GSSI SIR 4000, Manual

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Geophysical Survey Systems, Inc. SIR® 4000
Manual
MN72-433 Rev F 139
Appendix C: The How-To’s of Field Survey
As the old saying goes: “Garbage in, Garbage out”. The accuracy of your data collection is the biggest
single factor affecting data quality and your ability to make decisions based on it. This appendix has
instructions and helpful hints to get you into the habit of collecting quality data from the beginning. While
many of these points are only relevant to data collection over the ground, as opposed to concrete
structures, you may find this section helpful regardless of your application.
C.1: Site Selection
Radar is not the proper technique for every situation. If you are unable to inspect the site, have
prospective clients send a photograph of the area. As you gain experience, you will find it easy to judge
an area’s or an application’s suitability. In the meantime, consider the four following issues before
deciding whether you should conduct work at an area:
•
Topography
•
Ground Cover
•
Subsurface Conditions
•
Site Accessibility
Topography
Topography is one of the first things you should consider about a new survey area. In the first place, you
need to be able to physically move the antenna over the ground surface in a fairly smooth fashion. Areas
that are full of trenches or extreme slopes are not ideal. You can still survey an area of broken terrain, but
it may require collecting point data rather than continuous data profiles. Radar energy travels into the
ground perpendicular to the surface. This means that if the antenna is flat on the ground, and level, you
are reading right under the antenna. This forward-scanning of the antenna could lead to serious position
errors.
Ground Cover
If your antenna is floating on top of thick grass or a layer of gravel, you may get errors in your data
because the signal is taking too long to couple (penetrate) with the ground. When this happens, more of
the signal than was intended bounces off of the ground surface instead of going into the ground. Always
try to keep your antenna flat on the ground surface. The systems will have no problem penetrating
carpeting or low grass. A good rule of thumb for the 1500 MHz is no thicker than low carpet, and for the
400 MHz, no more than 1 inch. NEVER survey through standing water, no matter how shallow the
puddle.
Subsurface Conditions
If you are working with concrete, try to make sure that the concrete has had some curing time. Three
months is usually adequate for a standard slab on grade, while a suspended slab may cure faster. The best
solution is to practice on slabs of different ages so that you have a first-hand feel of the way they will
look. Concrete that is not well cured will be difficult to see into.
Try and find out some information about the area’s soil and water content. Generally speaking, clay and
water cause attenuation and impede penetration. Finding out the soil grain size (sand, silt, clay) will help
you to guess the dielectric constant of the material to help you set up survey parameters and make time-