Probing the Reno-Area Basin to 2 km Depth with Ultra-Deep Refraction Microtremor Surveys: A New Development Tool for Basin Plays of the Intermountain West.

Abstract

Prior application of the refraction microtremor (ReMi) shear-wave sounding technique to determine the thickness of the clastic sediments in the Reno-area basin of western Nevada has produced results with less ambiguity than is associated with other geophysical methods. Shear-wave velocity contrasts are distinct, being derived from the analysis of fundamental-mode dispersion of Rayleigh waves. Everyday activities in the city are the passive sources of the Rayleigh waves. A class at the University of Nevada, Reno, with the support of the NSF-funded PASSCAL Instrument Center at New Mexico Tech, conducted an ‘ultra-deep’ ReMi survey across the Reno-area basin in March 2016. The objective of the survey is to locate the floor of the Miocene volcanic basin that underlies the more recent sedimentary basin. The two ReMi arrays were longer than any attempted previously. The north-south array was 21 km long; and the west-east array was 17 km in length; each spanning the entire extent of the Plio-Pleistocene sedimentary basin. The two arrays consisted of ninety 4.5-Hz vertical geophones coupled to Reftek RT-125A ‘Texan’ portable single-channel recorders. Given the unusual length of the arrays, they recorded continuously for 30 minutes (minus 5 seconds), over a period of more than 200 minutes. The length of each transect across the basin determined its geophone spacing, approximately 235 m for the north-south array and 190 m for the west-east array. SeisOpt® ReMi™ software produced wavefield transformations of the array records that allow picking of the fundamental-mode Rayleigh dispersion curves. The picked dispersion curves are consistent with a sharp contrast of shear-wave velocity, from about 2.3 km/s to 3.2 km/s at 1.0 to 2.0 km depth. The long arrays allow us to perform a two-dimensional (2D) ReMi analysis and build a cross-sectional view of this deep interface. To date, the base of the hidden Miocene volcanics has not been recognized by a multitude of previous geophysical studies. The Reno-area basin has previously been defined by the thickness of Plio-Pleistocene sedimentary fill, to the top of the Miocene volcanics. The underlying Miocene volcanic basin can be defined as the depth of the fill (sedimentary and volcanic) to Mesozoic basement rocks. These volcanic deposits may have a significant seismic response compared to the Cretaceous bedrock beneath. This is a significant milestone because 2D ReMi analysis has never been extended to this depth. Correlating these ultra-deep ReMi models with results from prior shallow and deep ReMi surveys, active-source seismic reflection studies, gravity, H/V, and well-log data place further constraints on the depth to the deep basin interface. Development of this new, deeper capability for ReMi surveys has potential for improving seismic hazard models and offers petroleum and geothermal developers a more efficient and economic exploration tool for mapping deep basin topography.

AAPG Datapages/Search and Discovery Article #90266 © 2016 AAPG Pacific Section and Rocky Mountain Section Joint Meeting, Las Vegas, Nevada, October 2-5, 2016