hello,
I'm going to do a report on 'method of measuring sea bed bathymetry',
I know
nothing about it, can someone help me please?  i want some information
from scratch.
I'm a 2nd year degree student.

checkout

http://www.umesc.usgs.gov/aquatic/bathymetry/methods.html

or

http://www.epa.gov/region5fields/htm/methods/bathymetry/

In short, to measure depths at sea you first need to know tides at the
required location. Depending on the scale of the survey and the
accuracy required, the method would be choosen. For depths, you need
to refer the measurements to known tides. Measurements can be taken
using lead line, single echo sounder or multi beam. For objects
detection you would need to use sidescan sonar. At the same time, you
have to have a positioning method to determine your horizantal
position using different methods or instruments like GPS. An important
element in depth measurement is Sound Velocity (SV) which has to be
obtained or known prior any survey to feed in echo sounder. SV can be
measured by various methods like bar check or using sound vilocity
probes. The survey should be planned to use main lines perpindicular
to the contours and cross lines at right angles. Line spacing depends
on the accuracy and the scale required. In summary, bathymetry survey
need to have tides and horizontal positioning. I hope that my short
comment would help