CRUSTAL SCALE SEISMIC IMAGING IN DENSELY POPULATED REGIONS: THE ENAM-CSE
The ENAM study area encompasses the rifted margin, from the continental lithosphere onshore to the mature oceanic lithosphere offshore, including numerous features that highlight the post-rift margin modification by slope instability and fluid flow. The experiment consisted of embedded onshore and offshore, active- and passive-source surveys, designed to record simultaneously in order to image the region at different scales with a 3D approach. Here we describe the data acquisition, analysis and results of the land explosion survey.
The initial design of the onshore portion of the ENAM CSE, located along the coastal plain of North Carolina and Virginia, consisted of two margin-normal ~220 km-long profiles, shot using four (two per profile) large (500-900 kg) explosions recorded by 1400 Reftek 125 seismographs with 4.5Hz vertical component geophones (“Texans”) with a consistent receiver spacing of 250 m. Due to the density of population in the region and the ubiquitous presence of plastic clays underneath the plains, a pilot test was conducted to identify ideal shooting strategies to minimize well distortion and to preclude infrastructure damage, while maximizing energy penetration performance. Based on the results of the test, the land survey was redesigned with eleven 182 kg explosive charges, with an average spacing of 40 km. In the dataset so acquired Pg are visible to 170 km offset, and PmP arrivals starting at 80 km can be followed to 150 km offset with distinctive reverberations. Pn are traceable on the shots with longer offsets. Modeling of the data indicates crustal thickness between 36-43 km and the presence of a high velocity (7.0-7.3 km/s) layer, 5-11 km-thick at the base of the crust. This layer is interpreted as the onshore mafic magmatic addition equivalent to the high velocity lower crust layer observed offshore along the rift axis.