f Seismic impedance mapping

The last decades have shown a growing interest in the characterisation of the shallow subsurface using geophysical techniques. In environmental- and geotechnical engineering, as well as in the exploration for oil and gas, detailed knowledge on near surface structures and soil properties has become of critical importance. In this respect the primary interest relates to the distribution of elastic parameters and to the hydrologic properties of the shallow subsurface. Existing geophysical techniques that map near-surface structures and soil properties are either expensive (e.g., reflection and refraction seismic methods) or do not directly address the physical properties of interest (e.g., E- and EM-methods). A requirement for cheap and fast mapping techniques that directly address the elastic properties of the shallow subsurface is apparent. In this paper a technique is proposed, based on the measurement of the mechanical impedance of the earth. An external force is applied to the earth’s surface, using a small portable vibrator, and the resulting displacement velocity of the vibrator baseplate is determined. The ratio of force and baseplate velocity (the radiation impedance) is considered to be a measure for the mechanical impedance of the subsurface though it is also related to the geometry of the specific seismic source used. Similar to frequency domain EM methods, the proposed technique determines the complex radiation impedance at a suite of discrete frequencies.