You’re on the right track; the difference comes down to how it’s used. Grenades and the seismic survey charge are treated as ranged weapons when thrown, so you make a normal ranged attack roll and, on a hit, everyone in the same zone takes the listed “E” damage. Blast Power only applies to placed explosives, which don’t use an attack roll. Since the seismic charge has a range, it’s meant to be thrown by default, but if your group wants to treat it as a placed “set and detonate” charge, that would be a table ruling rather than the standard use.
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Hi everyone (UscssRookie, malignant11, Fenhorn). Fenhorn explained all of this very well. And I love malignant11's contribution; you should answer technical questions about the rules more often. You have a knack for it, and you're very welcome to do so.
Yes, in my book, seismic charges can be treated primarily as "ranged attack" launched with 3E damage (if the attack succeeds) to everyone in the area where the charge explodes... OR... consider seismic charges with a timer as "placed explosives" and use blast power instead of the attack (ranged attack) roll.
Arthur.