f The origin of reflections from above surface objects in ground penetrating radar data

The interpreter of ground penetrating radar (gpr) data should keep in mind that reflections coming from objects from above the surface can be present in the data. Due to the high velocity of these reflected waves the arrival time is usually in the time window of interest. The largest reflections occur when the polarization of the electric field is parallel to the object causing the reflection. Hence, radiation patterns indicate the orientation and location of possible unwanted reflections coming from above the surface. In this way possible unwanted reflectors can be identified and the acquisition parameters can be adapted to prevent these unwanted reflections. To determine the origin of the above surface reflections we investigated the radiation patterns of a horizontal dipole in the presence of a lossy halfspace. Figure 1 shows the directional characteristic and radiation pattern of an electric dipole directed in the x-direction for the xzand the yz-plane in a lossy halfspace, which have a geometrical spreading of 1/r. The arrows and circles indicate the direction of the electric field, where a circle indicates that the electric field is directed perpendicular to the plane of reference. The only component of the electric field in the yz-plane is in the x-direction, while the components in the xz-plane are in the xand z-direction. Note that in both cases the radiation pattern has nulls at the interface. However, we know that the air wave is present propagating close to the interface, which has a geometrical spreading of 1/r2, so it is not expected that a null is present at the interface. Therefore a far field expression for the air wave is derived to investigate its relative amplitude compared with the electric field close to the vertical. The validity of this far field expression for the air wave is investigated by evaluating the exact expression for a horizontal dipole in the presence of a lossy halfspace (σ = 0.01 S/m, εr = 4). In Figure 2 the exact radiation pattern is depicted together with the far field amplitudes of the air wave along the earth surface (*,+,x,o) for different frequencies. For f = 250 and 500 MHz (r > 5λ) the far field amplitudes of the air wave are similar to the exact amplitudes, while for f = 50 and 100 MHz (r < 5λ) the far field amplitudes of the air wave are larger than the exact amplitudes. For r < 5λ (f = 50 and 100 MHz), the amplitude ratio of the air wave and the wave near the vertical is larger than 0.5 for this specific case. Note that the orientation of the electric field at the interface in the xz-plane is directed in the z-direction. These results show that the vertical polarized air wave has the largest amplitude on the interface in the xz-plane and can cause unwanted reflections in gpr data. Numerical modeling is carried out using a 3D modeling package (Remis, 1998). The configuration is depicted in Figure 3a and shows a vertical object, representing a tree, which is present in the upper halfspace at 7 meters distance of the source and receiver. The reflection for the yy-configuration was negligible. For different height of the vertical object the reflection for the xx-configuration is depicted in Figure 3b. These results show that it is recommended to measure using the yy-configuration when for example trees are present along the survey line in the xz-plane.