Depth sensitivity of seismic coda waves to velocity perturbations in an elastic heterogeneous medium

Abstract

Numerous monitoring applications make use of seismic coda waves to evaluate velocity changes in the Earth. This raises the question of the spatial sensitivity of coda wave-based measurements. Here, we investigate the depth sensitivity of coda waves to local velocity perturbations using 2-D numerical wavefield simulations. We calculate the impulse response at the surface before and after a slight perturbation of the velocity within a thin layer at depth is introduced. We perform a parametric analysis of the observed apparent relative velocity changes, ε obs , versus the depth of the thin perturbed layer. Through the analysis of the decay of ε obs , we can discriminate two different regimes: one for a shallow perturbation and the other for a deep perturbation. We interpret the first regime as the footprint of the 1-D depth sensitivity of the fundamental surface wave mode. To interpret the second regime, we need to model the sensitivity of the multiply scattered body waves in the bulk. We show that the depth sensitivity of coda waves can be modelled as a combination of bulk wave sensitivity and surface wave sensitivity. The transition between these two regimes is governed by mode conversions due to scattering. We indicate the importance of surface waves for the sensitivity of coda waves at shallow depths and at early times in the coda. At later times, bulk waves clearly dominate the depth sensitivity and offer the possibility of monitoring changes at depths below the sensitivity of the surface waves. Based on the transition between the two regimes, we can discriminate a change that occurs at the surface from a change that occurs at depth. This is illustrated for shallow depth perturbations through an example from lunar data.

Response of hydrothermal system to stress transients at Lassen Volcanic Center, California, inferred from seismic interferometry with ambient noise

“…We also expect that this coda window will minimize the contribution of scattered body waves. Obermann et al (2013a) showed with numerical experiments that the body wave contribution will be dominant after six times the normalized transport mean free time t* = l/c where l is the transport mean free path and c is the surface wave velocity. As mentioned in Obermann et al (2013a), the estimation of the transport mean free path is often difficult.…”

Field observations of seismic velocity changes caused by shaking-induced damage and healing due to mesoscopic nonlinearity

“…A precise analysis of the depth where the velocity changes occur would require the correct identification of the wavefield properties. Based on the assumption that the seismic coda is dominated by surface waves (Obermann et al 2013), we can obtain a rough depth estimate of the volume influencing the measurements. As there is no local velocity model available for the surroundings of station PATCX, we use an idealized half-space model with p-wave velocity v p = 2000 ms −1 , s-wave velocity v s = 1000 ms −1 and density ρ = 2.5 g cm −3 .…”

Monitoring of environmental influences on seismic velocity at the geological storage site for CO2 in Ketzin (Germany) with ambient seismic noise

“…In general the sensitivity of coda waves is a combination of surface wave and body wave sensitivities with gradual transition from surface to body waves with increasing lapse time (Obermann et al 2013). This transition is faster for stronger medium heterogeneity favouring the conversion between the wave types.…”

“…In general, amplitudes of the velocity variations systematically decrease for time windows in later parts of the coda. Obermann et al (2013) showed that the depth sensitivity of coda waves can be related to a combination of bulk wave sensitivity and surface wave sensitivity. At early times in the coda, most of the waves propagate as surface waves at shallow depths, and are therefore sensitive to shallow changes.…”

“…Obermann et al (2013) showed that the depth sensitivity in the early time windows of the coda is dominated by surface waves and in later time windows dominated by bulk waves, which leads to the possibility to discriminate between changes close to the surface and changes at depth. Xu et al (2012) cross-correlated noise between two profiles, that were designed for an active survey at the test-site for CO 2 storage in Ketzin, Germany.…”