In Kajaani castle area were made GPR surveys to determine structure of ground and bedrock topography on island and to find possible buried objects in the ground.
2. GPR method
Ground penetrating radar is electromagnetic method that uses high frequencies from 10 MHz to 1 GHz. Transmitter antenna sends a short electromagnetic wave pulse to the ground, and reflected wave is received by receiver antenna, which measures time elapsed and the amplitude of wave. When pulse is repeated along the measured line, a continuous profile is formed. The behaviour of the electromagnetic wave in these frequencies is mostly dependent on dielectric properties of material and electric conductivity. High electric conductivity attenuates waves quickly, so GPR is not suitable for high conductivity areas. But if only a part of the area has a high electric conductivity (for example clay) it can be seen on the profile as a place with no information, no reflections. Dielectric properties of soil change the velocity of electromagnetic waves, and when dielectric properties change in ground quickly, it creates a reflection. The bigger the change in dielectric properties, the stronger the reflection. Most important factor in soil and bedrock dielectric properties is the amount of water. When dielectric constant (r) for air is 1, it is for water 80-81. The porosity of soil material and its’ filling are important. Single small objects (for example boulder in sand) in the ground can be found, if their electrical properties differ from surroundings (as they usually do). Objects create a hyperbola on the profile, and with some measurements from hyperbola, some estimates of the depth and size of the objects can be done. Metal objects create ‘ringing’ which is caused by resonance with antenna, and this can be seen on profile as non-attenuating repeated reflection. Usually under metallic object no information can be gathered. Basic principle of the antennas used is that the higher the frequency of the antenna is, the higher is resolution and lower the depth penetration. For example 50 MHz antenna is used on geological mapping (with depth penetration approx. 20 - 40 m) and 800-1000Mhz antennas are used on road surface mapping (with depth penetration approx. 0,3 -0,7 m).
3. Field arrangements
In this survey Malå Geosciences Ramac GPR was used with 100 MHz shielded antenna. Distance was measured with hipchain system. Different time windows and measuring intervals were used (see app. 1). Measured lines are drawn in appendix 1.
The interpreted profiles are attached as pictures at the end of report. Above in the picture is the radar image, processed by different methods (background removal, DC-correction, gain control, band-pass filtering etc.). On y-axis is two-way travel time and x-axis is the place in profile. Below is the interpretation of the reflection with corrected depth scale.
Interpreted bedrock topography and depth is presented in profiles. Bedrock depth is bigger in the eastern and middle part of the island, approx. 8 meters, and it is raising to about 5 meters in western part of island. There is of course uncertainty in depth estimation due to lack of reference that could provide knowledge of electrical properties of the soil.
The soil material itself is hard to define. Probably it is till or silty till.
Most noticeable single feature was found on line Ca4, at point 20.5-22 m. It is a metal object causing strong ‘ringing’, but it is impossible to say what metal it is. Depth is about 3.5-4 m, and it starts couple of meters above interpreted bedrock. Size of the object is 1.5-2 m among the line. Probably same object is seen on line Capoi2, and in this direction object seems smaller, about 0.5 m among the line. Lines Ca4 is presented in couple of different colourcharts, with black and white picture exactly the same as other lines. Depth dimension of object cannot be solved with this method, neither the details . Little noise is seen also on lines Ca5 and Ca6 on the same spot.
In some places there are also traces of some smaller disturbances in profiles, but it is impossible to say for what they are. Those locations are marked in interpreted profiles. There were also big disturbances in beginning of all Ca-lines caused by big steel-made stairway, or its’ basement.
GPR surveys at Kajaani castle
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