LITMIR.BIZ

Figure 2.2 Schematic diagram of the new method for sampling and measuring potential field at the surface. Current dipole is fixed at depth closer to the anomalous zone of interest.

Before continuing further, another advantage of a fixed-electrode strategy must be noted. Methods such as “4-D” seismic are inherently expensive because the excitation source typically involves large surface equipment (vibroseis units). Therefore, surveys tend to be at long time intervals, months or years. In resistivity monitoring, excitation is easily achieved using an economical power source, therefore repeated surveys can be taken within a day or a week, examined to see if sufficient changes have occurred, and if so, analyzed in greater detail. Furthermore, we note that arrays of electrodes at the surface are undoubtedly cheaper than geophones or accelerometer arrays. If appropriate well designs are used, such as fiber-glass casings, electrode arrays can be directly combined with seismic receivers, as well as with temperature and pressure monitoring sensors. These advantages and their inherent flexibility imply that resistivity methods can be extremely economic to operate once the array is in place.

2.7 Mathematical Quantification of Resistivity Resolution and Detection

      A good description of resolution, or resolvability, may be explained in terms of a minimum separation of two geological features or anomalies to be delineated [28]. Detectability is usually defined in terms of the size of the anomalous zone or electrical contrast of the anomalous zone that can produce a measurable potential field response [28]. On-site geological noise level must be considered in defining detectability. To delineate and quantify changes within the anomalous zone of interest, changes in geophysical parameters (e.g., electrical conductivity or resistivity) must occur over a distance. A detailed description of “resolvability” and “detectability” may be found in the geophysical literature. Greaves et al. [29] have discussed about the geometry and spatial distribution of current dipoles and potential dipoles giving a better resolution and detectability of the anomalous zone to be monitored.

Resolution may be further improved if the transmitting dipole or receiving dipole is placed into a borehole. Furthermore, it reduces the range of possible solutions and makes the mathematical analysis and interpretation more tractable and concrete. As indicated above in Figure 2.2, ∆-monitoring by inverting the differenced vector of potential field measurements may further enhance detectability if there are no systematic and non-random sources of noise. Narayan [3] has given a through description of theoretical sensitivity analysis and study for the resolvability and detectability of resistivity anomalies. Accordingly, a large amount of the current must flow through the anomalous region of interest. In this way, potential field measurements at the surface are detected effectively. It is important to focus on maximizing current flow through the anomalous zone. Current flow depends on several factors (e.g., spacing of current dipole, location of current dipole, dimension of anomalous region, electrical resistivity contrasts, etc.). It may be evaluated mathematically via theoretical model simulations and using other available data.

Figure 2.3 illustrates that the changes in potential field response as a function of electrical resistivity contrast [3, 28]. This also verifies equation 2.31 numerically. Also, the Figure shows that electrical potential field response varies exponentially with increasing resistivity contrast. A linear relationship has been found for small resistivity changes. The following important observations from this numerically computed result are summarized below: