Total number of echoes                           

Jan| 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Jan| 15 16 17 18 19 20 21 22 23 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14
---+------------------------------------------------------------------------
01 | -- -- -- -- -- -- 21 15 20 22 18 17 20 10%16 %3  8  8  6 %6 %8 IF%14%11
02 |  4 11 10 21 18%10 25 29 21 20 22 27 24 20%20 12  5  9  9  8 16 16%11 13
03 |%15 21 25 34%38 29 38 37%36%33%21 31 14 22 17  9 10  7  4  5 11 16 10 16
04 | 22 31 52 69 71 76 104 107 89 74 64%44 54 67%44%37%16%10 %7%10%13  8%11 IF
05 |%15%16%15%30%31%31%14%28%33%19%22%27%23%20 19 11 16  8  8 10  6 10  6%10
06 |%12%27%41%28%41%35%36 33 40 29 30 29 20 23 14 11  6 10 %3  5 11 -- -- --

                       Echoes longer than 20 seconds                                                     

Jan| 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Jan| 15 16 17 18 19 20 21 22 23 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14
---+------------------------------------------------------------------------
01 |                    1  1     1     1  1  1                              
02 |        1                                                               
03 |                                1                                       
04 |     1     1     2  1  3  1  1              1     1                     
05 |                                1  1     1                              
06 |                                                              1