Hydrophone . Field testing is vital to determination of the accuracy, 

 ease of deployment, and reliability characteristics of any new technique 

 because of the need for testing in an uncontrolled, anticipated opera- 

 tional environment. The hydrophone tests conducted at the corrosion 

 pier amplified the need for a standardized, uncomplicated inspection 

 technique. Explicit direction had to be given, and followed, for uni- 

 formity during testing. 



During several periods in the hydrophone tests, subjective human 

 reasoning and variations in procedures affected the results. The hydro- 

 phone test procedures varied in the following ways: (1) the hammer 

 impact was inconsistent due to the differences in each individual blow 

 and (2) at first, the hydrophone was touching the pile, but later was 

 held approximately 1-1/2 inches away from the pile. When these vari- 

 ations were recognized, a standard procedure was devised for the divers. 

 The divers were explicitly instructed to hold the hydrophone 1-1/2 inches 

 away from the timber pile being tested at the line marking below the 

 impact region. In addition, a consistent hammer impact was needed. The 

 same exact strength behind each hammer blow was impossible, but a more 

 consistent impact was achieved. 



The data recorded for each series of impact tests performed on the 

 four harbor test piles were the same as that outlined for the laboratory 

 impact tests. Minor variations in the data collected included: (1) the 

 digitized impact waveform was not recorded and (2) a description of each 

 test pile and damage in test locations were recorded on video tape as 

 well as in sketches. 



Accelerometer . The procedure followed for the accelerometer impact 

 tests was nearly the same as that outlined for the laboratory impact 

 tests. The accelerometer tests were conducted on only two quadrants of 

 each of the four harbor test piles. 



Field Test Results 



The impact test equipment, like the ultrasonic test equipment, could 

 only be evaluated in the field for its ability to detect external damage. 

 Figures 28 and 29 present test results showing the frequency shifts in 

 damaged and undamaged regions of the harbor test piles for the hydrophone 

 and accelerometer impact field tests, respectively. 



Figure 28a shows the shift in frequency (in quadrant 1) of the 

 highest amplitude peak in the frequency spectrum from a range of 500 to 

 700 Hertz in the solid regions to 400 Hertz in the damaged region of 

 pile no. 8. The accelerometer data obtained for the same pile (Fig- 

 ure 29a) were inconsistent with the hydrophone data. No correlation 

 could be established between the frequency (corresponding to the highest 

 energy peak) and the amount of external damage. 



The greatest amount of damage on pile no. 5 was in quadrant 2 below 

 line 3; the lowest frequency is observed in this region (Figure 28b). 

 The damage in quadrant 3 of pile no. 5 was insignificant, and the accel- 

 erometer data collected here show no shift in frequency (Figure 29b) . 



The data collected on harbor pile no. 1 show an increase in fre- 

 quency in traveling from a damaged region to a solid region (Figure 28c) . 



The Support pile results show that the frequency remains constant 

 along the pile (Figure 28d) . 



47 



