CLIAMS:1. An apparatus for 2-dimensional and 3-dimensional active seismic survey to determine the subsurface oil and gas in strata, said apparatus comprising:
- more active seismic energy source;
- plural broadband sensors;
- one or more recording instrument;
-filters
wherein the said active seismic energy source generates active seismic waves by inducing mechanical vibrations at or in proximity to the earth’s surface and said seismic waves reflects back from the strata boundaries;
wherein said broadband sensors detect the said seismic waves; and wherein said recording instrument recording said seismic waves as representative electrical signals and generating full wave seismic records; and
processing said signals into images;
characterized in that the said apparatus comprising:
- extracting resonant low frequency seismic signals at around 2 to 5 Hz frequency as hyperbolic events at late part of the seismic records combined with active seismic reflection signals at around 6 to 125 Hz in one seismic record collected by the sensors through said filters after the primary waves are gone from said seismic record;
mapping the mutually perpendicular apex locations of resonant hyperbolic events containing unique velocity or phase slope from said seismic records on a base map; and
identifying and/or discriminating the subsurface oil and gas in terms of surface location and depth by stack imaging in time and depth domain at phase velocity as hyperbolic events at later part of said seismic records.
2. The apparatus for seismic survey as claimed in claim 1, wherein the said broadband sensors acquire full wave reflections (Primary and Secondary wave reflections) and low frequency resonant energy in seismic record.
3. The apparatus for seismic survey as claimed in claim 1, wherein the recording time of said full wave seismic record is around 6 to 18 seconds depending on the depth range of said strata.
4. The apparatus for seismic survey as claimed in claim 1, wherein the said mapping of apex locations comprises forming a polygon by joining closely spaced resonant events on said base map and determining the centre point of the said polygon.
5. The apparatus for seismic survey as claimed in claim 1, wherein said apparatus generates common mid source (CMS) gathers over oil and gas reservoirs.
6. The apparatus for seismic survey as claimed in claim 5, wherein said apparatus move-out said common mid source CMS gathers with phase velocity.
7. The apparatus for seismic survey as claimed in claim 1, wherein said identifying and/or discriminating the subsurface oil and gas further comprises determining the centre point of maximum amplitude zone in said stack image in depth domain.
,TagSPECI:TECHNICAL FIELD OF THE INVENTION
The present invention belongs to seismic exploration for oil and gas, and processing seismic data. More particularly the present invention relates to apparatus and method of seismic data analysis for low frequency resonant seismic energy in active seismic data.

BACKGROUND OF THE INVENTION
There are two types of seismic operations which are carried out in oil industry for hydrocarbon exploration. They are active seismic and passive seismic. In active seismic, seismic energy source is under operator control and it is not in case of passive seismic.
Active Seismic exploration for hydrocarbons is carried out by using a seismic energy source, seismic energy detectors or sensors and recording instrument.
On land, the seismic energy source may be an explosive charge or another energy source having the capacity to generate mechanical vibrations at or near the earth’s surface. Seismic waves generated by these sources travel into the Earth’s subsurface and are reflected back from strata boundaries and reach the surface of the earth at varying intervals of time depending on the distance traveled and the heterogeneity of the subsurface. The return waves are detected by the sensors and representations of the seismic waves as representative electrical signals are recorded with recording instrument for processing into images. Normally, signals from sensors located at varying distances from the source are added together during processing to produce “stacked” or migrated stack seismic traces.
In marine seismic surveys, the source of seismic energy is typically air guns. Marine seismic surveys typically employ an array of sources and/or a few numbers of streamer cables, in which seismic sensors are mounted, to gather three dimensional data.
The process of exploring for and exploiting subsurface hydrocarbon reservoirs is often costly and insufficient because operators have imperfect information from geophysical and geological characteristics about reservoir locations. Furthermore, a reservoir’s characteristics may change as it is produced.
Geophysical and geological methods are used to determine Well locations. Expensive exploration investment is often focused in the most promising areas using reflection seismic data acquisition and processing. The acquired data are used for mapping potential hydrocarbon-bearing areas within a survey area to optimize exploratory Well locations and to minimize costly non-productive Wells.
The time from hydrocarbon discovery to production may be shortened if the total time required to evaluate and explore a survey area can be reduced by applying selected methods alone or in combination with other geophysical methods. Some methods may be used as a standalone decision tool for oil and gas development decisions when no other data is available.
Geophysical and geological methods are used to maximize production after reservoir discovery as Well. Reservoirs are analyzed using time lapse surveys (i.e. repeat applications of geophysical methods over time) to understand reservoir changes during production. Repeated data acquisition for oil exploration may have a negative impact on the environment. The impact of oil exploration methods on the environment may be reduced by using low impact methods such as potential methods and/or by narrowing the scope of methods requiring an active source, including refection seismic and electromagnetic surveying methods.
Low-impact methods include gravity and magnetic surveys that maybe used to enrich or corroborate structural images and/ or integrate with other geophysical data, such as reflection seismic data, to delineate hydrocarbon-bearing zones within promising formations and clarify ambiguities in lower quality data, e.g. where geological or near-surface conditions reduce the effectiveness of reflection seismic methods.
Passive seismic technology has come in industry in early year 2000 and can be used as Direct Hydrocarbon Indicator (DHI) based on noticed spectral high between 1 Hz to 6 Hz frequency. It is entirely new technology where seismic source is considered as passive seismic source like standing waves of the ocean, eigenmodus of the terrestrial crust, earthquakes, tremors from volcanoes/glaciers, etc., Sensors are planted on the surface and continuous recording is going on for several days (100-200 hrs). These sensors are recording below 10 Hz seismic data and there is no concept of reflections to get subsurface image.
Passive Seismic survey is entirely new methodology to identifying or monitoring the hydrocarbons in the subsurface. Since seismic source (Passive seismic source: Earthquake, standing waves of the ocean, eigenmodus of the terrestrial crust, tremors at nearby volcanoes/glaciers etc.,) is not under operator control, it is required continuous recording of seismic data for several days to localize the hydrocarbons only.
Since seismic source is not under operator control, localization of hydrocarbons can be done as Direct Hydrocarbon Indicator (DHI). However, depth estimation of hydrocarbon is still challenging.
United States Patent Number US 5,414,674 discloses method and apparatus for conducting seismic analysis of underground formations that offers improved accuracy in locating strata that contain natural gas in paying quantities. The method analyzes the resonant response generated when a seismic wave passes through a given stratum. Seismic data are collected using conventional field acquisition methods. Following conventional preprocessing, the seismic response data are mapped onto the frequency domain in order to separate the resonant and non-resonant components of the reflected energy. It discloses only for paying quantity of gas from subsurface rock strata at lower end of seismic frequencies in seismic data. This is associated with geophones only, not with broadband sensors.
Seismic reflection method is being widely used in Oil and Gas Industry since the year 1960. The objective of seismic reflection method is to get the good quality reliable subsurface image in order to identify the structural and stratigraphic traps that may contain hydrocarbons. However, subsurface image issues are quite challenging in seismic reflection technique for lateral complex geologic setting (Example :Thrust belt areas) and vertical complex geologic setting (Example: Mapping of low velocity sediments below the high velocity thick Deccan traps, below the thick salt / anhydrate strata and thick coal shales etc.)
Sedimentary basins may contains oil and gas which are identified by potential methods like gravity and magnetic methods. However, it is proven that seismic reflection method is the best geophysical method to see the subsurface image in order to identify structural and stratigrafic traps that may contains the hydrocarbons. It is normal practice in the industry that regional 2D seismic survey is the first geophysical operation in any unapprised area where no seismic, no well information is available.
A raw seismic record (<125 Hz) in active seismic is a product of complex seismic energy communication between seismic source, subsurface and receivers. In the same seismic record, geoscientists are being mainly looked for energy communication between seismic source, subsurface rock layers and receivers through seismic reflection technique since the year 1960. However, it is also even possible to see the simple communication of seismic energy between seismic source, hydrocarbon reservoir and receivers in the seismic data as seismic energy interact with hydrocarbon reservoir also in the subsurface. Therefore, a new seismic anomaly will be possible or hidden or invisible in seismic record that was recorded over the hydrocarbon reservoir with sufficient record length.
The present invention provides solution to the above mentioned problems and provides a method and apparatus of locating paying quantities of subsurface oil and gas & reduce present seismic exploration risk, includes seismic survey designing, acquiring seismic data with broadband sensors, extracting low frequency resonant seismic data attributes from the seismic data, mapping of right positions of resonant seismic attributes on base map and its stack imaging in time and depth domain at its right phase velocity to determine the likelihood of subsurface oil and gas.

OBJECT OF THE INVENTION
The prime object of the present invention is to overcome the drawbacks of the prior art.
It is an object of the present invention to provide a method and apparatus to acquire active seismic data for both low frequency resonant seismic data at 2-5 Hz and active seismic reflection data at 6-125 Hz together at one go of seismic survey.
It is an object of the present invention to provide a method and apparatus with less record time to determine the hydrocarbon depth.
Another object of the present invention is to provide an apparatus to produce resonant amplitude in the seismic record in time domain that helps to determine the hydrocarbon depth.
It is a further object of the present invention is to provide an apparatus and method to determine the subsurface oil and gas in terms of both surface position and its depth.
Yet another object of the present invention is to reduce the risk of cost effective 3D Seismic surveys.
It is an object of the invention is to reduce the risk of drill dry well for all geologic settings.
These and other objects of the present invention will become readily apparent from the following detailed description read in conjunction with the accompanying drawings.

SUMMARY OF INVENTION
The following presents a simplified summary of the invention in order to provide a basic understanding of the some aspects of the invention.
When the frequency of external force (seismic source acts as driver) interact with low velocity media (multi-fluid phase hydrocarbon strata) in the subsurface, a part of seismic energy is trapped in hydrocarbon strata and generates circumferential waves (surface types of waves) in hydrocarbon reservoir. These circumferential waves move with a particular frequency further act as secondary / buried source (driver) at the hydrocarbon reservoir. If the frequency of these circumferential waves meet with natural frequency of hydrocarbons (driven), then the resonance phenomena occurs. In the state of resonance, there occurs maximum transfer of energy from driver to driven and the amplitude of motion reaches to maximum at or proximity of the reservoir. These resonant seismic emission spreads out spherically in all directions from the multi-fluid phase strata which travel as low frequency, low velocity (Phase Velocity) of common body waves & appear as hyperbolic events at later part of raw seismic shot gathers (after all primary waves are gone). Since earth acts as low pass frequency filter, low frequency energy travel longer distances without significant attenuation and seismic sensors are capable to acquire these frequencies also.
The present invention is very clear to identify subsurface hydrocarbons (oil and gas) and reduce present seismic exploration risk. It is purely related to active seismic data at low frequency and Resonant Frequency (2-5 Hz). The present invention identifies both oil and gas in the subsurface. The present inventors found that the low frequency energy in any seismic data (passive seismic or active seismic) fit for all geologic settings to identify the subsurface oil and gas. Active seismic data also have low frequency (2-5 Hz) content, analyzing these frequencies are value addition to using active seismic reflection data (6-125 Hz). Seismic source is under operator control. Therefore, resonant amplitude in the seismic record in time domain helps to determine the subsurface oil and gas in terms of both surface position and its depth.

According to one of the aspect of the present invention here is provided An apparatus for 2-dimensional and/or 3-dimensional active seismic survey to determine the subsurface oil and gas in strata, said apparatus comprising:
- one or more active seismic energy source;
- plural broadband sensors; and
- one or more recording instrument;
wherein the said active seismic energy source generates active seismic waves by inducing mechanical vibrations at or in proximity to the earth’s surface and said seismic waves reflects back from the strata boundaries;
wherein said broadband sensors detect the said seismic waves; and wherein said recording instrument recording said seismic waves as representative electrical signals and generating full wave seismic records; and
processing said signals into images;
characterized in that the said apparatus comprising:
- extracting resonant low frequency active seismic electrical signals at around 2 to 5 Hz frequency as hyperbolic events at late part of the seismic records combined with active seismic reflection signals at around 6 to 125 Hz in one seismic record collected by the sensors through said electrical filters after the primary waves are gone from said seismic record;
mapping the mutually perpendicular apex locations of resonant hyperbolic events containing unique velocity or phase slope from said seismic records on a base map; and
identifying and/or discriminating the subsurface oil and gas in terms of surface location and depth by stack imaging in time and depth domain at phase velocity as hyperbolic events at later part of said seismic records.

To enable the invention to be more clearly understood and carried into practice reference is now made to the accompanying drawing in which like references denote like parts throughout the description.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
An embodiment of the invention will now be described, by way of example with reference to the accompanying drawings, in which:
Figure-01 represents a schematic diagram illustrating active seismic survey method.
Figure-02 depicts a schematic seismic shot gather (1-125 Hz) that contains direct energy, refracted energy, reflections, resonant energy(2-5 Hz) and ground roll etc.
Figure-03 represents a schematic seismic shot gather after applying auto gain control and filtering techniques.
Figure-04 illustrates the common mid source (CMS) gathers.
Figure-05 illustrates the CMS gather, Move out gather and Stack for interpretation subsurface oil gas in terms of its surface position and depth.
Figure-06 illustrates the tentative planning of 2D Seismic survey.
Figure-07 illustrates the low frequency resonant anomaly along 2D Seismic lines (N-S direction).
Figure-08 illustrates the low frequency resonant anomaly along 2D Seismic lines (E-W direction).
Figure-09 illustrates the low frequency resonant anomaly zones along 2D Seismic lines in any study area.
Figure-10 illustrates the planned 3D seismic on low frequency resonant anomaly zones in any study area in order to minimize the seismic exploration risk.
Figure 11 & 12 illustrates the methods steps involved in the active seismic survey data of the present invention.
Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure.
Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.

DETAILED DESCRIPTION OF THE INVENTION
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.
The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.
By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic is intended to provide.
Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments.
It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
The advantages, nature, and various additional features of the invention will appear more fully upon consideration of the illustrative experiment now to be described in detail in connection with accompanying drawings.
The present invention describes an apparatus for 2-dimensional and/or 3-dimensional active seismic survey to determine the subsurface oil and gas in strata where a resonant low frequency (2-5 Hz) seismic data 3 combined with active seismic reflection signals 8 at around 6 to 125 Hz in one seismic record collected by the broadband sensors 2 through the filter and auto gain application on seismic data for identification and discrimination pertaining to subsurface oil and gas with active seismic data or record. In the present invention low frequency (2-5 Hz) resonant seismic data analysis in active seismic data is enough to locate the subsurface oil and gas in terms of surface position and its depth. Analysis of both resonant energy 3 and reflections 8 in one seismic dataset for subsurface oil and gas mitigates the hydrocarbon exploration risk or reduce risk of drill dry well. Proper planning of active seismic survey designing in any study area 15 reduces the seismic exploration risk. Utilization of low frequency resonant energy 3 in the seismic data plays a very major role where seismic reflection image is not good enough to identify the structural and stratigrafic traps that may contain the hydrocarbons.
Figure 01 depicts a schematic diagram illustrating active seismic survey technique where the active seismic source 1 which is in under operator control generates mechanical vibrations at or near the earth’s surface. Seismic waves 7 generated by these sources 1 travel into the Earth’s subsurface and are reflected back from strata boundaries and reach the surface of the earth at varying intervals of time depending on the distance traveled and the heterogeneity of the subsurface. The return waves are detected by the sensors 2 and representations of the seismic waves as representative electrical signals are recorded with recording instrument 20 for processing into images. The Figure 01 further depicts the thick salt/anhydrate strata and thick coal shales 5 etc. and the reflected waves and ray path 4 for all primary body waves. The dotted curved lines represent the low frequency resonant energy 3 from oil which generates seismic profile 6 to the operator.
Figure 02 shows a schematic seismic shot gather (1-125 Hz) that contains direct energy 7, refracted energy 9, reflections 8, resonant energy (2-5 Hz) 3 and ground roll 10 etc. The straight line in Figure 02 represents direct energy 7 which is near offset first arrivals as linear events. The second straight line in figure 02 symbolically represents the refracted energy 9 which is long offset first arrivals as linear events. The curved line symbolically represents reflection energy 8 in range 6 to 125 Hz which is in form of hyperbolas of primary energy from different geo horizons. The straight line 10 symbolically represents ground roll as linear events. The dotted lines here represents the low frequency resonant energy 3 (2-5 Hz) which is the hyperbolic events after all the primary waves are gone as hidden energy.
Figure 02 diagrammatically shows that when the frequency of external force (seismic source acts as driver) interact with low velocity media (multi-fluid phase hydrocarbon strata) in the subsurface, a part of seismic energy is trapped in hydrocarbon strata and generates circumferential waves (surface types of waves) in hydrocarbon reservoir. These circumferential waves move with a particular frequency further act as secondary / buried source (driver) at the hydrocarbon reservoir. If the frequency of these circumferential waves meet with natural frequency of hydrocarbons (driven), then the resonance phenomena occurs. In the state of resonance, there occurs maximum transfer of energy from driver to driven and the amplitude of motion reaches to maximum at or proximity of the reservoir. These resonant seismic emission 3 spreads out spherically in all directions from the multi-fluid phase strata which travel as low frequency, low velocity (Phase Velocity) of common body waves and appear as hyperbolic events at later part of raw seismic shot gathers (after all primary waves are gone).
Figure 03 represents a schematic seismic shot gather after applying auto gain correction and filtering techniques. It is shown in the Figure 03 that the resonant energy 3 appears at late part of seismic record after all primary waves gone.
Figure 04 depicts the common mid source (CMS) gathers. The offset versus time plot is prepared which helps to decide the apex location of hyperbolic events and compute phase slope or velocity. Lateral position 11 of resonant source (oil and gas) is obtained by the hyperbolic events. Therefore the apparatus of present invention generates common mid source (CMS) gathers over oil and gas reservoirs.
Figure 05 represents the CMS gather, Move out gather and Stack 12 for interpretation of subsurface oil gas in terms of its surface position and depth. Centre point of maximum amplitude zone 13 in stack image 12 in depth domain corresponds to close to paying quantities of subsurface oil and gas. The aforesaid method is performed by the apparatus of the present invention to move-out the common mid source CMS gathers with phase velocity.
Figure 06 shows the tentative planning of 2D Seismic survey in any new area 15. The Figure 06 depicts 2D Seismic profile 1 unit × 1unit grid 14 as square boxes.
Figure 07 represents the symbolic 2D seismic survey where the dots represent the low frequency resonant 3 anomaly along the seismic profiles 6 (North-South). The Figure 07 as an exemplary embodiment depicts the two dominant low frequency resonant zones at west part of the study area as dotted circle.
Figure 08 illustrates the low frequency resonant anomaly along 2D Seismic lines 6 (East-West direction). The Figure 08 as an exemplary embodiment depicts the two dominant low frequency resonant zones at west part of the study area as dotted circle.
Figure 09 represents the merging of low frequency resonant anomaly along 2D Seismic lines 6 of Figure 07 and Figure 08. It illustrates the validating similar and same type of low frequency resonant energy 3 in seismic data that would be acquired mutually perpendicular direction of previous seismic data and map the apex locations of hyperbolic events on the base map. The mapping of apex locations comprises forming a polygon by joining closely spaced resonant events on the base map and determining the centre point of the said polygon. Thus, the apparatus and the method of the present invention determines the subsurface oil and gas in terms of right surface position (Latitude and longitude or easting and northing) and depth.
Figure 10 depicts the tentative planned 3D seismic survey over low frequency energy 3 covered zones in the study area 15. The method applied in the present invention reduces seismic exploration risk of the study area by around 60%. The dotted circle in the Figure 10 represents the seismic reflection image (structural high) where the dotted rectangle 16 here represents the Seismic 3D block. The present invention can also utilize for both resonant energy 3 and reflection energy 8 in the seismic data which reduces the risk of drilling dry well under integrated approach. The apparatus of the present invention discriminates the subsurface oil and gas by determining the centre point of maximum amplitude zone 13 in the stack image 12 in depth domain as shown in Figure 5.
Figure 11 and Figure 12 shows the step wise procedure involved in the active seismic survey the mapping of low frequency resonance spots in the base map.
The embodiments of the present invention utilize broadband sensors in active seismic operations to acquire full wave (Primary and Secondary wave reflections; PP, PSh & PSv reflections) information. In the present invention full wave seismic data acquisition is used where seismic record length is from 6 sec to 18 sec to get the subsurface image till the basement depending on the depth range of hydrocarbons. The seismic record length of 10-12 sec is enough to record low frequency, low velocity resonant seismic energy (after all primary waves gone) that is coming from hydrocarbon reservoir. For example for hydrocarbons at depth range 800 m to 1500 m, 6 sec seismic record length is required and for hydrocarbons at depth range 1500 m to 3500 m, 6 sec to 12 sec seismic record length is required. Similarly, for hydrocarbons at depth range 3500m to 6000 m, 12sec to 18 sec seismic record length is required.
In one of the aspect of the present invention low frequency resonant seismic data attributes is extracted, at around 2 Hz to 5 Hz frequency as hyperbolic events at late part of the seismic records after all primary waves are gone by using filters, from recorded active seismic data that contains in general dominant seismic reflection energy at around 6Hz to 125 Hz frequency and the mutually perpendicular apex locations of resonant hyperbolic events containing unique velocity or phase slope are mapped from said seismic records on a base map and the subsurface oil and gas is identified/discriminated in terms of surface location and depth by stack imaging in time and depth domain at phase velocity as hyperbolic events at later part of said seismic records.

Advantages of the invention:
The present invention provides use of low frequency seismic data (2-5 Hz) in active seismic data which has the following advantages:
a) Low frequency data analysis can be applied to all type of Sensors employed in the industry for hydrocarbon exploration.

b) Legacy dataset may also be reviewed. For shallow hydrocarbon prospects (1500 m thick sediments from surface), such seismic datasets can be analyzed by the present invention for their low frequency potential in discrimination and identification of oil and gas deposit.

c) Plays a very key role where seismic reflection image technique is not good enough to identify the structural and stratigrafic traps that may contain the hydrocarbons.

d) Reduces the present seismic exploration risk.

The present invention is now illustrated by way of non limiting examples. The invention is here described with reference to the following example which relates to an experiment. It will be appreciated that the invention as claimed is not intended to be limited in any way by the following examples.
Although the invention has been described with reference to particular examples of the invention, it should be appreciated that it may be exemplified in other forms. The invention qualifies to be adopted in a variety of other embodiments such modifications and alternatives obtaining the advantages and the benefits of the present invention will be apparent to those skilled in the art. All such modifications and alternatives will be obvious to a person skilled in art.
The description herein contains many specifics, these should not be construed as limiting the scope of the invention, but as merely providing illustrations of some of the embodiments of the invention. One of ordinary skill in the art will appreciate that elements and materials other than those specifically exemplified can be employed in the practice of the invention without resort to undue experimentation. All art-known functional equivalents, of any such elements and materials are intended to be included in this invention. Numerous variations, changes and substitutions may be made without departing from the invention herein.