the florida state university college of arts and sciences evolution of ...

time elapsed between when this energy is transmitted and received back at the surface is
measured. The delay between the transmitted pulse and the arrival of a reflection is proportional
to the depth of the subsurface feature that generated the reflection (Harari, 1996). A receiving
antenna detects the waves that are reflected back up to the ground surface when the transmitted
pulse encounters a subsurface interface (Harari, 1996). All sedimentary layers and other buried
materials have particular physical and chemical properties that affect the velocity of
electromagnetic energy propagation. Changes in these properties can be a result of changes in
water content, grain size, porosity or mineralogy (Davis and Annan, 1989; Van Dam and
Schlager, 2000; Best et al., 2003). The greater the change in velocity at an interface, the higher
the amplitude of the reflected wave. The antenna receives the reflected waves and stores them in
a digital control unit. The reflected signal is amplified, recorded and processed (Best et al.,
2003). Once thousands of reflections have been measured and recorded, a two-dimensional
picture is created. The depth of penetration is determined by the frequency of the antenna used.
Low frequency antennas (25-200 MHz) obtain subsurface reflections from great depths (30-100
ft) but have low resolution. High frequency antennas (300-1000 MHz) obtain reflections from
shallower depths and provide a higher resolution.
A GPR transect of the island using a MALA CU-II GPR system was conducted. A 100
MHz unshielded antenna with a 1 m separation was used. A repeat transect with a shielded 250
MHz antenna and with a 50 MHz unshielded antenna was later carried out over the same
transect. Figure 3.10 illustrates how the GPR surveys were carried out. All data was analyzed
using MALA Ground Vision Software. The data was filtered for DC removal and time varying
gain (TVG). The time to depth conversion assumed a velocity of 60 meters/microsecond. The
GPR profiles were not corrected for elevation. They were conducted along a road that cut
through, rather than over the beach ridges. The road was essentially horizontal, with a slight
southerly dip. Therefore, the profile is more or less representative of the actual depth of the
beach ridge reflectors. All GPR data was processed and stored at the Florida Geologic Survey.
A summary of the data is presented in chapter 4.
44