  A weakly-nonlinear potential theory is developed for the description of deep
penetrating pressure fields caused by single and colliding wave groups of
collinear waves due to the second-order nonlinear interactions. The result is
applied to the representative case of groups with the sech-shape of envelope
solitons in deep water. When solitary groups experience a head-on collision,
the induced due to nonlinearity dynamic pressure may have magnitude comparable
with the magnitude of the linear solution. It attenuates with depth with
characteristic length of the group, which may greatly exceed the individual
wave length. In general the picture of the dynamic pressure beneath intense
wave groups looks complicated. The qualitative difference in the structure of
the induced pressure field for unidirectional and opposite wave trains is
emphasized.
