Imaging the Shallow Crust Using Teleseismic Tomography

Teleseismic tomography is typically used to image the lithosphere or upper mantle with station spacing on the order of tens to hundreds of kilometers. It is generally presumed that, due to their low-frequency energy content (0.3-2 Hz), body waves generated by teleseismic earthquakes are unable to resolve small-scale (1-10 km) heterogeneity. Here, we take advantage of the 250-m station spacing of the LaBarge Passive Seismic Experiment to address two questions related to teleseismic tomography. First, we aim to determine whether relative delays in arrival times are present when the station spacing approaches hundreds of meters. Second, we aim to determine whether these measurements can be related to structure in the shallow crust, thus providing information about the subsurface at hydrocarbon exploration scale. Our results indicate that, even at this small station spacing, variability in relative delay-time measurements shows a clear relation to the shallow structure below the array (top 5 km). Inversions for subsurface velocity show that, while the lateral resolution of the inversion is on the order of the station spacing, the steep incidence of the incoming waves yields poor vertical resolution. We anticipate that additional measurements from local and regional earthquakes may be used to enhance vertical resolution.