons) representing a negative charge and the other kind (the pholes)
representing a positive charge. For both the number density inside the stressed rock
volume is higher than outside the stress fi led. Both would “like” to spread out of the
stressed rock volume, if they can.
Figure 4.
As one end of the granite slab is loaded (dotted line) electrons flow out of the stressed rock
volume to ground (lower curve) and from ground into the unstressed rock (upper curve). The two
currents are legs of one and the same circuit.
We conjecture that the boundary between stressed and unstressed rock acts like a
Schottky barrier: it lets holes pass but rejects electrons. This barrier function is illus-
trated in Figure 3 (lower right) by the symbol of a diode. Indeed, as shown in Figure 4,
a current carried by positive charge begins to fl ow as soon as a load is applied. A posi-
tive charge layer builds up on the rock surface (not shown), due to trapping of pholes.
In the case of the granite slab, a linear system, the currents increased approxi-
mately linearly with applied stress. In Figure 5, for a planar geometry, the currents
increase in a non-linear fashion, very steep at fi rst with strong fl uctuations, and then
saturate before the modest stress level of 48 MPa is reached. The cause of these
