FLORIDA GEOLOGICAL SURVEY


 appropriate based on local stratigraphy, hydrogeology, terrain, and cultural
 features. The spatial coverage of surface geophysical data should be
 sufficient to extend beyond the boundary of the possible sinkhole affected
 area. The data density should be close enough to define small localized
 sinkhole conditions. As one example, see ASTM Standard Guide for
 Selecting Surface Geophysical Methods D 6429-99.

 ix. There are other geophysical methods and technologies that are not
 typically used for shallow subsurface investigations; however they could
 have application in certain cases. Some of them are: Time-Domain
 Electromagnetic (TDEM) Surveys, Transient Electromagnetic (TEM)
 Soundings, Induced Polarization (IP), Seismic Refraction, Seismic
 Reflection, several cross-hole geophysical methods, and numerous remote
 sensing techniques. There may be other appropriate methods and new
 technologies are being developed by researchers continuously. If new
 technologies are utilized, they should be thoroughly documented in order
 to establish their acceptance in the geological / geophysical community,
 validity, and reproducibility of the method.

e. GPR is a commonly used method for Florida sinkhole investigations owing to its
 ability to resolve details of shallow soil and rock conditions. The main limitation
 of GPR is its site specific performance and the depth of penetration is limited by
 shallow clays, hardpan soils, or high conductivity pore fluids. Other methods can
 be used when they are appropriate to the problem and local subsurface geology.
 Note that electrical resistivity methods are less impacted by subsurface clays or
 groundwater quality; however, they are prone to unique interpretation problems
 when utilized in urban environments where conductive and/or resistive materials
 near buildings and other structures are present. Direct Current ER, however, has
 been shown to have depth capabilities much greater than GPR approaches. Also
 note CCR may have limited applications in Florida due to shallow water tables
 which yield higher conductivity reducing signal strength.

f. Ground penetrating radar (GPR) has best application in dry sandy soil
 conditions (depth of penetration impacted by clay layers, hardpan soils, and
 groundwater quality).

 i. A grid sufficient to ascertain near subsurface conditions should be
 designed by an experienced professional. Typically a maximum of an
 approximate 10-foot grid within the affected property is considered
 adequate.

 ii. Include the interior of the structure, where appropriate.

 iii. Identify affected areas on a site map for placement of subsurface tests (soil
 borings or soundings). The choice of boring location within an anomaly
 should consider proximity to the damaged structure and any significant
 surface or subsurface features located within an anomaly.