Survey Operations

This section will discuss specific issues about field operations using descriptions of notional and generally used equipment. Notional discussions will be general and can be applied to a class of equipment, while discussions about generally used equipment will discuss specific characteristics of brand name equipment. The following equipment will be discussed:

• Vertical Control Systems

+ Fathometers

+ Tide gages

+ Vertical motion sensors

• Horizontal Control and Navigation Systems

+ Optical Horizontal Control (Transits and Theodolites, including Hydro-I and II)

-l- Electronic Ranging Systems (Del Norte Trisponder and Mini-Ranger)

+ Global Positioning Systems and Differential GPS

-(- Acoustic Navigation Systems (Trackpoint II and Diver Navigation System)

• Side Scan Sonars

• Subbottom Profilers

4.4.1 Vertical Control Systems

When using a fathometer to conduct bathymétrie surveys, the following areas need special consideration to achieve maximum accuracy:

• Sound Velocity

• Draft Correction

• Vertical Motion Correction

The fathometer measures time of flight for an acoustic pulse to travel from a transducer to the bottom and return. The time is converted to depth using the velocity of sound, but the velocity of sound changes with temperature, depth, and salinity. In most cases, sound velocity will be measured using a Conductivity, Temperature, and Depth (CTD) instrument, which converts conductivity to salinity and computes velocity using the following equation:

where

C = Velocity of sound in m/sec T = Temperature in degrees centigrade S = Salinity in parts per thousand

For most coastal ocean situations, except near river mouths, the salinity can be assumed to be 35 percent. Thus, if a CTD

Table 4-4 Standard Chart Scales

Chart Scale

Nautical Miles Per Foot

Line Spacing for 1-Inch Plot Spacing (Rounded)

1:250

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