Q1) A radar record was taken in a tunnel in an attempt to map features such as changes in geology, fractures and voids around the tunnel located in granite. The radar record and the geologic section can be viewed here.
Although the structure is evident in both the geologic section and the radar record, it would be considerably more meaningful if depth scales could be attached to the two sections. To facilitate this a CMP survey was carried out and the reflection arrival times from a water filled fracture (seen as the strong reflector at a time of 300ns) are given in the table below:
| Rx/Tx Separation (m) | Arrival time of reflection (ns) |
0.0 |
300. |
2.5 |
300. |
5.0 |
302 |
7.5 |
305. |
10.0 |
309. |
12.5 |
314. |
15.0 |
320. |
17.5 |
327. |
20.0 |
335 |
22.5 |
344. |
25.0 |
353. |
a) Use the travel time information from the CMP gather (table above) to compute an average velocity for the region above the reflector.
b) What is the dielectric constant of the medium?
c) Print the radar and geological sections. Put a depth scale in meters on the right hand side of the radar section and on the geological section.
d) What is the dip of the dyke in degrees?
Read the paper 'Georadar for hydrogeology' by R.A. van Overmeeren. Use the GPR notes to answer the following questions on the paper:
Q2) Consider the georadar sections taken at different frequencies (fig 1).
a) The vertical axis is labeled in depth and time. What is the velocity used to make this conversion? What is the average value of epsilon for the upper region?
b) The obvious difference between the images is the resolution provided. Sketch an ideal georadar signal that corresponds to 25, 50, 100, and 200 MHz.
c) Although the images are small, attempt to use the reflection from the water table to quantify how the time width of the wavelet changes with frequency. Is this in accordance with your result in part b?
Q3) Consider the georadar data shown in figure 4.
a) What reason is given for the lack of radar signal beneath the water table?
b) Why is a step seen as a flat reflector plus a half of a hyperbola?
c) The size of the step is asserted to be about 2meters. What is the vertical resolution expected from a 25 MHz system? (Use the formula in the handwritten notes ).
Q4) Consider the example shown in figure 5.
a) Why could a 100 MHz radar be used for finding the water table for this example whereas it was not useful for the above example where a 25MHz radar was needed.
b) What is the velocity and associated epsilon used to convert time to depth?
c) How large, in time and depth, is the step in Profile 3 near station 280? What is the vertical resolution for the 100 MHz system? Should we be able to resolve the step with this system?
d) Consider the statement: “To exclude the possibility that the observed offset is caused by a sudden change in wave propagation velocity rather than by a groundwater step, CMP measurements were performed on both sides of the feature.” Suppose there was no physical step in profile 3 but rather that the water table was flat-lying. What is the distribution of the dielectric constant that could give rise to the same step structure?