Distance Amplitude Correction (DAC)

As mentioned previously, as a result of beam spread and attenuation, echo heights observed from equivalent defects decrease with increased distance. Consequently, a technique known as distance amplitude correction (DAC) is commonly employed to adjust signals generated at different distances for comparison purposes. This technique consists of generating a DAC curve that essentially indicates that a smaller echo at a greater distance may have similar properties to a larger echo at a lesser distance.
With straight beam transducers, blocks with flat bottom-hole specimens typically are used to generate the DAC curve. However, generating the same curve with an angle transducer is typically completed using a specimen with side-drilled holes. Regardless of the technique used to generate a DAC curve, the material used in the calibration block should be the same as the material in the test specimen due to potential differences in attenuation characteristics.
Figures 15a and 15b conceptually illustrate how a DAC curve would be generated for an angle beam transducer. In figure 15a, a side-drilled hole is shown in a test block that can be scanned with four different scanning patterns. Note that one could double the number of points on the DAC curve by using a second equivalent side-drilled hole at a different depth. When the echo signals are plotted together, the DAC curve shown in figure 15b results. This curve is referred to as "100 percent DAC." This means that for an equivalent defect in the test specimen, the echo signal will fall on this line. Smaller or larger defects in the test specimen will lie below or above the 100 percent DAC curve, respectively. The most accurate way to assess these defects is to repeat the DAC curve generation with a series of diameter holes. The result will be a series of curves that should allow for more accurate defect assessment. figures..