TY  - JOUR
A1  - Zhang, Tuo
A1  - Sens‐Schönfelder, Christoph
A1  - Epple, Niklas
A1  - Niederleithinger, Ernst
T1  - Imaging of Small‐Scale Heterogeneity and Absorption Using Adjoint Envelope Tomography: Results From Laboratory Experiments
Y1  - 2022-11-11
VL  - 127
IS  - 11
JF  - Journal of Geophysical Research: Solid Earth
DO  - 10.1029/2022JB024972
PB  - 
N2  - To complement the information provided by deterministic seismic imaging at length scales above a certain resolution limit we present the first application of adjoint envelope tomography (AET) to experimental data. AET uses the full envelopes of seismic records including scattered coda waves to obtain information about the distribution of absorption and small‐scale heterogeneity which provide complementary information about the investigated medium. Being below the resolution limit this small‐scale structure cannot be resolved by conventional tomography but still affects wave propagation by attenuating ballistic waves and generating scattered waves. Using ultrasound data from embedded sensors in a meter‐sized concrete specimen we image the distribution of absorption and heterogeneity expressed by the intrinsic quality factor Q<sup>−1</sup> and the fluctuation strength ɛ that characterizes the strength of the heterogeneity. The forward problem is solved by modeling the 2‐D multiple nonisotropic scattering in an acoustic medium with spatially variable heterogeneity and attenuation using the Monte‐Carlo method. Gradients for the model updates are obtained by convolution with the back‐propagated envelope misfit using the adjoint formalism in analogy to full waveform inversion. We use a late coda time window to invert for absorption and an earlier time window to infer the distribution of heterogeneity. The results successfully locate an area of salt concrete with increased scattering and concentric anomalies of intrinsic attenuation. The resolution test shows that the recovered anomalies constitute reasonable representations of internal structure of the specimen.
N2  - Plain Language Summary; No matter how small the structures are that a seismic imaging method is able to resolve, there is structure with smaller length scale. On the one hand this small‐scale structure causes unwanted signals for conventional imaging approaches. But on the other hand it provides complementary information about the investigated medium. To turn this to our advantage we, for the first time, apply a new imaging method that uses the waves which are caused by the small‐scale structure. Using data of an experiment in a concrete block we demonstrate that we can identify areas of anomalous small‐scale structure. The results may help in the future to locate minute perturbations in the medium as they occur in the advent of volcanic eruptions or after earthquakes and to obtain new information about the geologic history of subsurface materials. The approach can be transferred to investigate man‐made materials and structures, such as deteriorating concrete constructions.
N2  - Key Points: Adjoint envelope tomography is for the first time applied to image the scattering and absorption structure with the real data. The iterative inversion uses envelopes of ultrasound signals from embedded transducers in a meter‐sized reinforced concrete specimen. We successfully image the distribution of statistical parameters characterizing the small‐scale heterogeneity and attenuation.
UR  - http://resolver.sub.uni-goettingen.de/purl?gldocs-11858/11668
ER  -