2-D velocity structure in Kerala-Konkan basin using traveltime inversion of seismic data

JOURNAL GEOLOGICAL SOCIETY OF INDIAVol.79, January 2012, pp.53-60

0016-7622/2012-79-1-53/$ 1.00 © GEOL. SOC. INDIA

2-D Velocity Structure in Kerala-Konkan Basin UsingTraveltime Inversion of Seismic Data

PRAVEEN KUMAR SINGH and KALACHAND SAIN

CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad – 500 007Email: [email protected]

Abstract: The existence of gas-hydrates in marine sediments increases the seismic velocity, whereas even a smallamount of underlying free-gas reduces the velocity considerably. The change in velocities against the background (withoutgas-hydrates and free-gas) velocity can be used for identification and assessment of gas-hydrates. Traveltime inversionof identifiable reflections from large offset multi channel seismic (MCS) experiment is an effective method to derive the2-D velocity structure in an area. We apply this method along a seismic line in the Kerala-Konkan (KK) offshore basinfor delineating the gas-hydrates and free-gas bearing sediments across a bottom simulating reflector (BSR). The resultreveals a four layer 2-D shallow velocity model with the topmost sedimentary layer having velocity of 1,680-1,740 m/s andthickness of 140-190 m. The velocity of the second layer of uniform thickness (110 m) varies from 1,890 to 1,950 m/s.The third layer, exhibiting higher velocity of 2,100-2,180 m/s, is interpreted as the gas-hydrates bearing sediment, thethickness of which is estimated as 100 to 150 m. The underlying sedimentary layer shows a reduction in seismic velocitybetween 1,620 to 1,720 m/s. This low-velocity layer with 160-200 m thickness may be due to the presence of free-gasbelow the gas-hydrates layer.

Keywords: Gas-hydrates, Free-gas, Velocity, BSR, Traveltime inversion.

INTRODUCTION

Gas-hydrates are ice like crystalline substance composedof water and light hydrocarbons (mainly methane), foundin shallow sediments in the permafrost and the outercontinental margins of the world where methaneconcentration exceeds the solubility limit (Sloan, 1998; Paulland Dillon, 2001; Taylor and Kwan, 2004). The occurrenceof gas-hydrates is often inferred from the identification ofan anomalous reflector, known as the bottom simulatingreflector (BSR) on seismic section (Shipley et al. 1979).The presence of gas-hydrates increases the seismic velocityof sediments, whereas presence of free-gas below thehydrate-bearing sediments decreases the seismic velocity(Lee et al. 1996; Helgerud et al. 1999; Sain et al. 2000; Geiand Carcione, 2003; Chand and Minshull, 2003). Thischange in velocity, in turn, changes the impedance contrastacross the gas-hydrate/free-gas interface, which gives riseto negative polarity reflection on the seismic section as BSRwith reference to the seafloor reflection event. Distributionof gas-hydrates in sediment may be localized which produceslateral and vertical velocity variations (Pecher and Holbrook,2003). Detailed velocity analysis of seismic data constrainsthe extension (Ojha and Sain, 2009) of hydrate- and gas-

bearing sediments as well as estimation of gas-hydrates andfree-gas (Ojha and Sain, 2008; Ojha et al. 2010; Sain andOjha, 2008a; Sain et al. 2010) across the BSR. Hence, adetailed velocity analysis in an area may infer the presenceof gas-hydrates and underlying free-gas zone. A review forthe identification and quantification of gas-hydrates isavailable in recent literatures (Sain and Gupta, 2008; Sainand Ojha, 2008b). Different markers or proxies have beenbrought out in the KK in the western Indian margin by severalworkers (Rao et al. 2001; Satyavani et al. 2002; Shankar etal. 2005; Ojha and Sain, 2007; Sain et al. 2009). Details ofseismic data acquisition and processing are available in theseliteratures. Here we build a 2-D velocity model along aseismic line in the KK basin to demarcate the zones of gas-hydrate- and free gas-bearing sediments where BSR hasalready been identified.

STUDY AREA

The western continental margin of India (WCMI)evolved through rifting of India from Madagascar andSeychelles at late Cretaceous period (Norton and Sclater,1979). Biswas and Singh (1988) identified six major tectonic

JOUR.GEOL.SOC.INDIA, VOL.79, JAN. 2012

54 PRAVEEN KUMAR SINGH AND KALACHAND SAIN

elements in the western margin up to the Arabian abyssalplain. The shelfal part of the margin is divided into fivebasins of which Kerala-Konkan basin is the southernmost.The basin lies in the south-eastern part of WCMI (Fig. 1)and has attracted attention due to its hydrocarbon potential.The basin lies to the south of the Bombay offshore basin,which is the major hydrocarbon producer of India.The resource potential of this basin is speculated to behigh, most of which lies in bathymetry greater than 200 m.It is characterized by a complex ridge–graben regime. Theregional seismic section of Ratnagiri–Kerala shows that thethickness of the sediments gradually increases to 1500–2000m on the Miocene shelf edge – a few kilometres landwardsof the present-day shelf edge. This zone is marked by asudden thickening of sediments up to 3500–4000 m andshows progradational depositional features (Veeraya et al.1998). The shelf edge, which forms a prominent tectoniczone, appears faulted in places with possible deformationof sedimentary cover (Raju et al. 1981). Most of thesediments of the WCMI possess >4% total organic content(Paropkari et al. 1992), have sedimentation rates of 0.44–0.88 mm/yr and a high degree of preservation of organicmatter (Nambiar and Rajagopalan, 1991). The northern partof the basin is known as the Konkan basin and the southernpart is known as Kerala basin. Structural style in Keralabasin is mainly controlled by a NNW-SSE trending faultparallel to the coast line and mostly confined to the shelfalpart and a NNE-SSW fault oblique to the Miocene shelfedge and confined to the basinal part. Well data show thatsedimentation in the basin took place in three phases – the

coarser clastics which is divided into Cochin formation (lateCretaceous-Paleocene) and Kasargod formation (latePaleocene - early Eocene) unconformably overlying Cochinformation. This is followed by the carbonates (early Eocene- middle Miocene) which in turn is overlain by finer clasticsranging from middle Miocene to recent. Shallow seismicdata of the continental slope show a typical sub-bottompenetration of 200–500 m with seaward-dipping clinoforms,outbuilding and up-building.

METHODOLOGY

The reflection multi channel seismic (MCS) data werecollected in the Kerala-Konkan (KK) basin in 1990 for theexploration of hydrocarbons by the Oil and Natural GasCorporation Limited (ONGCL). The data were recorded ona 96 channel Digital Field Station (DFS-V) instrument,giving a maximum foldage of 48, with a maximum offset of2,575 m. The shot and hydrophone group intervals wereboth 25 m, and the data was recorded at 4 ms samplinginterval with streamer depth of 10 m. The air-gun sourcehad a tuned array with total volume of 1,650 cubic inchesand depth of 6 m.

We identify discernible reflected phases on shot gathersof this MCS data, and employ the ray-based traveltimeinversion method (Zelt and Smith, 1992; Zelt, 1999) to thetraveltimes of these phases to derive a 2-D velocity modelalong a seismic line. Each layer of an isotropic 2-D velocitymodel is parameterized by a number of velocity andboundary nodes, which are connected by linear interpolation.A smooth layer boundary simulation is applied to avoid thescattering and focusing effects of ray paths. The numberand position of the velocity and boundary nodes may varyfor different layers. The upper and lower boundary velocitynodes specify the P-wave velocity field within a layer.Traveltimes and their partial derivatives with respect to thevelocity and boundary nodes are calculated, and thetheoretical traveltimes are compared with the observed data.The difference between them is minimized iteratively, untilan acceptable match with a normalized �2 value close to 1is achieved. �2 is a statistical calculation used to test howwell the observed data matches with the calculated responseof the model. The calculated value of �2 is equal to the sumof the squares of the differences between observed traveltimeand calculated traveltime divided by the square ofuncertainty assigned to each traveltime pick. The equationfor ‘n’ number of observed traveltimes is given as

�

Fig.1. Location of the study area showing the seismic line (thin).Data along the small portion (thick solid line) have beenused in deriving 2-D velocity model. The bathymetry withcontour interval of 200 m is also shown.

�� =(Toi – Tci)

2

�i2

n

i=1

1n


2-D VELOCITY STRUCTURE IN KERALA-KONKAN BASIN USING TRAVELTIME INVERSION OF SEISMIC DATA 55

Where, Toi and Tci are the observed and calculatedtraveltime at the ith location, and �i is the uncertainty ormeasurement error at ith traveltime measurement.

The value of �2 greater than one implies that thetraveltime fit is poor and the model is underparametrizedand needs additional parameters (velocity and boundarynodes). The �2 value less than one indicates that the modelis overparametrized. A ‘layer stripping’ modeling approachis applied whereby successively deeper layers aredetermined by holding fixed the parameters of overlyinglayers in a layer-by-layer fashion.

MODELLING

We look into 26 single-ended shots with 250 m spacings,equivalent to 6,250 m of profile length, and identify fivephases including the seafloor reflection and BSR.Traveltimes of these phases from all shots (1st shot at0.0 km, 26th shot at 6.25 km, and in-between other shots at0.25 km intervals) are picked, and 25 ms uncertainties areassigned to these. The assignment of uncertainties issubjective and is based on half (20 ms) of the dominantperiod of the data. Figure 2 shows representative shotgathers that demonstrate various identifiable reflectedphases such as the reflections from the seafloor, BSR, oneabove and one below the BSR. From slope of the time-distance curve of direct arrivals, we find the water velocityof 1450 m/s. The modelling approach is similar to that of

Vijayarao et al. (2007), which have been applied to thefirst arrival traveltime data of closely-spaced single-ended shots of a deep seismic reflection experiment in theMarwar basin of NW Indian shield. To avoid timeconsumption for ray-tracing and computing traveltimes fordirect arrivals, we constrain the velocity of water as 1450m/s and determine the seafloor configuration (thickness ofwater layer) by inverting the traveltimes of the first phase(seafloor reflections) from all shots simultaneously. In thenext step, we keep fixed the water velocity and seafloorconfiguration, and determine the velocity and thicknessvariation of the first sedimentary layer by inverting thetraveltimes of second phase from all shots. Keeping fixedthe model parameters of above two layers, the traveltimesof the third phase from all shots are inverted simultaneouslyto derive the velocity and thickness of the secondsedimentary layer. Similarly, we determine the velocity-structure of the third and fourth sedimentary layers fromthe traveltime inversion of the fourth and fifth reflectedphases, respectively. The traveltime fit and the rays generatedthrough the final velocity model corresponding to all 26shots are shown in Figs. 3-7. Rays corresponding to allobservations have been generated with an RMS traveltimeresidual is 0.021 s and normalized �2 value of 1.087. Forclarity, we display the ray diagrams and traveltime fit atfifth shot intervals. The final 2-D velocity model consistingof water layer and 4 sedimentary layers is shown separatelyin Fig.8.

Fig.2. Representative shot gathers at the 1st, 8th, 16th and 24th shot locations with five identifiable reflected phases. Traveltimes of thesephases from all 26 shots are utilized in deriving the velocity-structure. Seafloor and BSR are marked.



�

�

Fig.3. Rays corresponding to all phases from six shot points (1st, 6th, 11th, 16th, 21st and 26th) through the final velocity model. Comparisonbetween observed (vertical bars) and the theoretical line (black lines) for various reflected phases is shown below.

Fig.4. Rays corresponding to all phases from six shot points (2nd, 7th, 12h, 17th and 22nd) through the final velocity model. Comparisonbetween observed (vertical bars) and the theoretical line (black lines) for various reflected phases is shown below.



�

�

Fig.5. Rays corresponding to all phases from five shot points (3rd, 8th, 13th, 18th and 23rd) through the final velocity model. Comparisonbetween observed (vertical bars) and the theoretical line (black lines) for various reflected phases is shown below.

Fig.6. Rays corresponding to all phases from five shot points (4th, 9th, 14th, 19th and 24th) through the final velocity model. Comparisonbetween observed (vertical bars) and the theoretical line (black lines) for various reflected phases is shown below.



RESULTS AND DISCUSSION

The thickness of the first sedimentary layer varies from140-190 m with velocity ranging between 1,680 m/s to 1,740m/s. The second layer has a velocity of 1,890–1,950 m/sand a uniform thickness of ~110 m. The next deper layerwith high velocity of 2,100-2,180 m/s is interpreted as thegas-hydrate-bearing sediments, the thickness of which variesfrom 100 to 150 m. The fourth layer with a low velocity of1,620–1,720 m/s represents the gas-bearing sediments withthickness varying between 160-200 m.

The seismic velocity of gas-hydrates bearing sediments,reported from different regions of the world, shows a largevariation. The waveform inversion of seismic data in theoffshore Vancouver Island shows a value of 1,700 m/s forthe hydrate layer (Singh et al. 1993). The velocity of 2,500m/s represents the gas-hydrates layer in the Blake Ridgeregion (Rowe and Gettrust, 1993). The velocity estimatedfor the free-gas layer is also found to be varying between1,240 m/s for the Cascadian margin (Mackay et al. 1994) to1,850 m/s in the western Indian margin (Satyavani et al.2002). Cordero et al. (2009) relate 2,200 m/s velocity to thegas-hydrates layer in the Chilean margin, and 1,250 m/svelocity to the free gas zone. Recent study of seismic qualityfactor (Sain et al. 2009) indicates presence of gas-hydratesin the KK basin. Based on proxies such as the pock marks,seismic transparency, mud diapirism, gas-chimneys, etc,Shankar and Sain (2007) are also of the opinion that the

�

Fig.7. Rays corresponding to all phases from five shot points (5th, 10th, 15th, 20th and 25th) through the final velocity model. Comparisonbetween observed (vertical bars) and the theoretical line (black lines) for various reflected phases is shown below.

KK basin has good prospects of gas-hydrates. The drillingand coring results by the Indian National Gas HydrateProgram (NGHP) EXPEDITION 01 (Collett et al. 2008)have confirmed the presence of gas-hydrates in the Krishna-Godavari (KG), Mahanadi and Andaman regions in theeastern Indian margin but could not find any gas-hydratesample in the KK basin. Out of 15 sites drilled in the KGbasin, massive gas-hydrates were recovered in fracturedshale only at site 10. At some sites, gas-hydrates were foundin disseminated form in pore spaces of shales and sands.Gas-hydrates could not be recovered from remaining sitesin the KG basin. Therefore, from drilling at one location(15.3° N and 70.6° E) in the KK basin, which is far away(>150 km) from the present study area, presence of gas-hydrates cannot be ruled out from the entire WCMI. In fact,using multidisciplinary geophysical data, Ramana et al.(2006) showed that both the KG and KK basins areprospective for gas-hydrates.

CONCLUSION

From traveltime inversion of the reflected phases ofmarine MCS data, we derive a four-layered velocity modelof shallow sediments. The maximum velocity of 2,100 m/s-2,180 m/s in the third layer underlain by a considerablylow velocity (1,620 m/s-1,720 m/s) zone represent the gas-hydrates and free-gas bearing sediments respectively.



permission to publish this work. The first author wishes tothank the Council of Scientific & Industrial Research, Delhifor providing him with a Senior Research Fellowship andthe Ministry of Earth Sciences is thanked for sponsoringthe gas hydrate research at NGRI. We also acknowledge theanonymous reviewer for his comments and suggestions toimprove the quality.

The study delineates the lateral variation (100-150 m) ofthe gas-hydrate-bearing sediments followed by 160-200 mthick zone of low-velocity free-gas bearing sediments inthe KK basin.

Acknowledgements: We thank the Director, NationalGeophysical Research Institute, Hyderabad for his

Fig.8. Final velocity model depicting the high velocity gas-hydrate layer and low velocity free-gas layer across the BSR.

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(Received: 22 June 2010; Revised form accepted: 13 May 2011)

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2-D velocity structure in Kerala-Konkan basin using traveltime inversion of seismic data