Geoscience Reference
In-Depth Information
26
Ground-Penetrating Radar
Mapping of Near-Surface
Preferential Flow
Robert S. Freeland
ContentS
26.1 Introduction ......................................................................................................................... 337
26.1.1 Radargram Characteristics of Saturated Soil ........................................................ 338
26.2 Case History........................................................................................................................ 339
26.2.1 Survey Protocol ..................................................................................................... 339
26.2.2 Data Processing ..................................................................................................... 340
26.3 Results and Discussion........................................................................................................ 343
26.4 Conclusion........................................................................................................................... 343
26.5 Acknowledgments ...............................................................................................................344
References ......................................................................................................................................344
26.1 IntRodUCtIon
A formidable challenge is determining the preferential-flow pathways of near-surface water. A tra-
ditional approach involves installing monitoring-well networks, which may be inadequate due to
the invasive act of coring and subsequent sampling. Physical probing into the subsurface may com-
promise delicate pathway integrity and distort matrix flow. Furthermore, improper or insufficient
well-layout patterns will produce spatial sampling errors, as the true morphological complexity of
the subsurface may have been misperceived or oversimplified.
Spatial sampling error is reduced by increasing spatial sampling frequency. One technology
capable of increasing spatial resolution is generally referred to as ground-penetrating radar (GPR),
which uses ground-injected electromagnetic impulses. This nonintrusive technology can scan the
subsurface in near spatial continuum, thereby yielding significantly higher spatial sampling resolu-
tions than that of point-specific wells.
GPR's basic operating procedure is by towing a ground-coupled antenna across the surface. A
basic GPR-antenna configuration contains both a transmitter and a receiver antenna, with its elec-
tronics continuously firing electromagnetic impulses into the subsurface and receiving the ensuing
echoes off subsurface strata (Figure 26.1). Assigning color values to the digitized samples of the
reflected waveform produces a pixel matrix of one column by
N
-rows of digitized samples (i.e.,
N
=
512, 1024, 2048, etc.). Columns are vertically stacked at approximately 60 Hz to form a scrolling
pseudo-image of the subsurface profile. When time stacked, the series of the returning waveforms
will profile the substrata as an image scrolling across the display as the antenna travels across the
surface (Figure 26.2). Called a radargram, this is a composite display exhibiting line profiles for
337
