Cover mapping using ambient noise surface wave tomography

An understanding of the thickness of the cover is important for many aspects of human activities such as seismic hazard characterization, infrastructure projects, extraction of different types of mineral resources or fossil fuel (coal, oil and natural gas), and characterization of groundwater aquifers in bedrock formations. Many mineral deposits are overlain by younger cover (sediments, soil or alluvium) which complicates exploration. Most investigation of cover thickness is done by drilling but the demand for more data is increasing. Fulfilling this need with existing methods is either imprecise or expensive, cumbersome and sometimes risky. Passive seismic imaging is a low-impact, low-cost technique that can be used for exploration and evaluation of cover thickness. Recent development of autonomous seismic recorders (“nodes”) allows for reliable continuous recording of seismic data for weeks or months at a time. In addition to complementing traditional 3D active-source reflection surveys, autonomous nodes greatly increase the flexibility of seismic survey design and, most importantly, permit low-cost collection of dense passive seismic data with minimal impact on the local environment. In this abstract, we present a few applications of our ambient noise surface wave tomography (ANSWT) for cover mapping at different scales. Two examples of mineral exploration are presented as well as an application used for seismic hazard characterization. All three results are groundtruthed using borehole information or other geophysical results. The different applications of ANSWT presented were successful for cover mapping because of the sharp seismic wave velocity contrasts usually encountered at the interface between cover and basement. The ease in the field deployment and the low cost and environmental impact makes this imaging method particularly suitable for large cover-mapping surveys.