A method for forming 3D image data representative of the subsurface of infrastructure located in the vicinity of a moving vehicle, the method including: rotating a directional antenna (14, 15), mounted to the moving vehicle, about an antenna rotation axis ( Ω); performing, using the directional antenna whilst it is rotated about the antenna rotation axis ( Ω), a plurality of collection cycles in which the directional antenna emits RF energy and receives reflected RF energy; collecting, during each of the plurality of collection cycles performed by the directional antenna: (i) radar data representative of reflected RF energy received by the directional antenna during the collection cycle; (ii) angular position data representative of an angular position of the directional antenna about the antenna rotation axis during the collection cycle; and (iii) translational position data regarding a translational position of the directional antenna during the collection cycle; processing the radar data, the angular position data and the translational position data collected during each of the plurality of collection cycles to form 3D image data representative of the subsurface of infrastructure located in the vicinity of the moving vehicle; characterized in that the method includes varying an angle θ between the antenna rotation axis ( Ω) and a direction of movement of the vehicle ( x 2) to a non-zero value such that a primary radiation axis ( d) of the directional antenna (14, 15) has a component of movement that is opposite to the direction of movement of the vehicle ( x 2) whilst the primary radiation axis ( d) of the directional antenna (14, 15) points beneath the moving vehicle. rotating a directional antenna (14, 15), mounted to the moving vehicle, about an antenna rotation axis ( Ω); performing, using the directional antenna whilst it is rotated about the antenna rotation axis ( Ω), a plurality of collection cycles in which the directional antenna emits RF energy and receives reflected RF energy; collecting, during each of the plurality of collection cycles performed by the directional antenna: (i) radar data representative of reflected RF energy received by the directional antenna during the collection cycle; (ii) angular position data representative of an angular position of the directional antenna about the antenna rotation axis during the collection cycle; and (iii) translational position data regarding a translational position of the directional antenna during the collection cycle; (i) radar data representative of reflected RF energy received by the directional antenna during the collection cycle; (ii) angular position data representative of an angular position of the directional antenna about the antenna rotation axis during the collection cycle; and (iii) translational position data regarding a translational position of the directional antenna during the collection cycle; processing the radar data, the angular position data and the translational position data collected during each of the plurality of collection cycles to form 3D image data representative of the subsurface of infrastructure located in the vicinity of the moving vehicle; characterized in that the method includes varying an angle θ between the antenna rotation axis ( Ω) and a direction of movement of the vehicle ( x 2) to a non-zero value such that a primary radiation axis ( d) of the directional antenna (14, 15) has a component of movement that is opposite to the direction of movement of the vehicle ( x 2) whilst the primary radiation axis ( d) of the directional antenna (14, 15) points beneath the moving vehicle.