Blenheim sits on deep alluvial gravels of the Wairau Plain, a complex fluvial deposit from the Wairau River. Quaternary sediments here can exceed 300 meters in thickness, overlying greywacke basement. Groundwater is shallow, often within 2-5 meters of the surface. Seismic tomography cuts through these heterogeneous layers to map velocity contrasts. The method distinguishes loose gravels from dense, cemented horizons without a single borehole. For infrastructure on the plain, knowing the depth to engineering rock is non-negotiable. We combine P-wave refraction with high-resolution reflection to image the contact between alluvium and bedrock. This approach meets NZGS site investigation standards and provides the continuous profiles that spot drilling alone cannot deliver.
A 2D velocity cross-section reveals paleochannels, cemented layers, and bedrock depth in a single survey line.
