Extraction of P-wave reflections from microseisms

Abstract

The last few years there has been a growing number of body-wave observations in noise records. In 1973 Vinnik conjectured that P-waves would even be the dominant wavemode, at epicentral distances of about 40 degrees and onwards from an oceanic source. At arrays far from offshore storms, surface waves induced by nearby storms would not mask the body-wave signal and hence primarily P-waves would be recorded. We measured at such an array in Egypt and indeed found a large proportion of P-waves. At the same time, a new methodology is under development to characterize the lithosphere below an array of receivers, without active sources or local earthquakes. Instead, transmitted waves are used which are caused by distant sources. These sources may either be transient or more stationary. With this new methodology, called seismic interferometry, reflection responses are extracted from the coda of transmissions. Combining the two developments it is clear that there is a large potential for obtaining reflection responses from low-frequency noise. A potential practical advantage of using noise instead of earthquake responses would be that an array only needs to be deployed for a few days or weeks instead of months, to gather enough illumination. We used a few days of continuous noise, recorded with an array in the Abu Gharadig basin, Egypt. We split up the record in three distinct frequency bands and in many small time windows. Using array techniques and taking advantage of all three-component recordings we could unravel the dominant wavemodes arriving in each time window and in each frequency band. The recorded wavemodes, and hence the noise sources, varied significantly per frequency band, and -to a lesser extent-per time window. Primarily P-waves were detected on the vertical component for two of the three frequency bands. For these frequency bands, we only selected the time windows with a favorable illumination. By subsequently applying seismic interferometry, we retrieved P-wave reflection responses and delineated reflectors in the crust, the Moho and possibly the Lehmann discontinuity.

Retrieval of reflections from ambient noise recorded in the Mizil area, Romania

“…Roux et al (2005) succeed in retrieving diving P-waves in such a way. Crosscorrelation of recorded ambient noise has also resulted in the retrieval of body-wave reflections (Hohl and Mateeva, 2006;Draganov et al, 2007;Draganov et al, 2009;Zhan et al, 2010;Ruigrok et al, 2011;Xu et al, 2012). Note that retrieval of body waves by SI with ambient noise has proved more difficult, as the body-wave noise is normally drowned out by the much stronger surface-wave noise.…”

“…This has been done using teleseismic arrivals (Ravi Kumar and Bostock, 2006;Tonegawa et al, 2009;Ruigrok et al, 2011), but also using ambient noise. Roux et al (2005) succeed in retrieving diving P-waves in such a way.…”

Retrieving virtual reflection responses at drill‐bit positions using seismic interferometry with drill‐bit noise

“…In regional seismology, examples of retrieving body waves are shown by Roux et al (2005) and Gerstoft et al (2006) for diving P-waves, by Zhan et al (2010) for Moho-reflected S-waves, and by Ruigrok, Campman, and Wapenaar (2011) for Mohoreflected P-waves.…”

The potential of imaging subsurface heterogeneities by natural local earthquakes

“…However, acquiring these images using conventional active sources (e.g., explosives, vibroseis, and airgun) is not always easily achievable because of costs, environmental concerns, and other practical issues. Therefore, the use of passive seismic methods, such as microseism reflection imaging (e.g., Toksöz, 1964;Tamakawa et al, 2010;Reshetnikov et al, 2012) and seismic interferometry with ambient noise (e.g., Draganov et al, 2009;Ruigrok et al, 2011Ruigrok et al, , 2012Grechka and Zhao, 2012), has recently become more attractive. In this sense, despite the fact that earthquakes are usually seen as abhorrent phenomena in human communities (due to the risks they pose), it is natural to also view weaker earthquakes as attractive natural resources if one can use their information in appropriate analyses.…”