GSSI SIR 30, Manual

Page: 88 / 150

Geophysical Survey Systems, Inc. SIR® 30
Manual
MN 93-101 Rev E 82
the media under study. The antenna separation interval, or offset step spacing, should be at least one
quarter the wavelength of the antenna center frequency in the media.
The minimum offset distance is dictated by the physical characteristics of the antenna housing. Because of
the shielded enclosures, an initial offset position of ‘zero’ is not physically possible and some other, larger
initial offset distance is required.
The maximum antenna offset should be from one (1) to two (2) times the estimated depth to the reflector
of interest. When setting up the CMP or WARR survey the User must initially move the transmitter and
receiver antennas to close to the required maximum offset in order to assess the effects of signal
attenuation in the media. The signal gains must be adjusted so that the airwave, ground wave and reflected
wave fronts are detectable at the farthest required offset and that there is adequate time range to collect
data from the reflectors at depth.
Figure 123: Bi-Static fixed offset – Offset 30 cm. Gain Adjusted.