58 



Inferences can often be made using these measures of stratigraphic and litho- 

 logic characteristics and important discontinuities. 



Fathometers or depth sounders, side-scan sonar, and subbottom profilers 

 are three major types of equipment used to collect geophysical data in marine 

 exploration programs. All three systems use electrically powered acoustic 

 devices that function by propagating acoustic pulses in the water and measur- 

 ing the lapsed time between pulse initiation and the arrival of return signals 

 reflected from various features on or beneath the bottom. These systems are 

 used to obtain information on seafloor geomorphology, bottom features such 

 as ripple marks and rock outcrops, and the underlying rock and sediment 

 units. Acoustic depth sounders are used for conducting bathymetric surveys. 

 Side-scan sonar provides an image of the aerial distribution of sediment and 

 surface bed forms and larger features such as shoals and channels. It can thus 

 be helpful in mapping directions of sediment motion. Subbottom profilers are 

 used to examine the near-surface stratigraphy of features below the seafloor. 



A single geophysical method rarely provides enough information about 

 subsurface conditions to be used without actual sediment samples or additional 

 data from other geophysical methods. Each geophysical technique typically 

 responds to several different physical characteristics of earth materials, and 

 correlation of data from several methods has been found to provide the most 

 meaningful results. All geophysical methods rely heavily on experienced 

 operators and analysts. 



Bathymetric surveys are required for many studies of geology and geo- 

 morphology in coastal waters. Echo sounders are most often used to measure 

 water depths offshore. Errors in acoustic depth determination are caused by 

 several factors: 



• Velocity of sound in water. The velocity in near-surface water is about 

 1500 m/sec but varies with water density, which is a function of 

 temperature, depth, and salinity. For high-precision surveys, the 

 acoustic velocity should be measured onsite. 



• Boat-specific corrections. As the survey progresses, the vessel's draft 

 changes as fuel and water are used. Depth checks should be performed 

 several times per day to calibrate the echo sounders. 



• Survey vessel location with respect to known datums. An echo sounder 

 on a boat simply measures the depth of the water as the boat moves 

 over the seafloor. However, the boat is a platform that moves 

 vertically, depending on oceanographic conditions such as tides and 

 surges. To obtain water depths that are referenced to a known datum, 

 echo sounder data must be adjusted in one of two ways. First, tides can 

 be measured at a nearby station and the echo sounder data adjusted 

 accordingly. Second, the vertical position of the boat can be constantly 

 surveyed with respect to a known land datum and these results added to 



Chapter 3 Field Data Collection and Observation 



