AMEC will use the LISA series 1000 liner integrity survey equipment with data collection and voltage mapping on the 50-acre soil-covered portion of the Ghent project, which is slated for completion by early January 2013. Voltage mapping involves plotting recorded values of voltage potential on a local X and Y coordinate system to view the voltage potential in plan view of the survey area. With the addition of voltage mapping, the survey results can be reviewed by a third party and it will provide survey quality control documentation. Three dimensional voltage mapping provides much better data review than two-dimensional “slices” of the voltage potential field, a method which is often used. The qualitative shape of the voltage field as seen in plan view can weed out false positive signals more easily than viewing slices of the data. Also, smaller holes are more likely to be noticed. This is because the magnitude of the leak signal is not always easily distinguishable from background noise levels; but the characteristic shape of an anomaly caused by a hole location is hard to miss through three-dimensional mapping.
This is best illustrated by comparing both data analysis methods. A roughly 15 by 60 foot soil-covered test cell containing two leaks was surveyed using the dipole method. The data analysis of the survey results are shown in Figure 1, with the location of the current injector electrode shown to assist with data interpretation.
The Data shown in Figure 1 was created with the same set of data recorded in the small test cell. The data shown in the graph above represents slices along the Y-axis on the voltage map below. In the voltage map, the test cell is given a local X axis and Y axis, with the “0” values beginning at the bottom left corner of the cell. The test cell is shown in plan view, with the voltage potential measurements as recorded by a dipole probe plotted as isopotential lines, very similar to how a topographical map is portrayed. The tick marks show the locations of the measurements taken by the dipole probe. The positive voltage values are shown in green and yellow, with the yellow being the higher positive values. The negative voltage values are shown in red and blue, with the blue being the higher negative values. The colors change from green to red when the voltage field becomes negative.
In the voltage map, the highest positive values are shown in the upper right-hand corner, where the current injector electrode is located, and where the last slice also shows a voltage spike. The characteristic shape for a hole location on a voltage map is to have closely-spaced isopotential lines between two concentrated points of high and low voltage potential readings. The peaks will be oriented in the direction that the dipole took the voltage measurements; in this case, from left to right. There are two holes in the survey area: a large one that is very easy to see because the signal is strong enough to cause the voltage field to go negative, and a small one that has the characteristic shape of a leak signal but the strength is not strong enough to cause the voltage field to go negative, especially with the presence of an nearby leak that is much larger. Take a look at both ways of displaying the exact same data and try to see if you can spot where the two holes are located before scanning this article below.
It is typical to perform a survey by recording data throughout the survey area, then reviewing the data to locate potential leak locations. This makes a survey more efficient than trying to locate leaks in real-time as the survey proceeds. The operator then goes back to each potential leak location and scans the area until a leak can be pinpointed or the area can be dismissed as a false positive anomaly. Then, the leak is excavated and the area is rescanned to make sure that no further leak signals are registered in the surrounding area.
In this test cell, a large hole is clearly seen in the upper left-hand corner of the voltage map shown in Figure 2. That large leak was first located and excavated and thus removed from the survey area. Then the second potential leak location was scanned. This time, the smaller leak showed a relatively strong negative value on the side of the leak opposite the current injector electrode. It could then be easily pinpointed and excavated.
Slice 3 of Figure 3 clearly shows the characteristic sine-wave pattern of a leak, which one would expect to drop below zero on the Y-axis. Line 3 was taken at the location on the Y-axis where the leak signal is strongest in the Figure 2 voltage map. However, the small leak located closest to Line 1 cannot be distinguished from the oscillations in background voltage of the adjacent survey lines.
Only by recognizing the pattern of the leak signal as seen in the three-dimensional voltage map of Figure 2 can one “see” the influence of the second, smaller leak.
When comparing the two methods of analysis visually, it is clear that the survey becomes much more transparent when plotted in three dimensions as a voltage map, with only a minimal amount of expertise to recognize leak locations. The enormous volume of data to be analyzed for a site such as the Ghent project can be broken down into easily decipherable voltage maps, rather than having to tab through thousands of oscillating voltage slices displayed in Excel graphs.
In a field where this “black box” technology is viewed skeptically, an enormous burden is lifted with meaningful survey quality control documentation that can be easily reviewed by others. AMEC is providing the highest level of quality assurance possible by using the voltage mapping method at the Ghent project, assuring that their client is getting the best possible survey performance.