Comparative Study of Multi-channel and High Resolution Seismic data for Gas Hydrate Exploration in Krishna-Godavari basin, Bay of Bengal.

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1 Comparative Study of Multi-channel and High Resolution Seismic data for Gas Hydrate Exploration in Krishna-Godavari basin, Bay of Bengal. Pawan Dewangan Gas Hydrate Group National Institute of Oceanography, Goa Council for Scientific and Industrial Research (CSIR) Workshop on Shallow water acoustics February 3-5 th

2 Water molecules A Crystalline solid consisting of methane gas molecules, where each molecule is surrounded by a cage of water molecules. Gas molecule 1 m m m 3 + Hydrate Methane Gas Pure Water At Standard Temperature Pressure

3 Gas Hydrate Stability

4 Presence of petroleum and natural gas deposits in Indian offshore enhance the likely occurrence of gas hydrates deposits The initial result demanded a detail experimentation and high resolution surveys in the Indian offshore basins to assess the mechanism of gas hydrate formation and its distribution

5 Potential areas for gas hydrates exploration Probable reserve ~ 1894 x10 12 m 3

6 JOIDES Findings (NGHP Exp. 01) Geo-scientific investigations onboard D/V JOIDES Resolution (2006) at specific locations in collaboration with IODP under the aegis of NGHP KG Basin Andaman Basin Drilling confirmed the presence of gas hydrate deposits in KG, Mahanadi and Andaman Basins

7 Gas Hydrate (white) Krishna-Godavari basin (NGHP Exp-1)

8 Gas Hydrate (white) Krishna-Godavari basin (NGHP Exp-1)

9 Physical property of Gas Hydrates

10 Mount Elbert test site, Alaska Published in GeoHorizon, 2009

11 Velocity of GHBS Velocity increases due to Hydrate Velocity decrease due to Gas The transition of Hydrate-Gas will form a strong reflector (BSR)

12 Comparison of MCS to high resolution sparker data Data used for identification of BSRs Depth of penetration Frequency content Vertical Resolution Diameter of the Fresnel zone, (Velocity=1700m/s; at target travel time= 1s) BSR observed on the seismic data MCS data Upto 10 Km with in the top sediments 5-70 Hz; with dominant frequency of 25 Hz Dominant frequency of ~25 Hz for shallow reflectors with in the gas hydrates zone (VR ~17m) The high resolution seismic (single channel) data The depth of penetration varies from m The frequency bandwidth of the dataset is between Hz ~340 m (25 Hz) ~60m (800Hz) ~85m (400Hz) ~107m (250 Hz) BSRs are continuous and high amplitude, reverse polarity and crosscutting the sedimentary layers Operational cost Exorbitant Economical Dominant frequency of ~800 Hz for shallow (VR ~46 cm), ~400 Hz for intermediate (VR ~92 cm) ~250 Hz for deep (VR ~1.48 m). BSRs are discontinuous and appear as enhanced reflections

13 Geo-Power 10KJ Pulsed Power Supply Operates on 380V AC, 16A, 3 Phase 50/60Hz Generates upto V

14 Geo-Spark 800

15 Krishna-Godavari Offshore Basin

16 NGHP-01-5 Core location BSR AD Multi Channel Seismic section showing BSR. The NGHP core location is shown where the presence of hydrate was confirmed during drilling

17 NGHP-01-5 Core location b c SN21/ Line BSR a High resolution seismic data (SN21/Line 17-18) acquired along the track of MCS. The BSR appear as enhanced reflection in the HRS data. The locations of BSRs in HRS data is confined to the mound regions and show no continuity.

18 Seismic blanking due to gas hydrates a BSR Free gas below the BSR Zoom out of the CDPs from showing the characteristic of BSR. Note the prominent enhanced reflections below the BSR indicative of free gas below the BSR. The seismic blanking is observed in the gas hydrate stability zone above the BSR.

19 b BSR Free gas Zoom out of CDPs from showing the characteristic of BSR. The depth of the BSR observed in the HRS data matches with the depth observed in MCS data. Note that no seismic blanking was observed in this case suggesting disseminated gas hydrate deposits of low concentration.

20 BSR c Zoom out of CDPs from showing the characteristic of BSR in the HRS data. The depth of the BSR observed in the HRS data matches with the depth observed in MCS data.

21 NGHP-01 Site 14 AD BSR MCS Section showing the location of BSR (Line AD-94-39). The line crosses the NGHP core location where the presence of hydrate was confirmed during drilling

22 NGHP-01-Site 14 b SN21/ Line23-24 Amplitude Blanking BS R BS R a High resolution seismic data (SN21/Line 23-24) along the MCS track (AD-94-39). The BSR appear as enhanced reflection in the HRS data. The amplitude blanking is observed above the BSR. The BSR is confined to the mound regions and no continuity observed.

23 Observations BSR appears to be a zone of high reflectivity in high frequency data BSR appears to be of low amplitude and discontinuous in high frequency seismic data?

24 Possible explanation BSR represent a phase transition and may not be a sharp boundary Hydrates above the stability zone or gas below the stability zone may show gradational behavior Hydrates may be scatter rather than a continuous layer

25 a) b) Zero-offset seismic response of the scattered hydrate deposits. Thick solid line represents radius of the Fresnel zone. a, Reference velocity model. b,c,d,e are the unmigrated seismic sections with varied horizontal resolution (governed by Fresnel zone diameter, D) c) D= d) e) Fresnel Zone is defined as the lateral distance over which the reflection signal is averaged. Dewangan et al, 2007

26 Gradational velocity model Chapman (2002)

27 Reasons for weak BSRs (current model) Low BSR amplitudes at high frequency are not because of insufficient power in the HRS source or to scattering or attenuation losses Spence et al suggest that weak BSR are due to Fresnel zone. This is also highlighted in my earlier work (Dewangan et al., 2007) Chapman (2002) BSR amplitude reduction at high frequencies is because of a gradational velocity contrast at the BSR Fink and Spence (1999) suggest that scattering from a rough sea floor or from highly disrupted reflectors below the sea floor may preferentially reduce higherfrequency amplitudes

28 BSR MCS seismic section showing the location of BSR. Continuity of BSR is less pronounced.

29 SN21/Line BSR a The HRS data (SN21/Line15-16) along the MCS track shows BSRs much resolved between CDPS The BSR is seen as enhanced reflection

30 Amplitude blanking Slumping due to gas hydrate thawing? BSR Zoom out of the BSR location. Note the free gas below the BSR and the amplitude blanking due to the presence of hydrates.

31 Major Challenges None of the models predict an abnormal increase in the strength of BSR for high frequency data Better models for seismic reflection at the Base of Gas Hydrate stability zone is required