Total number of echoes                           

Dec| 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Dec| 15 16 17 18 19 20 21 22 23 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14
---+------------------------------------------------------------------------
12 |%13 14 27 23 41 28 35 36 37 36 23 14 23 12  5 14  8 14 13 15 23 34 40 34
13 | 20 17 15 45 65 59 46 55 51 33%31%24 38 15 14 11 12 14 11 34 34 37 41 62
14 |%23 12 23 44 63 71 66 65 52 30 15 22 11 13 13  9  9 12 13 30 39 67 46 50
15 | 41 31 28 40 62 50 55 46 27 22 26 19 12 13  4 13  9  9  8 24 16 16 18 15
16 | 27 17 24 24 29 21 21 22 17 23 25 26 15 22 14%13%12 10 %3 IF%11 12 13  8
17 |%14 21 22 17 17 29 28 27 37 24 40 13 15  8 16 11  8  3 13 17  6 -- -- --

                       Echoes longer than 20 seconds                                                     

Dec| 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Dec| 15 16 17 18 19 20 21 22 23 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14
---+------------------------------------------------------------------------
12 |                       1           1                                    
13 |                       1           1                                    
14 |        1     1     1                       2                       1   
15 |           1        2        1                                          
16 |                 1              1                                       
17 |                          1