Seismic Refraction Survey of a Volcanic Ash Deposit in Northwestern Ellis County, Kansas

Abstract

A low-energy (tamper) seismic refraction survey of a volcanic ash (Pearlette) deposit reveals that the deposit lies in a shallow, north-south trending, near-linear depression. The ash deposit is lenticular and is 1 to 16 feet thick. Seismic velocities within the ash are constant (?2%) and the ash is seismically distinguishable from an overburden of colluvium and soil and from the underlying Smoky Hill Chalk Member (Upper Cretaceous) of the Niobrara Chalk. Volcanic ash deposits occurring in western Kansas may be lenticular and exhibit various types of bedding, and all are considered fluvially reworked airfall sediments (Carey et al., 1952). The ash deposit is located on the Love Ranch in the SE V4, Section 2, TI IS, R20W (Fig. 1). Allied Incorporated of Hays, Kansas, quarries the ash deposit. The color and petrographic character of the ash is consistent with that of the Pearlette ash (Carey et al., 1952). The purpose of this investigation is to establish the geometry of the buried volcanic ash deposit using a low-energy (20-pound tamper) seismic refraction system. Reverse traverses were conducted to determine the slope of the subsurface discontinuity and to correct for apparent-velocity errors caused by sloping discontinuities (Linehan, 1952). If a discontinuity, such as bedrock, has an irregular surface, the arrival-time points plot along a straight line (Linehan, 1952). A best-fit line is drawn through nonlinear points if this occurs, as is the situation for the Smoky Hill Chalk in this investigation. SEISMIC RESULTS Four reverse-traverse seismic refraction lines were conducted, two trending north-south and two east-west (Fig. 1). The surface of the study area slopes gently, less than 3 degrees to the north, and any calculations of dipping discontinuities from apparent velocities and critical distances are negligible. The east-west traverses have less than 1 degree of slope variation. Because of the limitations of the tamper cable length, 200 feet, three of the traverses, S1, S3, and S4, were segmented. The velocity of the ash in the quarry is 1800 This content downloaded from 157.55.39.112 on Wed, 07 Sep 2016 04:53:00 UTC All use subject to http://about.jstor.org/terms 56 TRANSACTIONS OF THE KANSAS ACADEMY OF SCIENCE