Kansas Geological Survey, Open File Report 98-48
by Alex Martinez and James J. Butler, Jr.
Kansas Geological Survey
Open-File Report 98-48
Previous drilling and borehole geophysics work has shown that the shallow subsurface at GEMS consists of 11-12 meters of clay and silt underlain by 11-12 meters of sand and gravel. In areas where borehole GPR techniques are applicable, cross-borehole tomography can provide detailed images of the dielectric permittivity field between wells. Differences in dielectric permittivity are related to changes in lithology, sorting, and fluid content, and therefore may be related to changes in hydraulic conductivity. The primary objective of the GPR fieldwork at GEMS was to determine the maximum distances GPR signals could be propagated at the site. Knowledge of maximum propagation distances allows optimized design of cross-borehole tomography surveys.
Survey 1 consisted of three GPR zero-offset profiles and two common-source gathers collected from wells 4-1 and 0-6. Survey 2 consisted of two GPR zero-offset profiles and two common source gathers collected from wells Gems4N and Gems4S.
The GPR data were collected using a Sensors and Software PulseEkko 100 GPR unit with 50, 100, and 200 MHz borehole antennas. The GPR transmitter and receiver transducers were placed on opposite sides of the wells, approximately 1.5 m away from each borehole. For Survey 1, the console and portable laptop computer used to record the data were approximately 5 m south of the line connecting the two wells. For Survey 2, the console and portable laptop computer were approximately 5 m south of Gems4S.
The wells used for Surveys 1 and 2 were 5.39 m (17.68 ft), and 9.66 m (31.70 ft) apart, respectively. The GPR data datum was from the top of the well casing or casing protector of each well. For Survey 1, there was an elevation difference of 0.04 m (0.15 ft), which was deemed negligible. For Survey 2, there was an elevation difference of 0.025 m (0.083 ft), which was also deemed negligible. For Survey 1, the transmitter antenna was in well 4-1, and the receiver antenna was in well 0-6. For Survey 2, the transmitter antenna was in well Gems4S, and the receiver was in well Gems4N. The GPR data acquisition parameters are summarized in Table 1.
The calibration of the starting time on data gather (time-zero) was not adequately achieved during data acquisition. Each gather or profile was internally consistent (i.e. had the same time-zero), but time-zero changed with each survey.
Table 1. Borehole GPR data acquisition parameters
|Gzop-r4||Common-source gather||200||0.25||0.4||Source = 12.0
Receiver = 21.25-12.0
|Gzop-r5||Common-source gather||200||0.25||0.4||Source = 16.0
Receiver = 21.25-11.0
Receiver = 20.5-11.0
Receiver = 20.5-11.0
The GPR data were downloaded from a portable laptop onto a UNIX workstation for data processing and display with the program Seismic UNIX (SU). SU is a shareware program available on the World Wide Web.
The data were converted from DT1 data format to SEGY data format for signal processing and viewing purposes. In SEGY format, the trace header values sampling interval was multiplied by 1x106 for later processing. This was necessary because the SU program was developed for seismic data (which have sampling intervals of microseconds), and thus cannot deal with the picosecond-scale sampling interval of GPR data.
The air wave data recorded at the beginning and end of each profile and gather were used to correct the zero-offset data to a time-zero datum. This is necessary for interpretation of EM velocity from the arrival times. The air wave data were removed from the profiles and gathers, and the header values for offset were set according to the depth or offset of each trace. These steps facilitated later processing and display.
The Survey 1 GPR data were gained using the parameters discussed in Claerbout (1985): multiplying the data by time-squared, taking the square root of the scaled data, and clipping amplitude values of the upper 95% of the amplitude range. This provided trace balancing, and enhanced the first arrivals. The trace balanced zero-offset gathers are shown in Figure 1, and the frequency spectra of these data are shown in Figure 2. Figures 3 and 4 show the trace balanced common-source data and frequency spectra.
For Survey 2, the 50 MHz GPR data were gained using the parameters discussed above. The 200 MHz data were also gained using a gaussian-tapered automatic gain control (AGC) with a window of 50 ns. This was necessary to gain the low-amplitude first-arrival information. The trace balanced zero-offset gathers are shown in Figure 6, and the frequency spectra of these data are shown in Figure 7. Figures 8 and 9 show the trace balanced common-source data and frequency spectra.
In the sand and gravel interval at GEMS (>12 m depth), zero-offset borehole GPR imaged a distance of more than 5 m. In clay-rich, highly conductive regions (<12 m depth), GPR was not able to penetrate the 5 m distance. However, the 50 MHz antenna data showed a weak signal in the clay and silt interval at approximately 8 m below datum. The 100 MHz and 200 MHz antennas did not have a return signal at this depth (Figure 1). The two 200 MHz common-source gathers showed that signal penetration is greater than approximately 8-9 m for the 200 MHz antenna in the sand and gravel interval (Figure 3).
As expected, the GPR data showed differing scales of resolution and signal penetration, depending on antenna frequency. The 200 MHz antenna provided the highest resolution data, but the least signal penetration. Conversely, the 50 MHz antenna had lower resolution, but greater penetration. Subtle changes in arrival times in the lower portions of each profile revealed possible changes in lithology or sorting (Figure 5). The signal was highly attenuated at depths shallower than approximately 12 m. The attenuation is attributed to the transition from the sand and gravel to the overlying clay and silt.
Although the ambiguity of time-zero precluded calculation of permittivity, a possible time-zero location is shown on Figure 1. This time-zero location was calculated using a velocity of 0.06 m/ns and the distance between the two wells (5.39 m). The velocity of 0.06 m/ns is an average velocity for saturated sand (Davis and Annan, 1989).
The 50 and 100 MHz data showed a dominant frequency of 50 and 100 MHz, respectively (Figures 2a and 2b). The frequency spectra of the 200 MHz data (Figures 2c and 4) did not exhibit the expected dominant 200 MHz character of the 200 MHz antenna, the reason for which is not known.
Survey 2 Data Interpretation
The zero-offset profiles from the two wells show a change in dielectric properties with depth (Figure 6). The 50 MHz antenna data had a greater amplitude return across the distance, and required less gaining than the 200 MHz antenna data. The low-amplitude returns of the 200 MHz data resulted in a much lower signal to noise ratio than the 50 MHz data.
Based on the common-source gathers, the 50 and 200 MHz antennas were able to successfully penetrate approximately 13 m in the sand and gravel interval at GEMS (Figure 8). The 50 MHz antenna most likely can penetrate considerably farther (possibly 20 m) in this interval.
Davis, J. L., and A. P. Annan, 1989. Ground-penetrating radar for high-resolution mapping of soil and rock stratigraphy: Geophysical Prospecting 37, p. 531-551.