adg vol5 n02 Mor 259_271.pdf
ANNALS OF GEOPHYSICS, VOL. 45, N. 2, April 2002
259
Integrated interpretation of seismic
and resistivity images across
the «Val d’Agri» graben (Italy)
Sergio Morandi and Enrico Ceragioli
Enterprise Oil Italiana S.p.A., Roma, Italy
Abstract
Val d’Agri is a «recent SSW - NNE graben» located in the middle of the Southern Apennines thrust belt «chain»
and emplaced in Plio-Pleistocene. The recent sedimentation of the valley represents a local critical geophysical
problem. Several strong near surface velocity anomalies and scattering degrades seismic data in different ways
and compromises the seismic visibility. In 1998, ENI and Enterprise, with the contribution of the European
Community (ESIT R & D project – Enhance Seismic In Thrust Belt; EU Thermie fund) acquired two «experimental
seismic and Resistivity lines» across the valley. The purpose of the project was to look for methods able to
enhance seismic data quality and optimize the data processing flow for «thrust belt» areas. During the work, it
was clear that some part of the seismic data processing flow could be used for the detailed geological interpretation
of the near subsurface too. In fact, the integrated interpretation of the near surface tomography velocity/depth
seismic section, built for enhancing the resolution of static corrections, with the HR resistivity profile, acquired
for enhancing the seismic source coupling, allowed a quite detailed lithological interpretation of the main shallow
velocity changes and the 2D reconstruction of the structural setting of the valley.
1. Geological framework and geophysical
context
«Val d’ Agri» is located in the middle of the
Southern Apennines thrust belt «chain» and in
this context the «Val d’Agri graben» is a typical
recent «tectonic valley» controlled by SSW and
NNE dipping high angle Pleistocene faults.
This structural setting represents the Apen-
nine maximum extension direction associated
with the middle Pleistocene-Holocene normal
tectonic regime, where the youngest tectonic
elements are normal NW-SE trending faults with
associated N-S trending left – lateral strike – slip
faults, superposed onto the pre-existing Apennine
fold and thrust structures (Hippolyte et al.,
1994a,b).
The sediments of the valley are essentially
Plio-Quaternary marine, lacustrine and fluvial
deposits of highly variable lithology and thick-
ness (0 to several hundred meters). The young-
est stratigraphic units of the valley are coarse to
fine grained alluvial deposits and coarse slope
«breccias».
Below the «valley» the Apennine thrust belt
is characterised by «embricate» structures with
extreme vertical and later tectonic displacements.
Mailing address: Dr. Sergio Morandi, Enterprise Oil
Italiana S.p.A., Via dei Due Macelli 66, 00187 Roma,
Italy; e-mail: Sergio.Morandi@rome.entoil.com
Key words ESIT project seismic imaging tomo-
statics integrated interpretation
260
Sergio Morandi and Enrico Ceragioli
M. hianco e 1
le za 1 T reFo t n -1
A r moS NVal d'Agri
5000 m
10 km
Pliocene and recent
Apenn. Platform
Allochtonous
Basement
Apulian platform
Fig. 1. Geological cross section across Southern Apennines.
Val D' Agri
Fig. 2. Example of conventional seismic image across Val d’Agri.
261
Integrated interpretation of seismic and resistivity images across the «Val d’Agri» graben (Italy)
In this context the sediments and the tectonic
environment of the valley are a sort of recent and
thin «roofing» above the Apennine complex
geological setting (fig. 1).
This recent sedimentation is the cause of an
important local geophysical problem. In fact, the
presence of this recent sedimentation has a
negative impact on seismic data quality and
degrades the seismic image.
– Significant shallow velocity variations and
anomalies.
– Heavy wave scattering noise that masks
the seismic reflections.
And, in particular,
– serious problems for optimizing static cor-
rections,
– very poor quality of the seismic image, are
the usual geophysical consequence on seismic
of this «thin roofing» above the «Apennine
imbricate structures» (fig. 2).
2. 2D resistivity and seismic acquisition tests
Innovative methodologies have recently
been tested in Val D’Agri for enhancing
seismic visibility.
In late 1998, ENI and Enterprise, with the
contribution of the European Community,
acquired two experimental 2D seismic lines
associated with two relative high resolution
resistivity profiles across the valley (fig. 3).
The results obtained on line A (crossing
Tramutola village, Monticello hill and Villa
d’Agri village) are illustrated in this paper.
One of the purposes of the test performed
was the analysis of methods able to improve
seismic data quality by optimising acquisition
source coupling and static correction effi-
ciency.
The assumptions were that:
– The application of correct statics to the
seismic data may improve the continuity of
Pliocene and recent sediments
Km 10
1998 Seismic test lines
Line A
Fig. 3. Location of the 1998 2D seismic test lines.
262
Sergio Morandi and Enrico Ceragioli
events, produce a better subsurface image, and
give a more appropriate relative position in time
domain to each single reflection.
– The presence of an HR resistivity profile
along the seismic line may aid the individuation
of the shallowest «clay sequences» of the valley,
that represent the optimal lithology for seismic
source coupling during the acquisition of the
seismic data.
2.1. HR resistivity survey
The equipment used for the HR resistivity
survey and the recording parameters were the
following:
– Equipment: AGI - STING R!; 256 Elec-
trods; 1 to 599 mA; 320 to 800 V; reading cycle:
1.2 to 14.4 s.
– Recording parameters: AB/2 120 to 500
m; dipole length ranges: 5 to 12 m; max number
of electrodes deployed at any one time: 56; no.
of soundings along the seismic line: 15.
The interpretation flow was based on the
2D inversion of each single «automatic true
resistivity section» and their «stack» along the
line (fig. 4).
2.2. Seismic survey
The seismic survey was acquired by the
«Sercel 368 recording unit» using both together
«conventional near vertical roll along» and
«global offset» combined recording techniques
(Dell’Aversana et al., 1999, 2000). The seismic
was recorded using offsets of 3000 to 18 000 m,
max CMP fold of 12 000% and source depth and
coupling variable; optimised following the HR
resistivity data indications.
However, the near vertical seismic infor-
mation was only used for tomostatics and near
surface tomography processing.
The roll along 2D near vertical seismic
recording parameters and layout are schematised
in fig. 5.
3. Near surface seismic tomography processing
The roll along near vertical seismic shots have
been only used for near surface tomography
issues.
With the purpose to enhance the seismic static
corrections resolution, «Statics» have been
calculated by the near surface tomography
approach based on the travel time inversion of
the near vertical seismic first arrivals (Zhu et al.,
1992, 1998).
Tomographic inversion was based on analysis
of the automatic and/or manual picking arrival
time of the turning waves. This has been done
on each SP along the entire near vertical offset
(3600 m).
There are three distinct steps to perform the
above process:
1) Divide the model into initial grid cells.
2) Ray trace through the model and compute
source to receivers travel times.
3) Construct an inversion matrix from the ray-
traced result and solve the linear equations.
The commercial «H. Russel GLI3D soft-
ware» was used to perform the job.
The tomographic inversion method imple-
mented within GLI3D uses a new approach that
abandons rays and uses a wavefront method in
both the forward and inverse processes.
This has two advantages: the first is a finite-
difference algorithm for the rapid and accurateFig. 4. HR resistivity section – interpretation flow.
263
Integrated interpretation of seismic and resistivity images across the «Val d’Agri» graben (Italy)
forward modeling of travel times (previously done
using ray tracing); the second is the application of
a new much faster inversion procedure in order to
calculate the back propagation of errors and
thereby updating the velocity model (previously
done by the solution of linear equations). The
inversion process is initiated with an estimate of
the true velocity structure (Hampson and Russell,
1984).
The first arrival time is computed by solving
the eikonal equation, which calculates the first
arrival time from a source location to each point
within the model. The method is rapid and
accurate, and properly handles the various wave
types that comprise a first arrival (fig. 6a).
It can also be applied to a heterogeneous
medium with moderate velocity variations. The
errors between the observed and calculated travel
times are then calculated and back propagated to
the source and the model velocity is updated
during the back propagation. The velocity updates
are calculated by using a gradient search technique
that minimizes the discrepancy between the
observed and calculated travel times.
The back propagation algorithm is originally
introduced in the field of computer neural networks
as a training algorithm. It is a generalization of the
least square algorithm.
It uses the gradient search technique to minimize
the mean square difference between the desired and
the actual network output.
The inversion is done shot by shot. The iteration
«loop» is repeated until an acceptable match is
obtained between the observed and calculated travel
times (fig. 6b). There are two iterative techniques
available in GLI3D: ART (Algebraic Reconstruction
Technique) and SIRT (Simultaneous Iterative
Reconstruction Technique). With ART the velocity
model is updated for every shot following each
iteration, and with SIRT the model is updated after
all shot records have been modeled.
The inversion method implemented within the
«HR GLI3D software» is calibrated based upon the
pick time, such that the error between the observed
pick time and the modeled pick time is minimized.
Eight iterations were executed during the seismic
tomography processing. The iterative procedure was
stopped when:
by "Resistivity"
information
Fig. 5. Roll-along 2D near vertical seismic recording scheme.
264
Sergio Morandi and Enrico Ceragioli
– No substantial improvement or modi-
fication was obtained in the model by increasing
the number of iterations.
– The differences between observed and
calculated travel times were estimated as minima.
The resolution is maximum in the central area
of the seismic section and reduces sensibly in
function of spatial ray coverage and density. The
ray coverage is redundant and constant along the
entire section and it usefully investigates the first
400-500 m of the subsurface along the entire
profile. The maximum investigation of the
subsurface in the final velocity/depth model is
around 800 m.
4. Imaging improvements
The near surface tomography results have
been used in two different ways.
Tomostatics improved the static correction
resolution and consequently the conventional
seismic time image of the valley.
In fig. 7 conventional statics processing and
near vertical tomostatics are compared. It is easy
to note the improvement of the seismic image
obtained using tomostatics relate to the conven-
tional elevation statics based on the SP uphole
time correction.
However, despite the sensible improvement,
any detailed lithological and structural inter-
pretation of valley is possible using this con-
ventional TWT representation of the seismic
information.
Near vertical tomography velocity/depth
model showed surprising and significant details
inside the valley.
Fig. 6a. Automatic picking on LMO corrected shot gathers.
Fig. 6b. Near surface tomography processing flow.
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Integrated interpretation of seismic and resistivity images across the «Val d’Agri» graben (Italy)
A new integrated interpretation as for the
lithological than for the structural interpretation
of the valley was possible using the near surface
tomography representation of the seismic
information.
In fact, the «high resolution shallow velocity/
depth seismic model» displayed by the near
surface tomography of the first arrival allowed
at the same time (figs. 8 and 9):
– An unconventional representation of the
shallower part of the seismic information.
– A significant illumination of the structural
setting inside the «Val d’Agri graben» to 1000 m
at least, as it was never done in the past by con-
ventional seismic images (fig. 10).
– A detailed interpretation of the shallower
part of the seismic section, by removing noise
and time imaging limitations, that are typically
the limit of the conventional seismic TWT
representation.
– A new robust 2D seismic image able to
contribute to the knowledge of the Val d’Agri
significantly.
5. Integrated 2D interpretation of seismic
and resistivity images across the
«Val d’Agri graben»
The integrated interpretation of the 2D near
surface tomography velocity/depth seismic
section with the 2D HR resistivity profile
sensibly contributed to:
– The reconstruction of thicknesses and
lithology of the most recent sediments of the
valley (figs. 11 and 12).
Fig. 7. Two Way Time (TWT) image of the Val d’Agri valley using tomostatics or elevation statics processing.
266
Sergio Morandi and Enrico Ceragioli
Figs. 8-9. Near surface tomography – initial and final velocity/depth models.
267
Integrated interpretation of seismic and resistivity images across the «Val d’Agri» graben (Italy)
– The knowledge of the structural setting of
the valley (figs. 13 and 14).
In fig. 11, the 2D geoelectrical profile is
displayed and the lithological interpretation of
the main resistivity contrasts is reported. This
section was obtained by manual interpolation of
1D HR measurements.
The penetration of the resistivity profile is
about 200 m. The western border of the valley is
associated to the presence of a «carbonate bed
rock» > 2000 m (Apennine platform outcrop)
and its dipping high angle Pleistocene fault; while
the eastern border of the valley is masked by
slope breccias and derbis slope deposits 100-150
m.
Inside the valley, two coarse alluvial deposits
cycles can be separated interpreting the most
significant resistivity changes, and the progressive
and rapid migration of the Agri River to the western
border can be underlined.
In fig. 12, if the interpretation of the resistivity
profile is integrated with the near surface to-
mography velocity/depth section, a great cor-
respondence is present between shallow resistivity
and shallow velocity contrasts, but the resolution
and the penetration of the seismic image is sensibly
higher than in the resistivity profile. For this reason,
on the seismic tomography section it is possible
to note the presence of a probable third alluvial
deposits cycle (3) plus other 3 or 4 alluvial deeper
cycles to about 800 m of depth.
The «perfect» correspondence between
resistivity and velocity contrasts is probably due
to the progressive compaction of the sediments
Fig. 10. Conventional time seismic versus tomography velocity/depth section.
268
Sergio Morandi and Enrico Ceragioli
Fig. 11. HR resistivity versus lithology.
Agri River
200 m
Fig. 12. HR resistivity versus lithology.
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Integrated interpretation of seismic and resistivity images across the «Val d’Agri» graben (Italy)
Fig. 13. Near tomography velocity – depth section – structural interpretation of the Val d’Agri graben.
Fig. 14. Near surface tomography velocity – depth section – structural interpretation of the Val d’Agri graben in
the wider context of the Apennine trust setting.
270
Sergio Morandi and Enrico Ceragioli
and to the presence of a progressive higher coarse
deposit facies with the depth.
The variation of the sedimentary thickness
inside each alluvial cycle could indicate that
during the initial phase of the graben opening
(alluvial cycle 3) the eastern border of the valley
was deepening more quickly than the western
one. Instead, during the youngest phase of the
graben evolution there was an important in-
version of the phenomena and the valley seems
now deepening more quickly in the western ward.
This hypothesis is supported by the seismic
reflection TWT image of the valley (fig. 10) and
by the robust correlation with the HR resistivity
model (fig. 11).
The high ray coverage and the redundant
high data density are in favour of the below
lithological interpretation (fig. 12) and exclude
to have an apparent multi-layering «artifact
because of poor data resolution, or inadequate
modeling».
In fig. 13, a possible structural interpretation
of the seismic tomography image of the valley
is represented. The structural setting of the Val
d’Agri graben is quite characteristic. The faulting
system of the valley is now imaged while it is
still inadequately imaged on the TWT conven-
tional seismic (fig. 10).
The «graben» is now relatively symmetric
and clearly controlled by SSW and NNE dipping
high angle faults. The actually active faulting is
probably located along the western margin of the
valley, where this potential active tectonic tilting
could also explain the lateral migration of the
Agri river to west. In the central part of the valley
the faulting system can be designed following
the distribution of the velocity changes. This
faulting system was partially active until now and
just sealed by the most recent thin alluvial
sediments.
From the structural point of view, the Val
d’Agri graben could be interpreted as a «negative
flower thrust» due to the transpressive and
distensive late Apennine tectonic phase, having
its depocenter at around 1000 m of depth.
In fig.14, this is quite clear when the entire
«near surface seismic tomography profile» is
displayed and interpreted.
Here, it is possible to have a more regional
2D view of the «Val d’Agri graben« and insert it
in the wider geological context of the «Apennine
thrust setting». The graben is young and limited
in depth lateral extension.
Along the entire seismic near surface to-
mography section it is also possible to note that
some of the normal faults of the graben (in
particular along its western margin) could interest
the deep allochtonous «Lagonegro units» too. It
is clear that the hypothetical extension of the
main thrusts below the resolution limit of the
tomography image is purely speculative.
6. Conclusions
Conventional seismic data contain infor-
mation that can be lost, if they are not processed
and displayed properly. This is the case of the
Val d’Agri, where the quality of same seismic
data set can be considered average, if processed
and displayed by conventionally time wiggle –
variable area images; or very detailed and high
resolution, if processed differently. This is the
case of the conventional seismic first break arrival
tomography.
Near surface tomography of conventional
seismic first break arrivals can be used as an
innovative interpretation tool, where rough terrain,
seismic noise and structural complexity negatively
influence the time seismic image and the seismic
visibility.
It is certainly a very efficient high resolution
illumination of the shallower part of the seismic
information and it is a very robust representation
of the near subsurface, directly in depth.
Shallow noise, scattering, absorption; static
corrections and shallow imaging pitfalls can be
very well solved by near surface tomography data
processing.
The Val d’Agri subsurface was never illumi-
nated by conventional seismic images in the past.
As a consequence, seismic tomography has
incidentally given a strong contribution to the
knowledge of the structural setting and lithology
in «Val d’Agri graben».
A very detailed reconstruction of the Val d’Agri
graben could be made by the near surface
tomography of the most part of the large seismic
data set present in the area. However, a good near
271
Integrated interpretation of seismic and resistivity images across the «Val d’Agri» graben (Italy)
surface tomography resolution can be obtained
where the acquisition offsets are sufficiently
long.
Offsets > 2500-3000 m are absolutely re-
quired to allow an efficient near surface to-
mography processing of seismic first arrivals to
about 1000 m of depth. Unfortunately this is
not the case for a lot of vintage seismic data
acquired in the past in the area.
HR resistivity information mainly con-
tributed to the lithological identification of the
shallower velocity changes in the near surface
tomography seismic section. However, the
integrated interpretation of geoelectrical and
seismic tomography images was crucial to allow
the reconstruction of thickness, lithology and
alluvial cycles distribution of the most recent
sediments of the valley.
Acknowledgements
– ENI S.p.A. – Agip Division, that operated
the seismic acquisition and the HR resistivity
survey.
– Enterprise Oil Italiana S.p.A.
– European Union – «Thermie Fund».
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