«Case study for multiwave seismic in Russia. Problems and solutions.»
GDS
V.M. Kuznetsov, G.A. Shekhtman, A.P. Zhukov, I.P. Korotkov
Abstract. Technology of multiwave seismic (MWS) allows obtaining additional
information on the lithology and more safely exploring the fractured reservoir (in the 3D-3C
version). Because MWS is a more expensive method, its use requires careful planning and clear
formulation of the geological problems. There are very specific requirements for the field
technique, processing and interpretation of 3C MWS data. By examples of two MWS 2D-3C
projects in Russia the possibility is shown for structural model to be added by the information on
lithological changes in the target intervals of the section. The features of MWS data processing
and interpretation are considered.
Technique elements selection based on the geological problem
setting
MWS technology requires an increase in the costs of field work, processing and
interpretation of data. Therefore, the design of these studies suggests a more careful selection of
components (hardware and methods), taking into account all a priori geological and geophysical
information and the alleged criteria for exploration of target zones.
The tasks which are set for MWS could be divided into three groups - structural,
lithological and evaluation of fracture. For structural problems two main criteria might be
outlined - the presence of gas clouds and more meaningful jump in acoustic stiffness of the Swaves for the target zone. In all other cases, the task is successfully solved by P- waves. To solve
the lithological problems separating by the Poisson's ratio the target objects and the surrounding
rocks is assummed. Solving problems of fracture zones localization and evaluating parameters of
fracture systems is based on the presence of cracks with the dominant direction. This involves
the use of three-dimensional (3D) field geometry with sufficient and uniform density
measurements for the beans in the whole range of azimuths.
An important aspect of building the MWS techniques is combination of borehole and
surface observations. The main condition for successful solution of the problem is the correct
identification of different type waves for the same geological sites - identified reflection of the
longitudinal. waves and shear/converted waves must match the same spatial object. This is
achieved by obtaining information on the elastic parameters of well data (VSP, wave acoustics,
dipole acoustic logging), and linking them with surface data.
Surface MWS acquisition requires the most accurate restoration of the full displacement
vector (3C-recording), increasing the density of recording points two times comparably to the
density of shot points. Special sources with adjustable directivity might be used.
At the designing the works the peculiarities of the upper part of the section should be
taken into account. Soft and water-filled grounds are unfavorable for recording of shear
oscillations. Of the deep factors unfavorable for the formation and propagation of transverse
waves is presence of "flowing" salt..
The main MWS advantage is the possibility of obtaining additional information on the
transverse/converted waves, which allows getting data on the velocities and seismic attributes
both longitudinal and transverse waves. In such a case the probability of the useful geological
information extracting is multiplied.
Results of reconnaissance works in the Eastern Siberia.
As an example of reconnaissance work we present results of the MWS project in the
Eastern Siberia. The problem was formulated as follows - to implement prediction of lithology
changes in intervals of the section, prospective for the detection of hydrocarbons. Petrophysical
information on the relationship of elastic parameters with the lithology, porosity and fluid
saturation for this region was absent. As to the collector, it was known only that no structural
features for reservoir are present. The target interval itself is an alternation of thin layers of
mudstone, anhydrite and sandstone. The unmudded sandstones with porosity greater than 5%
(Kp ≥ 5%) can herein be productive.
MWS works were carried out on a sparse grid of lines (2D-3C) with the step between the
SP and RP of 20 m and end-on spread geometry. The quality of the surface survey records on
longitudinal waves causes no specific criticism, while the data on PS waves differ with weak
energy of useful reflections and correlation coefficient. This is due primarily to complex
seismogeological conditions.
The main procedures of the processing, allowing improved correlation of the reflected PS
waves are as follows: re-orientating the horizontal components in the SP-RP coordinates,
interactive static correction, the combined pre-stack migration with a detailed joint (for PP and
PS waves) velocity analysis (Fig. 1). The main features of the combined pre-stack migration are
as follows:
- obtaining the Vp and Vs velocity model is made using the compilation traces of
common source point, rather than common middle point. This is a better way for PS waves
because of asymmetry of their radiation pattern;
- there is a possibility of velocity analysis for the PS data, linking it with already "fixed"
model for longitudinal waves, i.e. with the known velocity of the incident beam. It allows to pick
up velocity model for the same (matching with the depth) reflection objects for waves of
different types.
Fig.1. A comparison of pre-stack migration of the longitudinal and converted waves in the deep
area.
At the first stage of integrated interpretation the depth and stratigraphic tie of reflections
of different wave types to the VSP and well logging data and their comparison with surface data
in time domain were made. The depth and stratigraphic tie of reflections of different wave types
showed coincidence of reflections positions in depth for all target horizons for PP and PS waves
and in the signs of the wavelet phases. At matching of borehole and surface data static shift
between them was taken into account, the shift was caused by different values of interval
velocity of near-surface section models in the VSP and surface data. Appropriately the vertical
travel time curves and tying of seismic horizons to well data were corrected.
The wave identification procedure using the nature of the wave picture is based on the
fact that the reflecting horizons for different waves types are often the same, the reflection
coefficients and the frequency characteristics of the layers sets, forming a reflection, differ
mainly because of differences in scale of velocities.
The identification was based on a comparison of kinematic (the ratio of the time
recording for dominant waves, the time intervals between the waves), amplitude (intensity ratio
of the major dominant waves on the section between themselves and the intensity ratio of core
and weaker waves) and frequency response (changing forms of records, relative intensities of the
waves phases) of the wave field. The correctness of the identification was controlled by
assessment ratio of the time interval PP and PS reflections (parameter = Vs/Vp). Horizon
correlation is consistently downward from the horizon on the PP waves, then the same horizon
on the PS waves. The points of intersections with wells were taken into account for bedboundary resolutions of different waves type. The next PS correlated horizon is controlled by
corridor t between the values of the time TPSmax2 and TPSmin2 , which are determined from the ratio 
= Vs / Vp ( min = 0.3 and max =0.6):
(  max  1)
TPSmin2 
TPP 1 2  TPS 1
2 max
(  min  1)
TPSmax2 
TPP 1 2  TPS 1
2 min
where Tpp1-2 is the time interval between the overlying (1) and correlated earlier (2) PP
horizon, Tps1 - the time overlying (1) horizon for PS waves.
In the case of leaving the correlation from the corridor, the possibility of changing the
correlation with PP or the sign of the extremum was analyzed. This allowed reducing the errors
in correlation to a minimum and increasing the reliability of the final result.
Identification of PP and PS reflections, that had been conducted, allowed a comparative
analysis of various characteristics of the wave fields belonging to task intervals of the section Poisson's ratio, amplitude and frequency attributes of identified reflections for the waves of
different types and their relationships etc. Weak statistical base of petrophysical research for the
area under study did not allow calculating the regression dependences for interrelations between
the seismic and geological attributes.
Calculations using well data showed that with increasing bed shaliness Poisson's ratio
increases as well. On the other hand, decrease of porosity leads to a decrease of Poisson's ratio.
When building the dependence of Poisson's ratio from shaliness with different porosity
coefficients for the sandstone, we can see that in the intervals of the section characterized by
Poisson's ratio, lying within the limits between 0.26 – 0.35, one can predict the prevalence of
largely unmuddy sandstone with fairly high porosity coefficients. The values of Poisson's ratio,
shown on the map for the target interval (Fig. 2) with highlighted blue-green area, forecast the
collector, which satisfies to the imposed conditions. It agrees well with the map of sediment
types for the area, built from the results of seismoformation interpretation in the laboratory SFI
VNIIGeofizika (Mushin IA).
Fig. 2 Map of the Poisson's ratio values for the target interval of Vend clastic section
Resume
Correct identification of PP and PS waves on surface MWS data according to well data
(VSP and well logging) allowed matching the reflections to the geological features at the
interpretation stage. The accuracy of identification should be controlled with unambiguous
geologic imaging of the structure factor for different types of waves, the ratio of their dynamic
parameters and physicality of Poisson's ratio estimates. This greatly increases the reliability of
interpretation. Perspectives for improving forecasting problem of geological section and its
changing are connected with the fact that only all set of parameters determined from PP, SS and
PS waves characterize physical properties of the geological environment with sufficient entirety.
Additionally, it is essential that the parameters defined from the waves of various types, are
known to be functionally not linked and complement each other. This is the base for more
reliable localization litho-stratigraphic units and structural-facies zones from the seismic data,
and also evaluation of the basic laws of spatial distribution of properties for reservoir rocks and
seals. This implies more efficient evaluation of expected resources.