Table 10-2 

 Calibration Constants 



instrument 



Gain 



Offset 



Regression 

 Coefficient 



Number of 

 Points 



OBS 9 cm 

 from the bed 



5.1570 mg// 



-11 25.7 mg// 



0.9845 



20 



OBS 56 cm 

 from the bed 



9.4681 mg// 



-2308.9 mg// 



0.9923 



20 



Pressure 



0.0084 

 m of H 2 



-11.0216 

 m of H 2 



1 .0000 



6 



Current (both 

 components) 



1.861 x 10 3 

 m/sec 



-3.81 m/sec 



• 



- 



Pore Pressure- 

 low-gain 



3.6 x 10 3 

 ft of H 2 



-1.3350 

 ft of H 2 



0.9999 



84 



Pore Pressure- 

 range 1 



7.302 x 10" 

 ft of H 2 



-1.3232 

 ft of H 2 



0.9995 



12 



Pore Pressure- 

 range 2 



7.133 x 10" 

 ft of H 2 



0.9117 

 ft of H 2 



1 .0000 



23 



Pore Pressure- 

 range 3 



7.196 x ^0^ 

 ft of H 2 



3.0889 

 ft of H 2 



1 .0000 



21 



Pore Pressure- 

 range 4 



7.564 x 10 J 

 ft of H 2 



5.2803 

 ft of H 2 



1 .0000 



17 



Pore Pressure- 

 range 5 



7.874 x 10" 1 

 ft of H 2 



7.5625 

 ft of H 2 



0.9999 



10 



After initial synchronization between the two systems, each system sampled 

 according to its own internal clock. Each of the internal clocks was set at the 

 beginning of the day to the international time standard. 



Preprocessing and filtering 



Because the ACP used in this experiment was designed for field use, the 

 return signal from it is amplitude modulated on a 455-kHz carrier. Such a 

 modulated signal can be sent over long distances with only small losses in data 

 quality. Because of this, low-pass filtering is necessary to separate the signal 

 from the carrier. This filtering of the acoustic data was performed with a 

 passive two-pole low-pass filter with a 100-kHz cut-off frequency. The OBS 

 and pressure sensor contain low-pass filters with cut-off frequencies of 5 Hz 

 to remove electronic noise and to make the measured data points repre- 

 sentative of the time-averaged signal between samplings. The EMCM signal 



196 



Chapter 10 Intermittent Near-Bed Sediment Suspension 



