  We identify all fast-mode forward shocks, whose sheath regions resulted in a
moderate (56 cases) or intense (38 cases) geomagnetic storm during 18.5 years
from January 1997 to June 2015. We study their main properties, interplanetary
causes and geo-effects. We find that half (49/94) such shocks are associated
with interacting coronal mass ejections (CMEs), as they are either shocks
propagating into a preceding CME (35 cases) or a shock propagating into the
sheath region of a preceding shock (14 cases). About half (22/45) of the shocks
driven by isolated transients and which have geo-effective sheaths compress
pre-existing southward Bz. Most of the remaining sheaths appear to have planar
structures with southward magnetic fields, including some with planar
structures consistent with field line draping ahead of the magnetic ejecta. A
typical (median) geo-effective shock-sheath structure drives a geomagnetic
storm with peak Dst of -88 nT, pushes the subsolar magnetopause location to 6.3
Re, i.e. below geosynchronous orbit and is associated with substorms with a
peak AL-index of -1350 nT. There are some important differences between sheaths
associated with CME-CME interaction (stronger storms) and those associated with
isolated CMEs (stronger compression of the magnetosphere). We detail six case
studies of different types of geo-effective shock-sheaths, as well as two
events for which there was no geomagnetic storm but other magnetospheric
effects. Finally, we discuss our results in terms of space weather forecasting,
and potential effects on Earth's radiation belts.
