170 



the reflected acoustic intensities, and X's represent 1 1 extra bytes inserted as 

 spacers and contain no useful information. 



BASS. The BASS data file format contained 38 bytes for each 0.25-sec 

 record. For each of the four vertically aligned BASS sensor arrays there are 

 four values of two bytes each, containing four hexadecimal digits, correspond- 

 ing to the velocity components along the four transducer axes. The four bytes 

 previous to the 16 double-byte velocity words are used to record a time coun- 

 ter which contains the day, hour, minute, and second of that data record. 

 Each data record also contains a constant-valued header byte and a checksum 

 byte which is the sum of the 16 velocity component values and can be repre- 

 sented as 



H TTTT VV VV VV VV 



VV VV VV VV / 9 _2) 



VV VV VV VV 

 VV VV VV VV s 



where each character represents one byte and each byte is made up of two 

 hexadecimal digits. H is used for the header byte, T represents the time 

 bytes, S is the checksum byte, and VV corresponds to the two-byte velocity 

 components. 



Sample raw data 



An example of a single 1-sec ACP data record, in the format discussed 

 above, is given in Table 9-2. Because each record is fairly lengthy, ACP data 

 can be separated from the pressure data and stored as a separate data file for 

 later interpretation. The pressure data can be stored as a 2-Hz time trace 

 from any desired starting time and duration. 



Each of the ACP data bytes represents an integer value from to 255, 

 where the actual stored data are from hexadecimal 00 to FF. A preliminary 

 examination of the raw data begins by reading a 10-min block of data and 

 averaging the values in each range bin which gives the average reflected 

 acoustic signal for that particular range bin. Figure 9-8 shows the distribution 

 of the average raw signal for a 10-min period from wave run A0608A; other 

 figures showing distributions of average raw signals for seven other wave runs 

 can be found in Appendix I (Figures 11-17), contained in Volume n of this 

 report. 



The distribution in the figure shows that at range bin 108 the average 

 signal value is the maximum of 255 for the 10-min averaging period. The 

 maximum value, which indicates a strongly reflected signal, is always seen to 

 occur in range bin 108 and can be interpreted as the reflected response from 

 the bottom. For a particular data record a maximum reflected signal may 

 occasionally occur at range bin 107 or 109, but it always occurs at range bin 

 108. The dip towards the bottom of the profile shows attenuation of the 



Chapter 9 The Ohio State University Measurements 



