plot of synthetic with dip of 8.333ms per trace and 
sioseis syn params:  
secs 3 x 21 xinc 12.5 ntrcs 48 
ttva 2 .0 1500 1 2 1.0 -1500 1 2 1.5 2500 1 2 2.5 -2500 1 end

To cut the steep dip (8.3ms/trace) and retain shallow dip:
plot with dipcut -6 6 dippas -5.0 5.0 

To cut the shallow dip and retain the deep dip:
plot with dipcut -6 6 dippas -7 7 end

To cut all negative dips and retain positive dips:
plot with dipcut -1 dippas -.01 end