Where should PIT hit the pile head?

We were approached by a customer with the following two questions:

1. We didn’t comply with the ASTM D 5882 standard's stipulation to test at least 3 points from each pile. In your opinion, as a producer of equipment for low-strain testing, do you consider this a problem?

2. If we place the accelerometer at one point and hit that point with the hammer in 3 triangular spots, 20 to 40 impacts per point (collecting them in the same file for the same average reflectogram of the PET), do you consider this method to yield reliable results? Or is the only method to place the accelerometer at one triangular point, strike it with the hammer a number of times, and then repeat this at the other triangular points, so that you have at least 3 points from the surface of the pile? (see the collected data below, and the average blue reflectogram)
   

Our answer The reasoning behind the very detailed procedure in the ASTM D5882 standard is to find a spot that yields a good response.

The top of the pile is often not level, homogeneous, or smooth... it can contain broken pieces and low-grade concrete. So there is logic behind testing multiple points.

There are also 3D effects: It takes the wave 2-3 diameters to collect all the revibrations and start going down the pile as a 1-dimensional wave. The entire Pulse Echo Method (PEM) for PIT is based on the assumption of a 1D stress wave and does not account for unpredictable 3D effects.

The ASTM standard attempts to impose some order on this unpredictability by using a rigid recipe. Hence, hitting at 3 points on the pile head is not an attempt to add more data; rather, it is an attempt to collect cleaner and more reliable data. Therefore, we believe you should do the following -

You should try different locations for both the hammer and the accelerometer. Once you find a good spot that yields a clean, consistent response. Delete everything and restart testing at that point only. There is no sense in mixing good data with bad data, just for the sake of following a rigid procedure that does not give room to your experience and judgment.

With PET, you can use AutoSort, which saves you the need to reset and restart. Once you find a good spot, keep hitting on that spot, and AutoSort will remove the noisy data.