Edgar Manukyan

Formation of aeolian ripples and sand sorting.

We present a continuous model capable of demonstrating some salient features of aeolian sand ripples: the realistic asymmetric ripple shape, coarsening of the ripple field at the nonlinear stage of ripple growth, saturation of ripple growth for homogeneous sand, typical size segregation of sand, and formation of armoring layers of coarse particles on ripple crests and windward slopes if the sand is inhomogeneous.

Appraisal of waveform repeatability for crosshole and hole-to-tunnel seismic monitoring of radioactive waste repositories

Countries worldwide are seeking solutions for the permanent removal of high-level radioactive waste from the environment. Surrounding the waste with multiple engineered barriers and emplacement in deep geological repositories is widely accepted as a safe means of isolating it from the biosphere for the necessary 105 – 106 years . As a precautionary measure, society demands that repositories be monitored for 100–300 years after they are backfilled and sealed. Effective monitoring that does not compromise the engineered and natural barriers is challenging. To address this issue, we investigate the viability of crosshole and hole-to-tunnel seismic methods for remotely monitoring high level radioactive waste repositories. Measurements are made at two underground rock laboratories in Switzerland, one within granitic rock and one within clay-rich sediments. Numerical simulations demonstrate that temporal changes of the monitored features (i.e., bentonite plug, excavation damage zone, sand-filled microtunnel) sh...

Seismic Reflection for Hardrock Mineral Exploration: Lessons from Numerical Modeling

ABSTRACT The seismic reflection technique is an essential tool that is extensively applied in hydrocarbon exploration and development, but in hardrock mining and mineral exploration its use has been restricted. Ore bodies present quite elusive targets, in that they are often of limited lateral extent, have steep boundaries and complex shape, lie within high velocity undifferentiated rock and are characterized by small reflection signatures. Additional complications are that they occur at relatively shallow depth and the weak diffracted signals from the mineralization are frequently masked by many overlapping and interfering arrivals, especially mode conversions and reverberations within the weathered surface layer. Conventional CDP (common depth point) reflection profiling and even pre-stack migration reflection approaches do not favor the imaging of such structures. We have undertaken numerical model studies to determine the response of ore bodies and related geological structures (contacts, intrusions) ...

Non-intrusive monitoring using seismic tomography at the Mont Terri rock laboratory

Summary In the framework of a nuclear waste disposal program, we have conducted seismic experiments at the Mont Terri underground rock laboratory in Switzerland. The objective was to explore the possibilities and limitations of seismic tomography for remotely monitoring the changing properties of material filling a small (1 m diameter) tunnel embedded in a highly anisotropic clay formation. Crosshole traveltime tomography allowed the gross properties of the host rock to be determined, but it failed to identify the microtunnel. By comparison, recordings with verticalcomponent geophones attached to the inside of the microtunnel walls revealed significant traveltime and waveform variations for different fill materials (air and sand) and experimental conditions (dry versus saturated). Data from these geophones were not only diagnostic of the fill, but also of the excavation damage zone (EDZ) surrounding the microtunnel.

Towards Robust Acoustic Full Waveform Inversion

Seismic full waveform inversion (FWI) is a powerful method for obtaining high resolution subsurface images, but the objective function of FWI algorithms typically contains many local minima, which may lead to erroneous solutions. This can be avoided by choosing an initial model that is close to the true model, and by incorporating only low frequencies at an early stage of an inversion run. Here, we demonstrate that the local minima problem can be mitigated by removing, at each iteration, all data that have more than half a cycle mismatch in the traveltimes compared with those predicted for the current model. Using a simple two parameter example, we show that filtering waveform data by this traveltime criterion can successfully circumvent local minimum trapping. Inversions of synthetic 2D acoustic data show that our novel traveltime-directed FWI approach is more robust and far less dependent on having an accurate starting model.

Understanding the Impact of Karst on Seismic Wave Propagation - A Multi-method Geophysical Study

Karstified areas are known to be difficult ground for seismic exploration. We conducted a combined numerical-modeling and field-experiment study with the objectives to study the impact of karst on seismic wave propagation and to advance geophysical characterization of karst with seismic as well as non-seismic methods (electric and electromagnetic techniques). Finite-difference simulations using models with realistic topography illustrate the pronounced impact of topographic variations in high-velocity carbonate-bedrock environments on the scattered surface/guided waves. Wavefield complexities such as strong lateral changes in the strength of surface/guided waves, which were observed in a Vibroseis gather from a karst terrain in the Middle East, were also evident in our data recorded in Switzerland. In the latter case, amplifications of surface/guided waves could be correlated with low-velocity zones, which are probably due to more intensively karstified zones. Our study demonstrates that because of the strong heterogeneity of karst terrains, dense sampling is required to properly comprehend and disentangle the observed wavefield. Furthermore, we observed in our field study that the electrical-resistivity models correlate more closely with the mapped lithology, whereas karstification seems to more strongly affect the P-wave velocity models.

Systematic errors in waveform inversions caused by variable receiver coupling

The quality of receiver-to-ground coupling can be highly variable in surface, surface-to-borehole, and crosshole seismic experiments. Poor coupling can affect the recorded seismic traces in a major way. Ignoring variable coupling conditions can lead to severe problems when waveform inversions are attempted. To address such problems, we have developed a novel scheme that estimates medium properties and frequency-dependent source functions and frequency-dependent receiver-coupling factors. We demonstrate the efficacy of the new scheme via a synthetic crosshole experiment in which realistic receiver-coupling factors are simulated.

Seismic Full Waveform Inversion for Characterizing Near-surface Structures - Potential Problems and Solutions

Full waveform inversion (FWI) of seismic data has great potential to image the shallow subsurface, but the specific nature of the associated data sets requires that several problems be first addressed. One problem is the predominance of high-amplitude surface waves, which enforces changes primarily in the shallowest part of the subsurface model when matching observed and predicted data. A more uniform model update over the full depth range of interest can be obtained using Jacobian matrix scaling techniques. A further issue with FWI concerns the occurrence of local minima in the model space. We demonstrate the use of joint inversion of travel times and waveforms to alleviate this problem. Finally, we address the problem of variable source and receiver coupling and propose an estimation procedure that allows such variations to be accommodated and subsequently determined during the inversion.

Combined Inversion of Seismic Waveforms and Traveltimes for Acoustic Waves

Seismic full waveform inversion (FWI) is a powerful method for obtaining high resolution subsurface images, but the objective function of waveform data typically contains many local minima that may lead to inappropriate solutions. This can be avoided by choosing an initial model that is close to the true model, and by incorporating only low frequencies at an early stage of an inversion run. Here, we demonstrate that the local minima problem can also be mitigated by simultaneously inverting waveforms and first arrival travel times. Using a simple two parameter example we show, how the addition of travel time data can remove a local minimum present in the waveform misfit function. Inversions of synthetic 2D acoustic data show that our novel combined travel time and waveform inversion approach leads to superior results compared with a pure FWI inversion.