Seismic interferometry versus spatial auto-correlation method on the regional coda of the NPE

Introduction

In an efort to ban all nuclear tests, a large majority at the United Nations General Assembly in New York passed the comprehensive test ban treaty (CTBT) in September, 1996. Although 180 states signed the treaty, only 145 ratified it, prohibiting the treaty from entering into force (CTBTO, 2008). The US Department of Energy detonated a 1.5 kiloton chemical explosive charge at the Nevada test site on September $22^{\mathrm{nd}}$, 1993. The experiment, named the non-proliferation experiment (NPE), was conducted in anticipation of the CTBT. The explosion was recorded by over 50 broadband seismic stations in the western United States (Tinker and Wallace, 1997). Scientists used these recordings and other measurements to learn to distinguish between nuclear and chemical explosions (Carr, 1994).

A less well known recording was made by the Subsurface Exploration Company of Pasadena, CA, which operated a 610-channel petroleum-exploration seismic array approximately 200 km distant. The array was oriented east-west in Railroad Valley, Nevada, which is located north of the Nevada Test Site, see Figure 1. This sign-bit equipment was activated at midnight and recorded an extraordinary coda over 5 minutes in length. The incoming waves are spatially coherent at early times, when all energy comes in as a single plane wave (see Figure 2a). Very rapidly after the first break, the wavefield becomes more chaotic, and at later arrivals the higher frequencies are lost, as observed in Figures 2b - 2d. At later times, recorded arrivals are incident for a large range of apparent slownesses. This phenomenon was studied by de Ridder (2008), who concluded that all admissible slownesses define a cone in the frequency-wavenumber domain, as seen in Figure 3. The slope of the cone is determined by the event with the slowest possible apparent velocity; a surface-wave traveling purely along the array.

map Figure 1. Map showing the location of the Nevada Test Site (arrow) and Railroad Valley (shaded area). [NR]

In this report, the coda of the NPE is studied using cross-correlation techniques. Two approaches are contrasted: the seismic interferometry (SI) method which potentially retrieves the full impulse response of the earth from the recorded background field (Wapenaar, 2004) and the spatial auto-correlation (SPAC) method, which retrieves dispersion curves of the fundamental mode of the surface-waves in a horizontally layered medium (Aki, 1957).

shot1frames
Figure 2. Four segments of the first NPE recording; a) segment containing the first arrival; b) segment 30 seconds after the first arrival; c) segment 60 seconds after the first arrival; d) segment 90 seconds after the first arrival. [ER]

shot1WKframes
Figure 3. Frequency-wavenumber spectra of the NPE recording, subfigures a, b, c and d display each the frequency-wavenumber spectra for the corresponding subfigures in Figure 2. [ER]

Seismic interferometry versus spatial auto-correlation method on the regional coda of the NPE