Signification and Use of A-scan Images
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QuintSonic 7
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7.3.3.4
Superimposed echoes
Echoes travelling in the same direction Echoes travelling in alternating directions
Example: Superimposed echoes
If a layer is not thick enough, i.e. if the travel time through this layer is too short, the input and
output echoes of this layer will overlap. The effects of this situation depend on whether the two
echoes take the same direction or not (positive or negative).
If both echoes take the same direction, they will more or less merge into one large echo so that a
detection of this layer will not be possible any more. A portion of the missing layer thickness will be
added to the layer located above this layer, another portion will be added to thickness of the layer
below. This explains why a reliable measurement of the complete layer system is not feasible any
more.
If input and output signal travel in different directions, they will be represented on the diagram as
directly adjacent echoes with different algebraic signs (positive / negative). In such case, a
detection is possible. However, a suitable least-square optimisation (section 8.4) must be applied
to make sure any incorrect echo position will be corrected accordingly. This will work within the
temporal resolution limits of the system (see data sheet).
When measuring extremely thin layers, the temporal resolution does not allow to separate the
input and output echoes any more. The travel times through such thin layers being very short, they
have no significant effect on the calculated travel times of the adjacent layers and the thickness of
the adjacent layers can be determined with sufficient accuracy. Only the extremely thin layer will
not be represented in the result. A special situation will be given if additional layer interfaces have
been produced due to interruptions of the manufacturing process (see section 7.3.3.5).