TAPPER-64 | Overall Schematic
The system consists of three (3) major components as illustrated in the Figure 1 below.
1. Source-Receiver Panel (SRP)
This is the component holding the seismic source-receiver assembly and related items such as GPS receiver and Intra-Red (IR) temperature sensor. The source is a small metal bolt with cap nut tightened in a stiff zip tie (Figure 2). It is attached to a vertical rod, hanging down on the pavement surface equidistance inline in front of the four (4) MEMS microphone arrays (MMA) (Figure 3). It spontaneously makes impacts when the panel moves forward.
It is an extremely simple device, yet invented after decades of research and playing the most critical role in acquiring good-quality data. Although it outperforms in this prototype system, it is the main item that needs an upgrade in the way it operates in the field as explained in more detail in the upgrade section.
Each MMA consists of 16 MEMS microphones spaced with a 2.25-cm interval, and all four (4) arrays are arranged radially with the impact point at the center. Two edge arrays (#1 & #4) are arranged perpendicular to the moving direction, while the other two (#2 & #3) are arranged in such a way that centers of array are evenly spaced along the transverse direction (see Figure 3). This ensures an even spatial sampling along the transverse direction within the covered width (0.5 m – 1.0 m).
A GPS receiver is attached on the connecting frame to the hitch receiver near the impact point of the source, recording LAT/LON data at impact time. Infra-Red (IR) sensor is also attached near the GPS receiver pointing down to the pavement surface to record asphalt temperature. The entire SRP frame is attached into the 2-inch hitch receiver on the back of a survey vehicle.
2. Data Recording Unit (DRU)
A 64-channel National Instrument (NI) PXI device is used to record data coming from all 64 microphones. Each impact-generated sound, detected by the 1st channel microphone, triggers the system recording with 1.5-ms and 3.5-ms pre- and post-trigger times, respectively, to generate a 5.0-ms 64-channel record in the system memory. Once twenty (20) of such records are generated, they are saved on the HD as one file of TDMS format. PXI is a stand-alone Windows computer that runs the acquisition-control software as a startup program. Several key recording parameters can be controlled by this program (e.g., trigger voltage level, total recording time, etc.). A WiFi router connects PXI wirelessly with a separate on-board laptop computer used for Data Processing Unit (DPU).
3. Data Processing Unit (DPU)
An on-board laptop computer is used to process recorded seismic data and display the results in real time. When a 64-channel TDMS file is saved in PXI, the ParkSEIS-HMA (PS-HMA) software installed in this computer processes it immediately and displays the results in velocity (Vs), thickness (H), Young’s (E), and shear (µ) moduli in X (distance) and Y (width) coordinates. Actual sampled points on the pavement are geo-referenced on a separate Google map using the recorded GPS data. Corresponding temperature data are displayed in a separate chart.
Figure 1. Overall schematic of TAPPER 64 system
Figure 2. (a) Components of impact source, impact source with (b) optimum length of zip tie, (c) excessively long zip tie, and (d) excessively short zip tie.
Figure 3. A schematic of source-receiver panel (SRP) that holds four (4) 16-channel MEMS microphone arrays (MMA). Each MMA is installed on a sliding bracket to adjust the survey width (dY) while maintaining the same inline distance from the source with other MMAs.
