CETN-III-16
Revised
11/83
b o a t being used for inspection.
The SSS printout will be marked as the boat
moves past positioning marks or other features on the structure which can be
u s e d as reference points.
Then,' if the boat is moving at a known constant
s p e e d , the locations of any structural features of interest as shown on the
record sheet can be determined.
The recommended range of towing speeds is from one to two knots. The
c l a r i t y of the record increases as the towing speed decreases.
SSS can be used to inspect both sloping structures and vertical struc-
G e n e r a l l y , inspection of vertical structures such as seawalls
requires
tures.
m o r e detail since features of interest are usually smaller (i.e., cracks and
misalignments) than those on a sloping structure such as a rubble-mound struc-
ture with large armor units.
F o r inspecting vertical walls where small fea-
tures need detection, the fish should be.boat-mounted so it can be moved
v e r t i c a l l y without rotation or horizontal motion.
The boat should be kept as
near motionless as possible (i.e., t i e d up to the wall) and the fish fixed to
About a 50- to l00-foot
length of wall can be
a rod mounted in a rigid frame.
surveyed in a few minutes by lowering and raising the rod from this fixed
Raising and lowering the fish is not recommended for sloping struc-
position.
t u r e inspections because of extreme distortions on the printouts.
V e r t i c a l surveying, where the fish is turned upright and moved verti-
c a l l y rather than in the normal horizontal direction, will give more detail of
vertical
features.
However, this is a specialized technique which has not
been widely used.
F u r t h e r research is needed to better quantify its direct
a p p l i c a b i l i t y to solving engineering problems.
SSS inspection from land may be possible when there is adequate access
t o the water.
The frame can be hung from the side of a truck which is driven
The main advantages of this land-
along along the structure to be inspected.
based system compared to a boat are the significant reduction in survey time
a n d the increase in accuracy due. to better stability of the fish.
INTERPRETATIONOFSIDE-SCANSURVEYRECORDS: Some people expect a SSS record
A
detailed
explana-
t o look like an aerial photograph; however, it does not.
t i o n of how to interpret SSS records is beyond the scope of this technical
note.
Basically,
thinking of the SSS beam as a flashlight in a dark room and
the oblique image shadow as your printed record will give the proper perspec-
tive for viewing SSS images.
The darkness of the printed image is a function
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