Primary Reflections Research Articles

Marchenko multiple elimination of a laboratory example

SUMMARY The Marchenko multiple elimination (MME) scheme is derived from the coupled Marchenko equations. It is proposed for filtering primary reflections with two-way traveltime from the measured acoustic data. The measured acoustic reflection data are used as its own filter and no model information or adaptive subtraction is required to apply the method. The data obtained after MME are better suited for velocity model construction and artefact-free migration than the measured data. We apply the MME scheme to a measured laboratory data set to evaluate the success of the method. The results suggest that the MME scheme can be the appropriate choice when high-quality pre-processing is performed successfully.

Development of 3-axis Precise Positioning Seismic Physical Modeling System for Seismic Imaging and Data Processing

A seismic physical modeling system capable of automatic 3-axis positioning control of seismic sources and receivers was developed to allow more accurate seismic imaging and data processing. To verify the applicability of the system, data were acquired for sources and receivers at various depths. Comparison to numerical data showed that the acquired data showed similar reflected wave trends. A multi-channel seismic survey was performed at a salt dome model to determine the applicability of the seismic physical modeling system, and data processing such as velocity analysis and Kirchhoff migration was carried out. The seismic cross-section image acquired was consistent with that of the geological model, and the primary reflection, multiple reflection, and diffraction data obtained agreed with real wave propagation data. The seismic physical modeling system developed allows the acquisition of modeling data that is similar to field data in an economical and efficient manner. This data can be used for various purposes, such as to simulate seismic data acquisition, design seismic explorations, and investigate data processing algorithms.

A field data example of Marchenko multiple elimination

Internal multiple reflections have been widely considered as coherent noise in measured seismic data, and many approaches have been developed for their attenuation. The Marchenko multiple elimination (MME) scheme eliminates internal multiple reflections without model information or adaptive subtraction. This scheme was originally derived from coupled Marchenko equations, but it was modified to make it model independent. It filters primary reflections with their two-way traveltimes and physical amplitudes from measured seismic data. The MME scheme is applied to a deepwater field data set from the Norwegian North Sea to evaluate its success in removing internal multiple reflections. The result indicates that most internal multiple reflections are successfully removed and primary reflections masked by overlapping internal multiple reflections are recovered.

A fast algorithm for multiple elimination and transmission compensation in primary reflections

SUMMARY The transmission compensated primary reflections can be obtained from the single-sided acoustic reflection response in the two-way traveltime domain. This is achieved by eliminating free-surface and internal multiple reflections and compensating for transmission losses in primary reflections without model information. The substantial computational cost of the proposed scheme can be reduced by an order of magnitude with a fast implementation version. This is achieved by using the previously computed filter functions as initial estimate for every new truncation time value. We evaluate the success of the scheme with simple and complex 2-D numerical examples. We find that the scheme has excellent performance in most cases, except for the case where strong reflectors are present. In such case, the current scheme suffers from lack of convergence.

Improvement of Gulf of Suez subsurface image under the salt layers through re-processing of seismic data- a case study

ABSTRACT Advanced technology in processing the old seismic data in the Gulf of Suez (G.O.S) has been used and applied using new parameters to emphasise the optimisation of geological structures and stratigraphic units as much as possible. The primary reflections in Gulf of Suez have low frequency while multiple reflections have higher frequency where there is a high reflection contrast in the shallow section represented by Zeit Formation due to alternation of shale and anhydrite that generates a lot of multiple reflections. The deeper formations will be affected by low-frequency reflections resulted from the incident energy itself with the common attenuation parameters such as transmission loss, adsorption, and spherical spreading of waves. Otherwise, the data in the Gulf of Suez suffer from a lot of loss mechanism from the salt of South Gharib Formation and anhydrite of Zeit Formation. In this study, new software are adapted to apply new technology in processing and analysis of seismic data such as VISTA 11 software from GEDCO company. After a lot of iterations of all seismic data parameters, seismic flows and parameters have been chosen to retain the primary frequency contents and relative amplitude while suppressing multiples energy. The reprocessing produced seismic sections with the increased temporal and spatial seismic resolution, facilitating more accurate interpretation of the data. Enhanced imaging of both the salt structures and the sub-salt sequence has been achieved, which in turn leads to a more and better understanding of salt deformation and improve the mapping of structural subdivisions within the study area.

The réflection seconde through duplex cognitio. Thomist basis of Gabriel Marcel’s thought

La distinción entre misterio y problema en la filosofía de Gabriel Marcel implica una doble manera de acceder a esas dos regiones del Ser: reflexiones primera y segunda. Esta doble vía de acceso al Ser comparte caracteres esenciales con la teoría de la duplex cognitio de anima de Tomás de Aquino. La intención del artículo es mostrar esas coincidencias a partir de la lectura del artículo de Jaume Bofill Para una metafísica del sentimiento.

Seismic velocity estimation: A deep recurrent neural-network approach

Applying deep learning to 3D velocity model building remains a challenge due to the sheer volume of data required to train large-scale artificial neural networks. Moreover, little is known about what types of network architectures are appropriate for such a complex task. To ease the development of a deep-learning approach for seismic velocity estimation, we have evaluated a simplified surrogate problem — the estimation of the root-mean-square (rms) and interval velocity in time from common-midpoint gathers — for 1D layered velocity models. We have developed a deep neural network, whose design was inspired by the information flow found in semblance analysis. The network replaces semblance estimation by a representation built with a deep convolutional neural network, and then it performs velocity estimation automatically with recurrent neural networks. The network is trained with synthetic data to identify primary reflection events, rms velocity, and interval velocity. For a synthetic test set containing 1D layered models, we find that rms and interval velocity are accurately estimated, with an error of less than [Formula: see text] for the rms velocity. We apply the neural network to a real 2D marine survey and obtain accurate rms velocity predictions leading to a coherent stacked section, in addition to an estimation of the interval velocity that reproduces the main structures in the stacked section. Our results provide strong evidence that neural networks can estimate velocity from seismic data and that good performance can be achieved on real data even if the training is based on synthetics. The findings for the 1D problem suggest that deep convolutional encoders and recurrent neural networks are promising components of more complex networks that can perform 2D and 3D velocity model building.

Tuning the magnetism and tribological behaviour of electrodeposited Ni/Cu bi-layer by selective reinforcement of carbon nanotubes

Nickel–Copper alternate layers have been a topic of considerable interest due to having several applications in data storage devices, giant magnetoresistive (GMR) sensors and magnetic recording heads. But the continuous use of these devices can initiate damage to their structural integrity because of wear and erosion. Therefore, herein, the selective mixing of carbon nanotubes (CNTs) was engineered either with Ni layer or Cu layer or with both Ni and Cu layers in the electrodeposited Ni/Cu system for enhancing the magnetic susceptibility with the secondary aim of improving the wear resistance of the material. The satellite reflections along with the primary reflections in x-ray diffraction measurements exhibited the configuration of super-lattice structure in the coatings, which are the indicative of multilayered films. Addition of CNT within Ni as well as Cu layer (Ni-CNT/Cu-CNT) shows permanent magnetism with higher magnetic coercivity (749.13 Oe), remanence (0.072 emu/g) and squareness (0.30) when compared to the CNT addition either with Ni (Ni-CNT/Cu 275.20 Oe, 0.011 emu/g and 0.02, respectively) or Cu only (Ni/Cu-CNT 589.71 Oe, 0.027 emu/g and 0.09, respectively) as well as bare Ni/Cu coating (126.89 Oe, 0.007 emu/g and 0.008, respectively). A strong permanent magnetism of CNT added alternate layer was due to the spin transfer between the ferromagnetic Ni and the reinforcing particles. Nonetheless, Ni-CNT/Cu-CNT with high coercivity, remanence and squareness have the maximum transfer of spin between the ferromagnetic catalyst and CNTs due to refined grain structure (23 nm). The significant enhancement in the tribological resistance of Ni-CNT/Cu-CNT (wear rate of 3.57 × 10−3 mm3/Nm) than that of Ni-CNT/Cu (4.00 × 10−3 mm3/Nm), Ni/Cu-CNT (3.94 × 10−3 mm3/Nm) as well as Ni/Cu (12.71 × 10−3 mm3/Nm) was also linked through the reduced contact depth (18.0 μm), contact radius (57.23 μm) and shear stress (0.08 GPa), calculated by different models. Thus, Ni-CNT/Cu-CNT alternate layers with permanent magnetism and restricted wear damage can be a potential coating for giant magnetoresistive sensors.

Bragg resonant reflection of surface waves from deep water to shallow water by a finite array of trapezoidal bars

For surface waves propagating over a finite array of trapezoidal artificial bars from deep water to shallow water, an analytical solution in terms of Taylor series to the modified mild-slope equation (MMSE) is constructed. It is shown that the reflection coefficient totally depends upon the bar number, N, and five dimensionless parameters with respect to the incident wavelength L, i.e., the bottom width of the bars, W, the top width of the bars, Wt, the water depth, H0 (or K0=2πH0), the bar submergence, H1 (or K1), and the bar spacing, D. It is revealed at first time that, when the four dimensionless parameters W, Wt, K0 and K1 are fixed, the reflection coefficient is a periodic function of 2D with the period being unit. Based on the present analytical solution, influence of the bar number, bar width and bar height on Bragg resonant reflection, including the peak value, peak phase, and resonance bandwidth, is intensively analyzed. It is found that the peak value of the primary Bragg resonant reflection is not always greater than the peak values of all the subharmonic Bragg resonant reflections. When the bar number is relatively big, the peak phase will remain unchanged even if the bar number increases. In addition, with the increase of the bar width or bar height, the peak phases of the primary and subharmonic Bragg resonances downward shift more to a low frequency.

Mirror reverse time migration using vertical cable seismic data for methane hydrate exploration

Gas hydrates are located in the permafrost and in deepwater shallow sediments, where low temperature and high pressure satisfy the stability conditions of methane clathrates to remain as solid compounds. Hydrates are found in a form of fine-layered or altered-layered structure with hiatuses and necessitate high-resolution surveys, which may not be achieved by conventional marine acquisition using towed streamers. We have developed a recent case study in which the vertical cable seismic (VCS) method has been used for high-resolution subseafloor imaging using a set of buoyed vertical-arrayed receivers that are anchored to the seafloor. The observation close to the target in the deepwater environment provides a higher signal-to-noise ratio and higher resolution. The primary reflections, however, could not achieve reliable depth images in the data processing due to their limited subsurface coverage. We used a reverse time migration (RTM) implementation of mirror imaging to extend the spatial subsurface coverage by using receiver ghost reflections. Because conventional velocity analysis methods are not applicable to the VCS survey due to the asymmetrical reflection path between the source and receiver, we implemented seismic interferometry and generated virtual surface seismic data from VCS data for velocity analysis. To preserve the resolution, amplitudes, and phase characteristics, we applied mirror RTM on the ghost reflections in the original VCS data rather than imaging the virtual data. The introduced case study using a VCS survey for identifying the methane hydrate system of the Umitaka Spur in the Sea of Japan led to high-resolution images, which suggest that a large gas chimney exists beneath a pockmark and is responsible for transferring methane gas from a deep hydrocarbon source to the shallow sediments. A bottom-simulating reflector as the base of the gas hydrate stability zone was also imaged.

AMBIENT SEISMIC NOISE FOOTPRINTS AND SPECTRA IN THE MIDDLE BENUE TROUGH, NIGERIA

Ambient noise was analysed from a two-dimensional (2D) seismic data acquired in the Middle Benue Trough, Nigeria for the purpose of characterizing the ambient seismic noise. Sercel 428XL recording instrument was deployed on 3 traverse lines where dynamite explosive sources and geophone detectors were used. The acquired data was processed using frequency wavenumber (FK) and wild amplitude attenuation (WAA) algorithms. The dominant amplitude of the primary reflection ranges between -20dB and -10dB, while those of the ambient seismic noise varies between -42dB and -3dB. The primary reflections have dominant frequency varying from 6Hz to 75Hz while that of ambient seismic noise varies between 4Hz and 70Hz. Analysis of the noise shows two distinct ground roll modes with velocities between 400 ms-1 and 810 ms-1 both of which are dispersive with wavelength (λ) of 61.5m and peak frequency at 6.5Hz. Analysis of passive noise records acquired showed that ambient seismic (background) noise level excluding source-generated noise average of 91.56% are below 25µV, which is the tolerance noise level limit. The combination of frequency wavenumber FK and WAA filters effectively attenuated the surface waves especially ground rolls and other high amplitude noise making the primary reflection very visible and better enhanced. The filtered amplitude values estimated from signal-to-noise (SNR) analysis using cross correlation (XC) method are much higher than the values of the unfiltered amplitudes indicating that SNR are highest when noises are attenuated from the data than when noise algorithm is not applied to the data. The attributes of these seismic noises will provide further information and solution for their suppression during seismic data acquisition and processing.

The necessity of reflection in the oral history of philosophy

The oral history of philosophy is in the process of establishing. The article argues that this discipline is fundamentally different from the oral history of philosophers because the oral history of philosophy has to produce besides the empirical data (interview texts) but also a theoretical reflection that is inherent in any form of philosophy. The core product of oral history is the living narrative of a witness, which is only a starting point for the oral history of philosophy that has to work for philosophical generalizations. This article describes for the first time four possible types of reflection in the oral history of philosophy at the empirical level: (1) at the stage of developing the plan and purpose of the interview, when the historian of philosophy identifies the type of empirical data needed to achieve the primary purpose of his research; (2) between views of an interviewer and a respondent, when the interview appears as a common reflection between those two not just as an adjustable conversation; (3) at the reflection level of the respondent (if he or she is a philosopher) preceded the interview; (4) in the mind of the reader / viewer of the interview, who is able to actively rethink ideas expressed in the interview. Types 1-3 of reflection are central to the oral history of philosophy as a discipline (type 4 is not scientific and belongs to the field of public opinion). Consequently, the result of the empirical research of the oral history of philosophy is not only the empirical data itself but also the primary reflections that need to be developed at the next theoretical stage of research. At the same time, the oral history of philosophy can be both a source of data (that is a special interviewing methodology relevant to any type of the history of philosophy), and a relatively independent discipline, the history of philosophy study primarily based on interview material.

Field detection and classification of citrus Huanglongbing based on hyperspectral reflectance

Citrus Huanglongbing (HLB), also called citrus greening, is the most destructive disease in the citrus industry. Detecting the disease as early as possible and then eradicating infected roots can effectively control its spread. For the Shatangju mandarin cultivar, a non-destructive citrus HLB field detection method based on hyperspectral reflectance is proposed in this study. A characteristic band extraction method based on entropy distance and sequential backward selection is explored. Several machine learning algorithms (logistic regression, decision tree, support vector machine, k-nearest neighbor, linear discriminant analysis, and ensemble learning) were used to discriminate between disease groups: healthy, symptomatic HLB-infected, and asymptomatic HLB-infected, based on leaf reflectance. The results showed that the use of primary hyperspectral reflectance is very feasible for such classification. The band selection method proposed in this study provides an option for dimensionality reduction while still providing high classification accuracy. In three-group classification, the SVM learner achieved 90.8% accuracy, while in two-group classification (healthy vs symptomatic HLB leaves), the accuracy reached to 96%. The results also show that using only a few bands is insufficient for classification. In this study, 13 characteristic bands extracted by the proposed method provided the best performance.

Methods of the matrix analysis and graphic ranking of the array of the experimental IR spectroscopy data in studying the inter-component processes in the mixture of pheniramine maleate and naproxen

A widespread use of complex pharmacotherapy entails the necessity of studying the processes occurred in pharmaceutical mixtures. IR spectroscopy is one of the methods used to assess the stability of substances. To increase the efficiency of interpretation of the IR spectra of mixtures, a method combining matrix methods and graphic ranking of the array of experimental data has been developed. This approach has been tested for the analysis of model mixtures of pharmaceutical substances naproxen and pheniramine maleate. Absorption spectra were obtained on an Avatar 360 FT-IR ESP Fourier transform spectrometer (Nicolet, USA) using an ATR (primary attenuation total reflection) attachment Smart Perfbmer (optical crystal ZnSe; spectral range 1.35 - 26.6 pm, 7400 - 375 cm4). The obtained IR spectra were processed using Thermo Scientific Specta embedded software. Graphs for graphical ranking were plotted in the coordinates of the mixture heating temperature —relative frequency intensity. The points on the graph represent the relative intensity of the bands for a mixture kept at a certain temperature, whereas the curves with which they are connected reflect the processes occurring in the structure of the substance. Common trends in the relative intensity for different frequencies (at the same temperature) and/or for a number of frequencies of a certain characteristic region, indicates the unidirectionality of the process. Graphic ranking is applied to the characteristic bands of pyridine and aliphatic nitrogen of pheniramine maleate and characteristic frequencies of the intermolecular hydrogen bond and carbonyl group of naproxen. We also used the method of mathematical ranking of the matrices of transmittance values. For the characteristic bands of the studied compounds, a combined use of graphical and mathematical ranking allowed us to infer a change in the structure of naproxen and pheniramine maleate under the impact of elevated temperatures: the salt of pheniramine and maleic acid in aliphatic nitrogen has one structural conformation, whereas the binding force depends on the temperature.

Modelling S‐wave radiation for SP‐waves in isotropic and anisotropic media

ABSTRACTUtilizing shear‐wave (S‐wave) data acquired with compressional waves (P‐waves) is becoming more common as joint imaging and inversion techniques improve. Interest in S‐waves radiated from vertical sources and buried explosives exploits conversion to P‐waves as primary reflections (SP‐waves) for reducing acquisition costs and for application to legacy data. However, recent investigations overstate the extent of SP‐wave illumination and show isotropic processing results with narrow bandwidth frequency and wavenumber data. I demonstrate that illumination with SP‐waves is limited in general to near vertical polar angles up to around 30° or 35° for VP/VS of 2 or 3, respectively. At greater angles, S‐waves are typically in the P‐wave evanescent range and cannot excite SP‐wave reflections. Contrary to recent claims, these sources for P‐wave do not radiate SH‐waves polarized in horizontal planes in all azimuths. I show these properties for isotropic media with radiation expressions for amplitude derived in vector slowness coordinates. Also, I extend these expressions to transversely isotropic media with a vertical symmetry axis to show agreement with synthetic seismic data that only quasi SV‐waves are radiated and become more narrowly focused towards 45°. Furthermore, in orthorhombic media, synthetic data show that fast S1‐ and slow S2‐waves polarized parallel and perpendicular to fractures may appear as SV‐ and SH‐waves. For the partially saturated fracture model studied here, S1‐wave radiation has broader azimuthal illumination than slow S2‐waves, which are more narrowly focused in azimuth. These produce SP‐wave splitting signatures on vertical component reflection data that are nearly identical to PS‐wave signatures on radial horizontal component data. Separating these fast and slow SP‐waves is an additional processing challenge.

THERMOGENIC PETROLEUM POTENTIAL OF THE NANKAI SUBDUCTION ZONE, OFFSHORE SW JAPAN

In the Kumano forearc basin, offshore SW Japan, the potential occurrence of hydrocarbons has been inferred from both geophysical surveys which point to the presence of natural gas hydrates, and geochemical analyses of gas samples recovered from boreholes and submarine mud volcanoes. The forearc basin is located in the inner part of the Nankai Trough where the Philippine Sea plate is subducting northwards beneath SW Japan. The basin succession unconformably overlies a deformed accretionary prism complex. Hydrocarbons with both microbial and thermogenic origins have been recorded in the Kumano Basin, but the thermogenic petroleum potential is poorly constrained because the regional‐scale geological architecture of the Nankai subduction zone is not well understood.In this paper, the regional‐scale petroleum potential of the Nankai Trough is investigated based on the geological interpretation of new seismic reflection images. The images were derived from legacy seismic survey data using advanced processing techniques such as reverse time migration, which make use of both primary and multiple reflections in wide‐angle seismic data collected with ocean‐bottom seismographs. In the study area, a seaward‐dipping reflector was identified which terminates at the plate boundary décollement and which is interpreted as part of the boundary between previously‐identified older (>14 Ma) and younger (<6 Ma) accretionary prisms. A triangle body at the base of the younger accretionary prism is interpreted to consist of underthrusted sediments including thick hemipelagic mudstones with source rock potential which were scraped off the down‐going oceanic plate. These tentatively identified, deeply‐buried mudstones may have the potential to generate thermogenic hydrocarbons which may then migrate upwards along dipping carrier beds or faults and fractures to the unconformity at the base of the Kumano forearc basin succession. The presence of hydrocarbons in the accretionary prism may be inferred from positive amplitude anomalies which have been identified in the reprocessed multi‐channel seismic data.The new reflection interpretations therefore allow the potential for thermogenic petroleum to be proposed in the study area. The hydrocarbon potential at this subduction margin may therefore rely not only on shallow microbial methane, but may also involve deeper, thermogenic petroleum generated within the underlying accretionary prism.

Influence of different BFO filler content on microwave absorption performances in BiFeO3/epoxy resin composites

Multiferroic BiFeO3 (BFO) has garnered interest in recent years due to its magneto-electric coupling between ferroelectric and magnetic ordering. This unique property offers some advantages when applied as electromagnetic (EM) wave absorbers. In work reported here, the microwave absorption properties of multiferroic BFO/epoxy resin composites with different weight percentage (wt%) of BFO fillers of various thicknesses were investigated. The BFO powders were synthesized using high energy ball milling (HEBM) and sintered at 775 °C in a furnace with an ambient air condition. The sintered BFO powders with different weight ratios (50 wt%, 60 wt% and 70 wt%) were mixed with epoxy resin as a matrix to form a composite with thicknesses of 1, 2 and 3 mm. Phase identification, grain size and morphology, magnetic and microwave absorption properties of prepared samples were characterized. The absorption performances of samples were measured in the frequency range of 8–18 GHz. As a result, the compressed BFO powders and BFO/epoxy resin composites with 50–70 wt% BFO filler showed dual-band microwave absorption resonance behavior. The best performances were demonstrated by a 3 mm thick BFO70 composite sample which exhibited a primary reflection loss (RL1) of −26.0 dB at 9.1 GHz and a secondary reflection loss (RL2) of −40.5 dB at 11.3 GHz, along with an associated −10 dB bandwidth of 1.31 GHz. The minimum reflection loss (RL) peaks were shifted to a lower frequency as the thicknesses were increased due to the λ4 condition. Theoretical studies on the absorbing wave mechanism reveal a unique combination of dielectric loss relaxations and antiferromagnetic resonance effects in the BFO absorbers.

Data-driven internal multiple elimination and its consequences for imaging: A comparison of strategies

We have compared three data-driven internal multiple reflection elimination schemes derived from the Marchenko equations and inverse scattering series (ISS). The two schemes derived from Marchenko equations are similar but use different truncation operators. The first scheme creates a new data set without internal multiple reflections. The second scheme does the same and compensates for transmission losses in the primary reflections. The scheme derived from ISS is equal to the result after the first iteration of the first Marchenko-based scheme. It can attenuate internal multiple reflections with residuals. We evaluate the success of these schemes with 2D numerical examples. It is shown that Marchenko-based data-driven schemes are relatively more robust for internal multiple reflection elimination at a higher computational cost.

Extracurricular Activities in Higher Education and the Promotion of Reflective Learning for Sustainability

The objective of higher education institutions is to integrate reflective learning that contributes to the development of a greater awareness among individuals of the importance of facing the 21st century’s sustainability challenges. This paper analyzes the impact of an extracurricular volunteer activity in Tangier, Morocco in the development of student reflection at a Spanish university. To this end, two objectives were proposed: (1) to explore the students’ primary reflections of the experience, and (2) analyze the students’ perceptions of the importance of participating in the experience in order to develop reflective learning. In the study, in-depth interviews were conducted with 23 students who participated in the volunteer activity. Data analysis was carried out using Iramuteq software to conduct a descending hierarchical classification (DHC), and MAXQDA software to conduct a constant comparison analysis. This research highlights the value of voluntary extracurricular activities in the development of reflections that guide change in the beliefs, attitudes, and daily behaviors of students that ultimately result in sustainability. Due to this, not only is it considered essential that students participate in social projects, but also that they undertake these projects with peers and instructors who can create environments of support and trust.

Transmission compensated primary reflection retrieval in the data domain and consequences for imaging

We have developed a scheme that retrieves primary reflections in the two-way traveltime domain by filtering the data. The data have their own filter that removes internal multiple reflections, whereas the amplitudes of the retrieved primary reflections are compensated for two-way transmission losses. Application of the filter does not require any model information. It consists of convolutions and correlations of the data with itself. A truncation in the time domain is applied after each convolution or correlation. The retrieved data set can be used as the input to construct a better velocity model than the one that would be obtained by working directly with the original data and to construct an enhanced subsurface image. Two 2D numerical examples indicate the effectiveness of the method. We have studied bandwidth limitations by analyzing the effects of a thin layer. The presence of refracted and scattered waves is a known limitation of the method, and we studied it as well. Our analysis indicates that a thin layer is treated as a more complicated reflector, and internal multiple reflections related to the thin layer are properly removed. We found that the presence of refracted and scattered waves generates artifacts in the retrieved data.