Andrea Tognarelli

Two-grid stochastic full waveform inversion of 2D marine seismic data

We apply stochastic Full Waveform Inversion (FWI) to 2D marine seismic data to estimate the macro-model velocity field which can be a suitable input for subsequent local (gradient based) FWI. Genetic Algorithms are used as the global optimization method. Our two-grid representation of the subsurface, made of a coarse grid for the inversion and of a fine grid for the modeling, allows us to reduce the number of unknowns to an acceptable number for the given computer resources and to perform a stable and reliable finite difference modeling. Thus, notwithstanding the known high computational costs that characterize global inversion methods, we are able to reconstruct a smooth, low wavenumber, acoustic velocity model of the subsurface. The reliability of the estimated velocity macro-model is checked through the inspection of prestack depth migrated gathers and through the superposition of observed and modeled seismograms. The method we propose is less affected by the risk of being trapped in local minima of the misfit functional than gradient based FWI methods, and can be a viable alternative to estimate proper starting models for gradient based full waveform inversions.

Estimation of a High Resolution P-wave Velocity Model of the Seabed Layers by Means of Global and a Gradient-based FWI

Summary We present a two-step procedure to full-waveform inversion (FWI) that combines a stochastic, genetic algorithm optimization and a subsequent gradient-based inversion with the aim to estimate a high-resolution P-wave velocity (Vp) model of the shallow seabed layers. In particular, we take advantage of the broad band frequency content of the seismic well-site (WSS) data to extend the frequency range up to 70 Hz. The first step is a genetic algorithm optimization aimed at deriving a reliable starting model for the subsequent gradient-based FWI. The lack of low frequencies and the limited maximum offset of the WSS acquisition, make the GA inversion particularly crucial as it provides a Vp field that contains the low-medium wavelengths of the subsurface compressional velocity field. These wavelengths are essential to attenuate the risk for the following gradient-based FWI of being trapped into local minima. The gradient-based FWI is performed in the acoustic approximation thus inverting for the Vp only. This two-step procedure yields a final Vp model characterized by an improved resolution with respect to the outcomes of GA-FWI and many fine details of the layering. The fair match between the reflections kinematics in the actual and predicted data supports the reliability of the final model.

S/N enhancement by means of array simulation for near surface seismic investigations

We present a procedure for enhancing the signal-to-noise ratio (S/N) of shallow seismic reflection data based on two different steps: 1) an acquisition step that requires the recording of closely spaced common source records with standard source and receiver equipment and 2) a processing step where weighted or un-weighted source and receiver arrays are simulated on the basis of required needs for source related noise attenuation and depth penetration. The data acquisition can be carried out employing single source-single geophone recordings, with a standard 24 or 48-channel equipment. Simple energy sources such as weight drop or sledgehammer are considered. The design and application of the spatial filters in the processing phase is very flexible and can be tailored to the specific needs. In fact, the simulated source and/or receiver arrays can be time and/or space variant and can be weighted to provide the desired responses. Optimal weights can be determined by means of Chebyshev polynomials. Real data examples show the increase in the data quality in terms of better coherent noise attenuation and of enhanced depth penetration.

Stochastic FWI on Wide-angle Land Data with Different Order of Approximation of the 2D Acoustic Wave Equation

We experience the application of a genetic algorithm driven full-waveform inversion (GA FWI) on two expanding spread records acquired in a forward-reverse configuration along a 2D seismic land profile. Maximum source to receiver offset reach up to 40 km for the forward record and up to 30 km for the reverse record. The FWI is performed in time domain and with the acoustic 2D approximation. The data area is characterized by rough topography and complicate near surface and the seismograms show a low S/N ratio. We test whether, given the poor data quality and using a simple data misfit computation based on waveform envelopes and L1norm, the order of approximation of the spatial derivatives can be reduced without losing anything significant with respect to higher order approximations. The GA FWI experiments consider only the direct and diving waves of the shot records and constitute part of a wider project in which GA FWI is applied to both the expanding spread and the standard data to estimate a low-resolution velocity model apt to be used as a starting model for gradient based FWI. It turns out that reducing the order of approximation in the spatial derivatives computation from the 4th to the 2nd order does not appreciably change the matching between observed and predicted data as well as the estimated velocity models, while the computing time is drastically reduced. Also, in such data conditions, the adoption of more sophisticated misfit functions does not seem to produce significant improvements in the results.

SH-wave Seismic Reflection at the Patigno Landslide and Integration with P-wave Reflection Data

In this work we describe the acquisition and processing, up to the depth migrated image, of an SH-wave reflection seismic survey carried out on a complex deep seated landslide located in the Northern Apennines in Italy. We also show a comparison with a recently acquired P-wave seismic reflection profile that investigates the same landslide body. The P-wave survey was able to delineate the deep sliding discontinuity, but was unable to give a detailed description of the small reactivation slip surfaces delineating minor landslides at shallow depths, that are responsible of the major observed damages. Our experience shows that the combined use of both P-waves and SH-waves offers the possibility to obtain detailed insights of the whole landslide body from the deepest discontinuity up to the very shallow portion of the landslide, overcoming the limitations due to the low resolution of P-wave method for imaging shallow horizons and the low investigation depth of SH-wave method. The deeper knowledge of the landslide internal setting that can be gained by a joint application of both methodologies is of primary importance to plan adequate and effective defence strategies.

Wavelet analysis of δ18O and δ13C time-series from an Holocene speleothem record from Corchia Cave (central Italy): insights for the recurrence of dry-wet periods in the Central Mediterraneans

In this work, the continuous wavelet transform (CWT) is used to analyse stable isotope (δ18O and δ13C) time-series from a speleothem from Corchia Cave (Apuan Alps, Tuscany, central Italy). The record spans the ca. 13.3- 0.4 ka period. Both proxies are thought to represent changes in the amount of precipitation and in the hydrological recharge over the cave catchment, and to indicate alternating wetter and drier periods. The CWT shows the presence of different dominant frequency components, changing across the Middle-Holocene. This can be interpreted as mostly related to long term variations in summer and winter insolation, resulting from changes in orbital parameters. The lower frequencies (millennial scale) individuated may have different origin, not always obvious, and are rarely reported in others records. Instead, the short period components (multidecadal to centennial scale) could be in the range of frequencies considered as expression of the solar variability. δ18O and δ13C time-series show very coherent response to lower frequencies, but δ18O time-series show the presence of higher frequencies, that are not reported in the δ13C record. This can be related to a different response of the soil-karst system compared to variability in the δ18O of precipitation and their recharge of the karst. A Matlab® code has been implemented to compute the wavelet transform and to generate all the results presented in this work.

A Global-Local Experience of 2D Acoustic FWI on a Real Data Set

In this work, we illustrate an example of acoustic 2D FWI on a real data set extracted from a 3D volume. The FWI is carried out in two successive steps. The first step is based on a global optimization procedure that makes use of genetic algorithms. It is aimed at the estimation of a velocity model close enough to the global minimum such that the second step, based on a local optimization, quickly converges to the optimal solution. The local step also increases the resolution of the estimated velocity field. An L1-norm misfit function, computed on the envelope of the observed and synthetic data to avoid cycle-skipping effects, is used to increase the robustness of the solution. A low-velocity region at approximately 1.2 km depth is observed in the global inversion that is better detailed by the local inversion. The alignment of the CIGs at shallow depth after Kirchhoff pre-stack depth migration supports these results that are obtained with a very limited manpower effort.

A two-step elastic full-waveform inversion applied to reflection seismic data for shallow hazard identification

We apply a two-step elastic full-waveform inversion (FWI) to well-site survey (WSS) marine seismic data to estimate high-resolution P-wave (Vp) and S-wave (Vs) velocity models. Our approach combines a first global, genetic-algorithm optimization and a subsequent gradient-based inversion. The broad-band frequency content of the available seismic data makes it possible to extend the frequency range considered in the inversion up to 70 Hz and thus to derive a high-resolution elastic characterization of the shallowest part of the subsurface. The lack of low frequencies and the limited maximum source-to-receiver offset of the WSS acquisition, make the GA inversion particularly crucial as it provides a starting model for the gradient-based FWI that contains the large-medium wavelengths of the seismic velocity field. The following gradient-based FWI yields Vp and Vs models characterized by an improved resolution with respect to the outcomes of GA-FWI. The match between the observed and the predicted seismic data proves the reliability of our predictions.

The Limits of Narrow and Wide Angle AVA Inversions in Case of High Vp/Vs Ratios - Application to Seabed Characterization

Since its introduction in the oil and gas industry, amplitude versus angle (AVA) inversion has become a standard tool in hydrocarbon exploration. However, with the intensification of off-shore construction activity, applications of this method have also been extended to evaluate the elastic properties of seabed sediments. These sediments are often undercompated and characterized by very low Vs and high Vp/Vs ratios. The importance of Vp/Vs ratios is usually underrated in AVA inversion which is often applied with few misgivings in case of high Vp/Vs ratios. In this work, we derive the elastic properties of the seabed interface by applying AVA inversion to a 2D well-site survey acquisition. The limited water depth, the maximum offset of the seismic acquisition and the high frequency content of the data make it possible to consider two different ranges of incidence angles: 0-30 and 0-60 degrees. The limits of AVA inversion in case of high Vp/Vs ratios are discussed together with the benefits introduced by wide angle reflections in constraining the inversion. In particular, these reflections are essential to decrease both the cross-talk between the inverted parameters and the uncertainties in the Vp and density estimations, but they do not guarantee a reliable Vs estimation.

Source and Receiver Array Simulations for Near Surface Seismic Investigations

We present a procedure for enhancing the data quality of shallow seismic data based on two different steps: 1) an acquisition step that requires the recording of closely spaced common source records with standard source and receiver equipment and 2) a processing step where weighted and/or un-weighted source and/or receiver arrays can be simulated on the basis of required needs for source related noise attenuation and depth penetration. The data acquisition can be carried out employing single source – single geophone recordings, with a standard 24 or 48 channels equipment. Simple energy sources such as weight drops or sledgehammer are considered. The design and application of the spatial filters in the processing phase is very flexible and can be tailored to the specific needs: in fact, the simulated source and/or receiver arrays can be time and/or space variant and, if needed, can be weighted to provide the desired responses. Optimal weights can be determined by means of Chebyshev polynomials. Real data examples show the increase in the data quality in terms of better coherent noise attenuation and of enhanced depth penetration.