Sensitivity of seismic refraction inversion to smooth and blocky velocity gradients

We utilize the term critical zone to denote the region near the surface of the earth that extends from the soil to the base of weathered rock. Geophysical studies designed to investigate this region have increased in recent years and one of the commonly used tools is seismic refraction tomography (SRT). While SRT provides valuable information, it is important to recognize the limitations of this approach. We examine the ability of SRT to distinguish between blocky and smooth velocity models in a critical zone like setting. We simulate a seismic refraction experiment over smooth, semi-blocky and blocky velocity models of the critical zone, then pick traveltimes and invert with a commonly used commercial SRT package. The synthetic inverted velocity profiles were found to be virtually indistinguishable, and in the blocky model the depth to consolidated bedrock was overestimated by 25%. Inability to distinguish between blocky and smooth gradients must be considered a significant source of uncertainty when utilizing such data, for example, to inform hydrologic models.