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A New Approach for Shallow Subsurface Imaging and its Application to the Dead Sea Sinkhole Problem

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73rd EAGE Conference & Exhibition incorporating SPE EUROPEC 2011
Vienna, Austria, 23-26 May 2011
A new approach for shallow subsurface imaging and its
application to the Dead Sea sinkhole problem
S. Keydar (1), L. Bodet (2), C. Camerlynck (2), A. Dhemaied (2), P-Y. Galibert (2), M.G. Ezersky
(1), E. Oz Dror Akkawi (1) and A. Al-Zoubi (3)
(1) Geophysical Institute of Israel, 6 Baal Shem-Tov Street, Lod 71100, Israel
(2) UMR CNRS 7619 Sisyphe, Université Pierre et Marie Curie-Paris 6, France
(3) Al-Balqa’ Applied University, Salt, Jordan
January 10, 2011
Summary
Zero-offset CSP stacking and diffraction imaging constitute complementary methods to reveal useful
information about the subsurface. As an illustration, this approach has been applied on the Ghor Al-
Haditha sinkholes site (Jordan), on which the seismic surface-wave profiling technique has been
previously performed. Surface-wave dispersion inversion results presented strong lateral variations,
characterized by low shear-wave velocity anomalies, correlated with the proximity of existing
sinkholes and the currently most active collapsing zone. Such anomalies have been explained by a
systematic decompaction of shallow sediment layers during the sinkhole development process.
Diffraction imaging sections computed from these surface-wave profiling data appear to correlate
with dispersion inversion results in terms of lateral variations. CSP stacked sections make it possible
to enhance deeper structures, possibly related to compacted sediments.
73rd EAGE Conference & Exhibition incorporating SPE EUROPEC 2011
Vienna, Austria, 23-26 May 2011
A new approach for shallow subsurface imaging and its
application to the Dead Sea sinkhole problem
S. Keydar (1), L. Bodet (2), C. Camerlynck (2), A. Dhemaied (2), P-Y. Galibert (2), M.G. Ezersky
(1), E. Oz Dror Akkawi (1) and A. Al-Zoubi (3)
(1) Geophysical Institute of Israel, 6 Baal Shem-Tov Street, Lod 71100, Israel
(2) UMR CNRS 7619 Sisyphe, Université Pierre et Marie Curie-Paris 6, France
(3) Al-Balqa’ Applied University, Salt, Jordan
January 10, 2011
Summary
Zero-offset CSP stacking and diffraction imaging constitute complementary methods to reveal useful
information about the subsurface. As an illustration, this approach has been applied on the Ghor Al-
Haditha sinkholes site (Jordan), on which the seismic surface-wave profiling technique has been
previously performed. Surface-wave dispersion inversion results presented strong lateral variations,
characterized by low shear-wave velocity anomalies, correlated with the proximity of existing
sinkholes and the currently most active collapsing zone. Such anomalies have been explained by a
systematic decompaction of shallow sediment layers during the sinkhole development process.
Diffraction imaging sections computed from these surface-wave profiling data appear to correlate
with dispersion inversion results in terms of lateral variations. CSP stacked sections make it possible
to enhance deeper structures, possibly related to compacted sediments.
Introduction
In the framework of the MERC project a new approach for imaging shallow subsurface has been
developed. This approach involves zero-offset Common Shot Point (CSP) stacking (Keydar et al.
1996) and diffraction imaging method (Landa and Keydar, 1998; Keydar et al. 2010). On the basis of
those methods a package of programs were written using a new weighted multipath summation
technique (Keydar, 2004; Shtivelman et al, 2009). The CSP stack and diffraction method are
complementary to each other and reveal useful information about the subsurface. The diffraction
method serves as a tool for detection of faults and voids, while the CSP stacking contains information
about the structure of the subsurface. These methods approach can be for instance applied in order to
better understand the phenomenon of sinkholes occurrences along the Dead Sea's shorelines. As an
example, the results provided using this approach are illustrated here by a real data-set from the Ghor
Al-Haditha sinkholes site (Jordan), on which surface-wave dispersion inversion technique has been
recently applied (Bodet et al., 2010).
The Common Shot Point stacking Method
The Common Shot Point (CSP) stacking is a special case of a Homeomorphic Imaging approach
(Gelchinsky, 1989). This approach is based on the fundamental principle of topological equivalence
of a reflector and its image as constructed from the stack parameters. The basis of the zero offset
Common Shot point stacking method is a new normal moveout (NMO) time correction formula. This
formula is a function of local parameters such as the radius of curvature and the angle of entry of the
arriving front at some fixed central point. The method has three main properties:
1. It enhances useful waves;
2. No knowledge of the overburden is required for stack procedure;
3. The resolution and the information concerning target objects are not diminished by a stack of
recorded data;
4. The time correction formula is stretch free.
73rd EAGE Conference & Exhibition incorporating SPE EUROPEC 2011
Vienna, Austria, 23-26 May 2011
The stacking can be implemented using either of two approaches. In the first, conventional, approach,
the target waves are stacked along the CSP time curve defined by some ‘optimal’ parameters values
(radius of curvature and angle of entry). These values are usually estimated by a time-consuming
procedure of correlation analysis involving visual examination and interpretation of intermediate
seismic images. In this study we have implemented an alternative, a more formal, multipath
summation approach (Keydar, 2004). The multipath summation is performed by stacking the target
waves along all possible time curves at the given central point and for a given sample. This approach
does not require any explicit information on the parameters (radius of curvature and angle of entry of
the wave), since the involved multipath summation is performed for all possible parameters values
within a wide specified range.
Diffraction imaging technique
The imaging algorithm for the detection of underground anomalies should emphasize scattering
objects and not be too sensitive to the presence of continuous and smooth interfaces, which generate
reflected/or refracted waves. For example, it is clear that so-called stacked section, conventionally
used in seismic reflection imaging, cannot be employed for faults and facies changes detection, since
the stacking procedure emphasizes reflected waves and tends to eliminate diffraction/scattering events
generated by faults and facies changes. For the detection of scattering objects, we propose to use so-
called diffraction imaging algorithm (Landa and Keydar, 1998), which is based on the phase
correlation of the diffraction signals on the observed records. The data are analyzed along different
diffraction curves to find the curve closest to the travel time curve of the signal. This technique has
been successfully used for the detection of and locating karst cavities and man-made tunnels (Keydar
et al., 2009). In this study the diffraction imaging technique has been improved using weighted
multipath summation (Shtivelman and Keydar, 2008).
Real example from the Ghor Al-Haditha sinkhole site seismic survey
The Ghor Al-Haditha site is located in Jordan, along the eastern shore of the Dead Sea, near its
southern tip. This site has been recently hit by sinkholes, causing a progressive loss of productive
arable land and farming surrender. To better understand this phenomenon (e.g. recently described by
Legchenko et al., 2008), an integrated geophysical survey has been conducted on this site in May
2007, in the framework of the NATO Science for Peace Programme (project SfP 981128). Two
seismic surface-wave profiling lines (Line 3 and Line 4) have been deployed nearly perpendicular to
the sinkholes well-defined N24° trend (Fig. 1a). Line 4 crosses this direction to the south of the main
subsidence area (only a few old sinkholes are mapped along this line), whereas Line 3 intersects the
currently most active area. During the survey (May 2007), a new sinkhole opened suddenly along
Line 3. From that location, an accelerated development of sinkholes occurred toward north in 2008,
where farms and one industrial estate were abandoned while in the same time, new sinkholes also
opened southerly.
Surface-wave dispersion inversion results
Active-source surface-wave dispersion measurements can be achieved using typical seismic shot
gathers to retrieve Shear-wave velocities (VS) of the subsurface. The recorded wavefield is basically
transformed to the frequency-wavenumber (or frequency-slowness) domain, in which surface-wave
propagation modes can be picked as dispersion curves. Dispersion curves are then inverted for a 1D
(VS) profile with depth. When the method is implemented along linear sections and when the seismic
set-up provides redundant data, specific processing techniques (offset moving windows and
dispersion stacking) can be successfully used so as to obtain a pseudo-2D VS section (Boiero and
Socco, 2008). Dense multifold acquisition geometries were then deployed in Ghor Al-Haditha to
enable surface-wave dispersion stacking and to estimate the ability of this technique in the imaging of
subsurface lateral heterogeneities related to sinkholes.
73rd EAGE Conference & Exhibition incorporating SPE EUROPEC 2011
Vienna, Austria, 23-26 May 2011
Figure 1 (a) Seismic surface-wave profiling lines and main sinkholes location in Ghor Al-Haditha
area up to May 2007 (the eastern end of each line, marked by a ‘o’, corresponds to position 0 m). (b)
2D representation of inverted VS profiles (down to a maximum Half-Space Depth (HSD) of 60 m)
along Line 3 and Line 4. These results show low velocity anomalies in the vicinity of exiting sinkholes
and as soon as both lines enter the currently active area (which was intact during the survey).
A 72 channels Geometrics seismic recorder has been used with a 24 geophones spread (4.5 Hz,
vertical component). Receiver spacing was 5 m, shot location was 5 m away from nearest trace (off-
end shooting East side), and move up between shots was one receiver interval. All seismograms
displayed coherent wavefields, sometimes strong surface scattering and lateral obvious velocity
variations (more details about used processing and inversion techniques can be found in Bodet et al.
2010). Dispersion data were extracted and inverted at each position for a 1D VS profile with depth,
using the neighbourhood algorithm as implemented by Wathelet et al. (2004). The parameterization of
the model was achieved here using a stack of 3 layers overlaying the half-space. We imposed a
maximum half-space depth of 60 m (based on the maximum wavelength (λmax) observed in the
dispersion data). No lateral constraints were applied during the inversions since we assumed surface-
wave dispersion stacking to ‘naturally’ smooth the data. Final sampled VS profiles with lowest misfits
were eventually extracted and re-sampled in depth, every 5 m down to the half-space, in order to
estimate an average velocity structure. Each 1D profile was then represented at its corresponding
position so as to obtain the pseudo-2D VS sections presented on Fig. 1b. The global results clearly
showed strong lateral variations, characterised by low VS anomalies interestingly correlated with the
proximity of sinkholes and the currently most active collapsing zone (which was intact during the
survey).
CSP stacking and diffraction imaging
Diffraction imaging and CSP stacked sections computed from these surface-wave profiling data are
presented on Figure 2.
73rd EAGE Conference & Exhibition incorporating SPE EUROPEC 2011
Vienna, Austria, 23-26 May 2011
Figure 2 (a) Diffraction image and, (b) CSP stacked section along Line 3. (c) Diffraction image and,
(d) CSP stacked section along Line 4
Both diffraction images (Fig. 2a and 2c) express strong lateral heterogeneities possibly correlated with
some of the low velocity anomalies depicted by the dispersion inversion results (around position 200
m along Line 3 and around position 220 m and 350 m along Line 4). The CSP section along Line 3
(Fig. 2b) shows a deep reflector, possibly related to compacted sediments and complicated by a fault
(at position interval 165-190 m). Along line 4, the CSP stack section (Fig. 2d) shows comprehensive
anomaly at position interval 270-290 m clearly expressed on the diffraction image (Fig. 2c). From this
position, the CSP stack as well shows a deep reflector, correlated with the lateral velocity change on
the dispersion inversion results.
Conclusions
Zero-offset CSP stacking and diffraction imaging constitute complementary methods to reveal useful
information about the subsurface. As an illustration, this approach has been applied on the Ghor Al-
Haditha sinkholes site (Jordan), on which the seismic surface-wave profiling technique has been
previously performed. Surface-wave dispersion inversion results presented strong lateral variations,
characterized by low shear-wave velocity anomalies, correlated with the proximity of existing
sinkholes and the currently most active collapsing zone. Such anomalies have been explained by a
systematic decompaction of shallow sediment layers during the sinkhole development process.
Diffraction imaging sections computed from these surface-wave profiling data appear to correlate
with dispersion inversion results in terms of lateral variations. CSP stacked sections make it possible
to enhance deeper structures, possibly related to compacted sediments.
Acknowledgements
73rd EAGE Conference & Exhibition incorporating SPE EUROPEC 2011
Vienna, Austria, 23-26 May 2011
CSP stacking and diffraction imaging studies have been performed within the framework of MERC
Project M27-050, sponsored by the USAID fund (the opinions expressed in this study are those of the
author(s) and do not necessary reflect the views of the USAID) and thanks to the support of the Israel
Ministry of Infrastructure. The field study in Jordan and surface-wave dispersion analyses were
carried out within the framework of the NATO Programme for Security through Science (Science for
Piece sub-Programme, project SfP 981128).
References
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Conference & Exhibition incorporating SPE EUROPEC 2010 Barcelona, Spain, 14-17 June 2010.
Boiero, D. and Socco, V. [2008] Retrieving lateral variations from surface-wave data. 70th EAGE Conference
& Exhibition Rome, Italy, 9 - 12 June 2008.
Keydar, S., Gelchinsky, B. and Berkovitch, A. [1996] Common shot-point stacking and imaging method.
Journal of Seismic Exploration, 5.
Keydar,S. [2004] Homeomorphic imaging using path integrals. 66th EAGE Conference & Exhibition Paris,
France, 7-10 June 2004.
Keydar, S., Pelman, D., and Ezersky, M. [2010] Application of seismic diffraction imaging for detecting near-
surface inhomogeneities in the Dead Sea area. Journal of Applied Geophysics, 71.
Legchenko, A., Ezersky, M., Boucher, M., Camerlync, C. and Al-Zoubi, A. [2008] Pre-existing caverns in salt
formations could be a major cause of sinkhole hazards along the coast of the Dead Sea. Geophys. Res. Lett.,
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in the shallow subsurface. Geophysics, 63.
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... In order to examine those two hypotheses a new 2D and 3D imaging approach had been developed ( Keydar et al. 2011. Based on this approach we have carried out several new seismic surveys along line crossing the sinkhole lineaments and reinterpreted number of previous reflection sections. ...
... In the framework of the MERC project a new approach for imaging shallow subsurface has been developed. This approach involves zero-offset Common Shot Point (CSP) stacking and diffraction imaging method ( Keydar et al, 2011). On the basis of those methods in combination with a new weighted multipath summation technique (Keydar, 2004;Keydar and Mikenberg, 2010) a package of programs was written. ...
... The 3D survey was conducted at the Mineral Beach located between the Dead Sea shoreline and Route #90 (Fig. 3a), where sinkholes develop in alluvial fan. Earlier, we have performed there 2-D reflection survey ( Keydar et al., 2011). The field acquisition covers 120m by 60m using 288 shots with 96 channels in 2.5m interval per shot. ...
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Results of a geophysical study of the sinkhole development sites in the Dead Sea coastal area in Israel and Jordan are presented. The updated Seismic Reflection methodology has studied the relationship between sinkhole lineaments and faults. It was shown that sinkhole lineaments are arranged along the salt layer edge. We suggest that sub-vertical displacements along faults generated shallow coastal basins where salt formed. Such suggestion would agree two major competition models of sinkhole formation: structural model, considering control of faults, and second one explaining sinkholes formation along the salt edge. Such a model suggests that the salt layer edge is conformed to faults, and sinkholes are arranged simultaneously along both salt edge and faults. This model will be verified shortly by analysis of boreholes and by other reflection sections carried out in the past. Note, models presented in the paper enable us to conclude that: (1) Salt edge can be considered an ancient shoreline was existing at the stage of salt unit formation (10,000 years ago). Modern sinkholes are formed along this ancient shoreline; and (2) Buried salt layer comes out far from the modern Dead Sea shoreline permitting its investigation from the surface.
... However, in the shallow subsurface the conventional CMP method causes a loss of information because of the problem of "stretching" of data caused by the NMO time correction formula. Therefore, a new 2D (Keydar et al., 2011(Keydar et al., , 2012 and 3D free stretch imaging approach has been applied in order to examine the tectonic hypothesis which associates the sinkholes with tectonic faults. This approach involves zero-offset Common Shot Point (CSP) stacking and diffraction imaging method (Keydar et al., 2012). ...
... Receiver spacing was 5 m, shot location was 5 m away from the nearest trace (off-end shooting east side), and the move up between shots was one receiver interval. The recorded seismograms were also compiled and processed by Keydar et al. (2011) in order to extract Common Shot Point (CSP) stacking and diffraction images along the lines. ...
... However, in the shallow subsurface the conventional CMP method causes a loss of information because of the problem of "stretching" of data caused by the NMO time correction formula. Therefore, a new 2D (Keydar et al., 2011(Keydar et al., , 2012 and 3D free stretch imaging approach has been applied in order to examine the tectonic hypothesis which associates the sinkholes with tectonic faults. This approach involves zero-offset Common Shot Point (CSP) stacking and diffraction imaging method (Keydar et al., 2012). ...
... Receiver spacing was 5 m, shot location was 5 m away from the nearest trace (off-end shooting east side), and the move up between shots was one receiver interval. The recorded seismograms were also compiled and processed by Keydar et al. (2011) in order to extract Common Shot Point (CSP) stacking and diffraction images along the lines. ...
... Barjous et al. (2004) (Camerlynck et al., 2012). In 2007, these included also a seismic survey along 4 205 profiles targeted to combined reflection, refraction tomography, and Multi Channel Analysis of Surface Wave (MASW) data analysis (e.g., Dhemaied, 2007, Bodet et al., 2010, Keydar et al., 2011, Ezersky et al., 2013a et al. (1999) and Rix and Leipski (1991), where L is the total source-receiver spread length of 120 m. Fig. 1b) show shear wave velocities of mainly less than 400 ms -1 215 from surface to Zmax, while profile 4 (magenta line 4 in Fig. 1b) images a high velocity layer of more than 800 ms -1 from nearly 30 m depth to Zmax, which was interpreted as the shallow salt layer. ...
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Near-surface geophysical imaging of alluvial fan settings is a challenging task, but crucial for understating geological processes in such settings. The alluvial fan of Ghor Al-Haditha at the southeast shore of the Dead Sea is strongly affected by localized subsidence and destructive sinkhole collapses, with a significantly increasing sinkhole formation rate since ca. 1983. A similar increase is observed also on the western shore of the Dead Sea, in correlation with an ongoing decline of the Dead Sea level. Since different structural models of the upper 50 m of the alluvial fan and varying hypothetical sinkhole processes have been suggested for the Ghor Al-Haditha area in the past, this study aimed to clarify the subsurface characteristics responsible for sinkhole development. For this purpose, high-frequency shear wave reflection vibratory seismic surveys were carried out in the Ghor Al-Haditha area along several crossing and parallel profiles with a total length of 1.8 km and 2.1 km in 2013 and 2014, respectively. The sedimentary architecture of the alluvial fan at Ghor Al-Haditha is resolved down to a depth of nearly 200 m in high-resolution, and is calibrated with the stratigraphic profiles of two boreholes located inside the survey area. The most surprising result of the survey is the absence of evidence for a thick (> 2–10 m) compacted salt layer formerly suggested to lie at ca. 35–40 m depth. Instead, seismic reflection amplitudes and velocities image with good continuity a complex interlocking of alluvial fan deposits and lacustrine sediments of the Dead Sea between 0–200 m depth. Furthermore, the underground of areas affected by sinkholes is characterized by highly-scattering wave fields and reduced seismic interval velocities. We propose that the Dead Sea mud layers, which comprise distributed inclusions or lenses of evaporitic chloride, sulphate, and carbonate minerals as well as clay silicates, become increasingly exposed to unsaturated water as the sea level declines, and are consequently destabilized and mobilized by both dissolution and physical erosion in the subsurface. This new interpretation of the underlying cause of sinkhole development is supported by surface observations in nearby channel systems. Overall this study shows that shear wave seismic reflection technique is a promising method for enhanced near-surface imaging in such challenging alluvial fan settings.
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