the water depths. For a class 1 survey, either method of data correction 

 requires meticulous attention to quality control. 



• Waves. As the survey boat pitches up and down, the seafloor is 



recorded as a wave surface. To obtain the true seafloor for the highest 

 quality surveys, transducers and receivers are now installed on heave- 

 compensating mounts. These allow the boat to move vertically while 

 the instruments remain fixed. The most common means of removing 

 the wave signal is by processing the data after the survey. Both 

 methods are effective, although some contractors claim one method is 

 superior to the other. 



Even with the best efforts at equipment calibration and data processing, the 

 maximum practicable achievable accuracy for nearshore depth surveys is about 

 + 0.15 m (HQUSACE 1991). The evaluation of these errors in volumetric 

 calculations is discussed in Chapter 5. Survey lines are typically run parallel 

 to one another, with spacing depending on the survey's purpose and the scale 

 of the features to be examined. 



In geophysical surveys, the distance between the sound source and reflector 

 is computed as velocity of sound in that medium (rock, sediment, or water) 

 divided by one half of the two-way travel time. This measurement is con- 

 verted to an equivalent depth and recorded digitally or on a strip chart. A 

 recent development that is valuable in bottom sediment interpretation is a 

 signal processing unit that can be interfaced with an echo sounder and used to 

 indicate the seafloor sediments in terms of Wentworth or other general classi- 

 fication schemes. This is accomplished by measuring two independent 

 variables, roughness and hardness, from the acoustic signal and interpreting 

 these data in terms of sediment type. 



The principles of subbottom seismic profiling are fundamentally the same 

 as those of acoustic depth sounding. Subbottom seismic devices employ a 

 lower frequency, higher power signal to penetrate the seafloor (Figure 25). 

 The transmission of the waves through earth materials depends upon the earth 

 material properties, such as density and composition. The signal is reflected 

 from interfaces between sediment layers of different acoustical impedance 

 (Sheriff 1980). Coarse sand and gravel, glacial till, and highly organic 

 sediments are often difficult to penetrate with conventional subbottom 

 profilers, resulting in poor records with data gaps. Digital signal processing 

 of multi-channel data can sometimes provide useful data despite poor signal 

 penetration. Spacing and grid dimensions again depend upon the nature of the 

 investigation and the desired resolution. 



Acoustic characteristics are usually related to lithology so that seismic 

 reflection profiles can be considered roughly analogous to a geological cross 

 section of the subbottom material. However, because of subtle changes in 

 acoustic impedance, reflections can appear on the record where there are 

 minor differences in the lithology of underlying and overlying material. Also, 

 significant lithologic differences may go unrecorded due to similarity of 



Chapter 3 Field Data Collection and Observation 



59 



