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TIPS: Noise, Stacking and DynaQ®

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ne of the factors that limits the depth of GPR signal penetration into the ground is random background noise from other radio frequency emitters, such as radio and TV stations, cell-phone towers and radio communications in the survey area. The “Depth of Penetration” for GPR is defined as the depth at which the ground materials attenuate the GPR signals down to the same level as the background noise floor, at which point they can no longer be differentiated from the noise (Figure 1, left). This means that, if the random noise floor can be lowered, more of the deeper GPR signals hidden in the noise can be detected (Figure 1, right).

Figure 1
Left – the “Depth of Penetration” is the depth where the GPR signals attenuate down to the background noise floor. Right – Stacking GPR traces increases the depth of Penetration by lowering the noise floor so that deeper, weaker GPR signals are detectable.

The noise floor can be reduced by stacking the GPR data traces. In other words, at each collection point on the ground, multiple GPR traces are collected. Those traces are then averaged, and the averaged trace is saved. The averaged trace has less noise, increasing the signal-to-noise ratio and thus, increases the depth of penetration (Figure 2).

Since the early 1980s, Sensors & Software GPR systems have had the capability of stacking data traces up to 2048 times; the operator set the number of stacks desired and moved the antennas after the GPR trace was stacked, averaged and saved.

Figure 2
Stacking traces multiple times and saving the average trace lowers the amount of noise (shown as red signal) in the average trace. The more you stack GPR traces, the lower the noise, the better the GPR signal and the deeper the depth of penetration.

The challenge for users in those days was to optimize the number of stacks to the speed the GPR system was moving. Too few stacks decreased the possibility of achieving a greater depth of penetration and too many stacks decreased survey speed, forcing the user to determine the optimal stacking interval. Following our motto of turning complex to simple, in 2005 we introduced Dynamic Quality technology, or DynaQ®, to eliminate the requirement for customers to self-select the number of stacks by determining optimal stacking dynamically and automatically.

Most of you know DynaQ® as the colored scroll bar under your GPR data image that tracks along as you collect GPR data (Figure 3), but that simple colored scroll bar has some powerful technology behind it that reduces random noise, increases data quality and increases the depth of penetration.

Figure 3
The DynaQ® bar, seen on all Sensors & Software GPR products with an odometer wheel, scrolls out under the GPR image as the GPR line is collected. The color is an indication of the number of stacks.

With DynaQ®, the GPR system stacks continuously until the odometer indicates that the system has moved one step size and the system needs to start collecting a new trace. At slower speeds the system stacks more times between odometer triggers than at higher speeds, but it always collects the maximum number of stacks possible (Figure 4).

Figure 4a

 

Figure 4b

 

Figure 4c

 

Figure 4d
With DynaQ®, traces are collected continuously over the Step Size distance (5 cm for the Noggin 250 shown). When the GPR system moves fast (4a and 4b), fewer traces are collected in each step size (in this example, 1 trace in 4a and 2 traces in 4b). The traces in each step size are averaged (stacked) and the average trace saved. When the GPR system moves slower (4c and 4d), more traces can be collected and averaged. The traces with higher numbers of stacks (4c and 4d) produce saved traces with less noise (red signal). These examples all show a relatively low number of stacks. Sensors & Software GPR systems can collect up to 2048 or even 65,536 stacks, numbers that are difficult to depict in simple illustrations.

Therefore, if you are struggling to see deep enough, remember that one simple strategy is to slow down and let DynaQ® do its work; stack the data-trace more times, decrease the noise floor and increase the depth of penetration.

DynaQ® is standard for all our GPR systems that use odometers – pulseEKKO®, NOGGIN®, CONQUEST®, FINDAR® and LMX®.

With the introduction of the NOGGIN® Ultra 100 capable of stacking up to 65,536 times, we have further enhanced the DynaQ® functionality to take advantage of this additional stacking. New DynaQ® colors – purple, light green, and dark green – represent stacks beyond 511, 2049 and 8192, respectively, giving you visibility into when you are hitting Ultra stacking levels.

DynaQ® is one of the many unique features of Sensors & Software products that help you to achieve the best data quality without needing to worry about complex settings during data collection.

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