Offshore Krishna-Godavari Basin Research Articles

Characterising the effect of gas hydrate in pore pressure and stress using NGHP-02 downhole logs in Krishna Godavari offshore Basin, India

The present study demonstrates the impact of gas hydrate concentration and its morphology on pore pressure and sediment stress. We analyse the well-logging data of the 2nd Indian National Gas Hydrate Program (NGHP-02) in Area B of the Krishna Godavari Basin. Overpressure zones are delineated by observing porosity with depth and log inferred compaction trends. Eaton, Bower, Miller and porosity-based models are used for predicting pore pressure. The predicted pore pressures are ranging between ∼3 and 4.5 MPa, and ∼2.8 and 4.4 MPa respectively at Hole 02–22A and 02–23A, where the estimated fracture pressures and horizontal stresses are nearly equal to vertical stress at both holes. Within the gas hydrate stability zone (GHSZ), overpressure is not found; rather, our findings show that pore pressure falls with increasing gas hydrates saturation at both holes. This could be due to a decrease in water concentration during hydrate formation. Consequently, the effective stress of sediments increases, causing gas hydrate-filled vertical fractures to heave apart in the minimum horizontal stress directions. We observe overpressure below the gas hydrate layers at two holes, which is due to the load of the overlying sediments, low permeability created by the high concentration of gas hydrate at the bottom of GHSZ (about 40% and 80% at Hole 02–22A and 02–23A, respectively), and may be the microbial gas being produced at a faster rate than they are being expelled due to the low permeability. It is noticed that the strike directions of fractures vary with depth at both holes, which shows the anisotropic effects of stress conditions on gas hydrate-filled fractures during the time of formation.

A Robust Mechanistic Model for Pore Pressure Prediction from Petrophysical Logs Aided by Machine Learning in the Gas Hydrate-Bearing Sediments over the Offshore Krishna–Godavari Basin, India

A Robust Mechanistic Model for Pore Pressure Prediction from Petrophysical Logs Aided by Machine Learning in the Gas Hydrate-Bearing Sediments over the Offshore Krishna–Godavari Basin, India

Modeling of borehole washout effects and gas hydrate-filled fractures using NGHP-02 downhole data in Krishna Godavari offshore basin, India

Modeling of borehole washout effects and gas hydrate-filled fractures using NGHP-02 downhole data in Krishna Godavari offshore basin, India

Joint inversion of resistivity and sonic logs using gradient descent method for gas hydrate saturation in the Krishna Godavari offshore basin, India

Joint inversion of resistivity and sonic logs using gradient descent method for gas hydrate saturation in the Krishna Godavari offshore basin, India

Modeling of seismic attenuation in fracture-filling gas hydrate-bearing sediments and its application to field observations in the Krishna-Godavari Offshore Basin, India

Understanding the attenuation mechanism in gas hydrate-bearing sediments (GHBS) is essential for accurate quantification of natural gas hydrates, yet previous rock physics models fail to explain the observed attenuation suppression at seismic frequencies when hydrates occur as fracture-filling morphology. Here we present a new model to decipher the viscoelastic performance of fracture-filling GHBS, by treating it as a composite of host sediment and pure gas hydrate. We first characterize the attenuation in the host sediment without hydrate by the viscous fluid flow in swelling clay minerals. Then, based on laboratory data, we quantify the mechanical performance of pure gas hydrate, exhibiting that hydrate is a consolidated aggregate with properties different from unconsolidated host sediment. After incorporating fracture-filling gas hydrates into the host sediment, the properties of host sediment are significantly modified and the resulting model reproduces the suppressed attenuation observed in field. We apply this model to the field attenuation extracted from multichannel seismic data in the Krishna-Godavari Offshore Basin in India, where gas hydrate was found filled in fractures. The results show that the attenuation and hydrate concentration predicted by our model agree well with the field measurements, which suggests the promise of our model in detection and quantification of nature gas hydrates in clay-dominated sediments using seismic data.

Rock Physics Modeling of Gas Hydrate Reservoirs Through Integrated Core and Well-Log Data in NGHP-02 Area in KG Offshore Basin, India

Since gas hydrates are unconventional reservoirs, they pose unique challenges for identification, characterization, quantification, and extraction. The conventional approach—elastic logs—can provide a better method for identification through attribute analysis. On the other hand, geomechanical studies for wellbore stability analysis pave the way for the effective exploitation of gas hydrates. It is crucial to predict elastic logs against gas-hydrate-bearing sediments, which requires an effective rock physics model. In the present work, a study pertaining to the National Gas Hydrate Program-02 (NGHP-02) campaign in the Krishna‐Godavari (KG) Offshore Basin, India, where gas hydrates are deposited primarily in two facies—a shale-dominated shallower one and a sand-dominated deeper one that has been identified by responses of conventional and spectroscopy logs—is discussed. It is commonly known that depositional heterogeneity impacts petrophysical and elastic properties. To address this issue, an innovative approach has been adopted to model compressional and shear log data using rock physics modeling of gas hydrate reservoirs based on the depositional type of gas hydrate. Guidance from the change of compressional velocity data from log and core with an increase of gas hydrate saturation shows gas hydrate deposition in the study area can be explained through a matrix/grain-supported model. The Jason grain-supported rock physics model appeared best suited among different available rock physics models, depending on the clay volume and porosity in our study area. Using input from a robust multimineral petrophysical evaluation and rock physics modeling, the finalized model is propagated to test wells for predicting compressional, shear, and density logs, with the predicted data validated by core-measured compressional and shear data. Model consistency is indicated by a high correlation from multiwell crossplots of modeled and recorded elastic logs (compressional and shear velocity) with acoustic impedance. The developed rock physics model better discriminates gas hydrate in the shaly sand layer and gas hydrate in the sand-dominated layer, calcite, and shale in the VpVs domain.

Biogeochemistry and trophic structure of a cold seep ecosystem, offshore Krishna-Godavari basin (east coast of India)

Active cold seep sites associated with shallow gas hydrates were recently reported from Krishna-Godavari (K-G) basin, off east coast of India. Benthic fauna including chemosymbiotic bivalves (genera Bathymodiolus, Acharax, Conchocele), polychaetes (genus Sclerolinum), and heterotrophs (gastropods: Provannidae family; squat lobster: Munidopsidae/Galatheidae; Goose barnacle: genus Neolepas; Brittle star: genus Amphiodia) were sampled from the cold seep locations. Here, we present the first δ13C and δ15N data sets of the soft tissues extracted from the organisms along with the sediment pore fluid composition. The isotope ratios of the soft tissues provide a glimpse of the potential food sources for the cold seep organisms and the trophic structure of the ecosystem. The δ13C values suggest the dependence of the Bathymodiolus on methanotrophic symbionts, whereas Acharax, Conchocele, and Sclerolinum show the tell-tale signature of the thiotrophic pathways. In contrast, the heterotrophic benthic fauna shows a wide range of δ13C values, a signature of a variety of carbon sources. The δ15N values of the studied organisms also show a broad range (−1 to +9 ‰) indicating variable food sources as well as variable nitrogen assimilation pathways followed by various symbionts.

Estimation of porosity and gas hydrate saturation by inverting 2D seismic data using very fast simulated Annealing in the Krishna Godavari offshore basin, India

ABSTRACTGas hydrate saturation and porosity are the two essential parameters for characterizing a gas hydrate reservoir. Generally, porosities determined at the well locations are interpolated and extrapolated over the seismic volume, which is not so appropriate to get an estimate of gas hydrate saturation. Here, we propose a method to predict both porosity and gas hydrate saturation directly by inverting seismic data using a global optimization technique known as Very Fast Simulated Annealing. Acoustic impedance, as a function of water saturation and porosity, is defined by a second‐degree polynomial equation used as a forward problem. These are used as the primary model parameters. Acoustic impedance derived from these values is then used to compute synthetic seismograms that are compared against observed traces. We demonstrate our approach on a 23.5 km two‐dimensional seismic line crossing four wells in the Krishna Godavari Basin, eastern Indian offshore. First, the proposed method is tested on well log data and then applied on post‐stack seismic data. The posterior probability density function and the correlation matrix between the model parameters (gas hydrate saturation and porosity) are calculated to measure the associated uncertainty in prediction. Inversion results illustrate that the approach can be efficiently used for the prediction of porosity and gas hydrate saturation directly from the seismic data.

Machine learning in the classification of lithology using downhole NMR data of the NGHP-02 expedition in the Krishna-Godavari offshore Basin, India

We apply machine learning techniques to classify the lithology using downhole nuclear magnetic resonance (NMR) data of gas hydrate reservoirs in the Krishna-Godavari (KG) Basin, eastern Indian offshore. We choose two sites of the second expedition of the Indian National Gas Hydrate Program (NGHP-02) for our study, one with clay dominated site NGHP-02-01 from Area E and another with silt/sand dominated site NGHP-02-05 from Area C. We obtain five distinct classes in NMR signals at two sites using unsupervised techniques such as Davies-Bouldin index, silhouette, self-organizing map, k-means clustering and emergent self-organizing map. Obtained classes are interpreted using the NMR T2 distribution pattern with time in terms of clay, silt, sand and pebble/gravels with depth and reconciled with gamma-ray, density, porosity, resistivity, velocity and photoelectric factor. Our results demonstrate the lithology as silty-clay with minor sand at Hole NGHP-02-01A, and silt, sand with less clay and minor pebble/gravel at Hole NGHP-02-05A. The presence of gas hydrate is identified by observing an overall low amplitude T2 distribution curve, which is due to the reduction of pore space by solid gas hydrate. The maximum concentration of gas hydrate of about 43% of pore space estimated using NMR- and density-porosity are distributed mainly in silty-clay at Hole 01A and of about 75% in clayey-silt and silty-sand at Hole 05A. Low amplitude NMR T2 signals observed in clay-dominated sediments at higher relaxation time may be due to the fracture-filled gas hydrates at both holes. Interestingly, high porosity (90–95%), high permeability (4 mD), low bulk-density (1.2 gm/cm3) and low photoelectric factor (0.8 b/electron) are found in sandy layers within the gas hydrate-stability zone at Hole 05A, which possibly comprising buried channels. Comparison of the lithology obtained from other log data with that from NMR data and core samples illustrates that the NMR data are most effective for characterization of reservoirs, except for a high concentration of gas hydrate in a sandy/coarse-grained reservoir, which needs further studies.

Subsurface porosity estimation: A case study from the Krishna Godavari offshore basin, eastern Indian margin

Porosity is one of the fundamental petrophysical parameters essential for appraisal of hydrocarbon reserve and planning production operations. Several interpretation techniques have evolved over time that allows geologists to meticulously estimate reservoir properties by integrating seismic and borehole data. The present study investigates the use of Multi-Attribute Transform Method (MATM) and Artificial Neural Networks (ANN) for the delineation of lateral variation of porosity along a 2D seismic reflection dataset in the gas hydrate inferred area of Krishna–Godavari basin, eastern Indian margin. The best set of attributes that can predict the porosity are derived using multi-attribute regression analysis. These attributes are applied in MATM and are also used to train the neural networks. This is achieved using different networks of neural computation like Probabilistic Neural Network (PNN), Multi-Layer Feedforward Network (MLFN) and Radial Basis Function (RBFN) with an aim to decipher an optimized approach best suits to predict porosity of the study area. The correlation between observed and predicted porosity logs are tested both by cascading and non-cascading the networks with the MATM. The predictive power is found improved as we progress from MATM to neural network cascaded with the trend of MATM. Finally, it is observed that PNN cascaded with the results of MATM shows the least prediction error than the other networks.

Amount of gas hydrate estimated from rock physics analysis based on morphology and intrinsic anisotropy in area B, Krishna Godavari offshore basin, expedition NGHP-02

India's National Gas Hydrate Program (NGHP) Expedition-02 was conducted in 2015 to investigate the prospective locations of sand-rich gas hydrate reservoirs along the eastern offshore, India. We select Area-B among four areas A, B, C and E of expedition NGHP-02 as it was completely unexplored during NGHP-01. Logging while Drilling (LWD) data were collected at twelve sites in area B in Krishna Godavari (KG) offshore basin, where a huge amount of gas hydrate (up to 90% of the pore volume) is present as pore-fill, fracture-fill and both pore- and fracture-filled (mixed) morphology. The resistivity log generally overestimates gas hydrate saturation because of the increase of resistivity of fresh water near gas hydrate accumulation and also in the presence of fractured-filled gas hydrate. Though gas hydrate saturation obtained from pressure cores are assumed to be ground truth but they are discrete. Hence, the application of an advanced rock physics model is required to obtain continuous quantification of gas hydrate throughout the depth considering the gas hydrate morphology and microstructure of sediment. We apply a combination of self-consistent approximation (SCA) and differential effective medium (DEM) theories to model biconnected marine gas hydrate reservoir that can deal with inherent anisotropy due to the presence of clay platelets and pore-filling, fracture-filling or mixture of both morphologies of gas hydrate. First, we determine the distribution pattern of clay platelets from the observed background velocity (water-saturated sediment) and then estimate the gas hydrate saturation at four holes of area B, where pressure core measurements are available. We predict maximum gas hydrate saturation as 50–90% of the pore space above the BSR in this area. Comparison with other rock physics model shows that prediction from the combined SCA-DEM theory matches better with the pressure core measurements.

Widespread occurrence of methane seeps in deep-water regions of Krishna-Godavari basin, Bay of Bengal

Drilling/coring activities during NGHP-Expeditions 01 and 02 discovered significant gas hydrate deposits in the Krishna-Godavari (KG) basin, a proven petroliferous basin located along the eastern continental margin of India. Active and paleo-cold seeps are also reported from deep waters (>1000 m), indicating methane venting episodes in the KG basin. In the present study, we analyzed geophysical data acquired onboard RV Sindhu Sadhana to study the spatial distribution of gas flares in the KG offshore basin. Twenty-two gas flares were observed in water-column images at seafloor depths ranging from 638 m to 1960 m. All flares except one are detected within the gas hydrate stability zone (GHSZ), which occurs beyond 700–720 m water depths. Most gas flares seem to terminate at these depths, indicating that the bubbles might be coated with a thin methane hydrate skin. Seabed sampling of flare regions shows the presence of chemosynthetic communities and shallow gas hydrates within 2–4 mbsf.The spatial distribution of gas flares is not random and tends to align with the toe-thrust zones and shale diapiric mounds. These compressive structures formed due to shale tectonism in the KG basin have provided an environment conducive for the formation of gas hydrate deposits and gas flares/cold seeps. The analysis of high-resolution seismic data across the flares shows the presence of seismic chimneys and faults, which facilitated migration of deep-seated fluid/gas through the GHSZ. Focused fluid flow along the fault zone and the perturbation of methane + seawater phase curve due to salinity increase owing to the continuous formation of methane hydrate are the most likely mechanisms to explain methane gas migration.

Spatial distribution of gas hydrate deposits in Krishna-Godavari offshore basin, Bay of Bengal

To delineate the occurrences of gas hydrates and free gas in the Krishna-Godavari (KG) basin, Bay of Bengal, we used 3D seismic data to construct a velocity model of high spatial density. We used an automated version of migration velocity analysis to estimate P-wave velocities using Kirchoff prestack time migration. We manually checked all common image gathers (CIGs) to assess the reliability of the auto-picking algorithm. The velocity model was validated against available downhole information from drilling Sites NGHP-01-02, NGHP-01-03, and NGHP-01-10. The model shows a) anomalously high velocities at Site NGHP-01-10 where significant gas hydrates were reported and b) background velocities at Sites NGHP-01-02 and NGHP-01-03 where a negligible amount of hydrate was recovered. A bottom simulating reflector (BSR) is identified in the 3D seismic data, and an average velocity map is prepared above the BSR. Background velocities of marine sediments in the KG basin vary from 1.43 km/s to 1.6 km/s; velocities higher than 1.65 km/s indicate gas hydrate bearing sediments while velocities lower than 1.43 km/s indicate free gas bearing sediments. High velocities observed below seafloor mounds and ridges suggest the presence of gas hydrate in a structurally deformed area. Slope basins are generally devoid of gas hydrates. Free gas is observed at depths of up to several hundred meters below the BSR suggesting the migration of deeper fluid/gas into the gas hydrate stability zone (GHSZ). Zones of gas hydrates and free gas are preferentially oriented along major faults indicating the role of faults in the transportation of free gas and formation of gas hydrates deposits. We identify a free gas bearing horizon at depths (>300 m) below the BSR, which might contribute to the lateral migration of free gas. Gas hydrate deposits in KG basin are classified into two categories: i) leaky deposits which occur in the areas of active seafloor faults, and are characterized by heterogeneous distribution of gas hydrates, and ii) non-leaky deposits which are formed in areas of subsurface faults, and show a relatively homogeneous distribution of gas hydrates. Site NGHP-01-10 is a good example of leaky gas hydrate deposits above a deep-seated gas reservoir.

Seismic evidence of free gas migration through the gas hydrate stability zone (GHSZ) and active methane seep in Krishna-Godavari offshore basin

Abstract Seismic amplitude versus offset (AVO) analysis was performed on the multichannel seismic data in the vicinity of Site NGHP-01-10, where thick fracture-filled gas hydrate is reported, to understand the fluid migration in fracture dominated gas hydrate system. Amplitude preserved common image gathers (CIG) are used in extracting the seismic amplitude with offset for seafloor, bottom simulating reflectors (BSR), and different horizons within the gas hydrate stability zone (GHSZ). The extracted amplitudes are fitted with three-term Shuey's AVO approximation to estimate the intercept and gradient values. Based on the intercept and gradient values, two distinct classes of seismic AVO responses are observed; class IV AVO (decreasing negative amplitudes with increasing offsets) and class III AVO (increasing negative amplitudes with increasing offsets) for the BSR and other horizons above the BSR. The large negative intercept and gradient values which indicate class III AVO ranges from −0.2 to −1.8 for the inline seismic profile and from −0.01 to −0.3 for the crossline profile. The presence of class III AVO anomaly within the gas hydrate stability zone (GHSZ) provides evidence of existence of free gas which may be migrating along the fault system. Some of the active faults are exposed on the surface and show gas venting signatures such as gas-flares and pockmarks in the multibeam data.

Characterization and rheology of Krishna-Godavari basin sediments

Abstract The Krishna Godavari offshore basin to the east of India is a proven reserve, rich in natural gas hydrate. The impact of sediment characteristics on the hydrate formation has already been well established in open literature. In the present study, we have attempted to investigate the mineralogy of sediments collected from KG basin by elemental analysis (energy dispersive spectroscopy) and x-ray diffraction. Various physicochemical characteristics of the clayey sediments such as TDS, salinity, pH, conductivity and resistivity were analysed at various concentrations and compared together. Also, this paper highlights the rheological behavior of the sediment samples with concentrations of 20, 35 and 50 wt% at different experimental temperatures (278.15 K, 283.15 K and 288.15 K). Viscosity measurements were performed for a wide range of shear rates for all concentrations and comparative studies have been conducted on their exhibited behavior. The viscosity of sediment sample were found to be varying from 76.3 to 0.003 Pa s depending on the sediment concentration, temperature and shear rate. In addition, viscoelastic measurements were carried out at various angular frequencies for all the sediment samples. The work aims to characterise the sediments of KG basin and analyse the rheological behavior of sediment solution which has not yet been reported in open literature. This will provide vital information for possible methane recovery from KG basin hydrate reservoir by manipulating the host sediment behavior.

Lithofacies characterization of gas hydrate prospects discovered during the National Gas Hydrate Program expedition 02, offshore Krishna-Godavari Basin, India

Abstract Lithofacies of host sediments associated with gas hydrate reservoirs may have a significant influence on the formation of hydrates. The type of sediments and the depositional processes responsible for the gas hydrate-bearing sediments along the eastern margin of the India have been studied by integrating core data and logging-while-drilling downhole logs. Borehole data correlated with seismic data were used for the interpretation of geological process responsible for gas hydrate formation. The acquired seismic profiles were correlated with the downhole logs by identifying the shape of the well logs curves that indicate finning up and blocky patterns. Turbidite sands and debris flows were also identified from the log and core sediment studies. This study identified two prominent types of depositional systems: (1) turbidity currents and (2) debris flows were mainly responsible for the deposition of gas hydrate-bearing reservoirs in the study area. Based on these studies two well-developed gas hydrate systems have been identified in the Krishna-Godavari Basin and hydrates were determined to be distributed mostly in sand dominated facies. Fine-grained sand in the identified turbidite deposits were shown to be a good hydrate reservoir. The hydrate-bearing sediments in the central part of the Krishna-Godavari Basin (Area C in D6 block) consist of mostly debris flows, which is consistent with scattered gravel- and sand/silt-sized grains in channel-levee systems.

Downhole physical property-based description of a gas hydrate petroleum system in NGHP-02 Area C: A channel, levee, fan complex in the Krishna-Godavari Basin offshore eastern India

India's second National Gas Hydrate Program expedition, NGHP-02, collected logging while drilling and sediment core data in Area C offshore eastern India, to investigate controls on the distribution and peak saturations of methane gas hydrate occurrences in buried channel, levee and fan deposits. Physical property results are presented here for the four Area C coring sites: NGHP-02-07, targeting an upper continental-slope channel deposit; NGHP-02-08 and −09, targeting levee deposits on either side of a channel further downslope, and NGHP-02-05, targeting a sequence of fan deposits extending out from the slope base. Coarse-grained sediment exists at each site, but site-specific differences in clay distribution provide significant controls on the gas hydrate distribution and saturation. At NGHP-02-07, only the upper ∼4 m of a ∼42 m-thick, relatively low clay-content, coarse-grained interval is inferred to be gas hydrate-bearing. NGHP-02-07 has a relatively thin, high-permeability overburden seal, and methane-rich fluid likely leaks from the primary reservoir. NGHP-02-08's levee deposit seal is similarly permeable near the reservoir, but becomes less permeable toward the seafloor. Relative to NGHP-02-07, methane is retained more effectively in the NGHP-02-08 reservoir, but that reservoir is interbedded with layers of high clay-content, low gas hydrate saturation sediment, limiting the maximum gas hydrate content for NGHP-02-08. NGHP-02-09, the second levee deposit site, has a thicker, less permeable overburden than NGHP-02-08, combined with >50 m-thick, low clay-content reservoir sediments. Correspondingly, NGHP-02-09 has a thicker gas hydrate-bearing reservoir with consistently higher gas hydrate saturations than NGHP-02-08. NGHP-02-05 has abundant coarse-grained material spread over nearly the entire drilling interval, but the sediment is poorly sorted. Gas hydrate is distributed among several primarily coarse-grained layers, but gas hydrate saturations are limited by relatively high clay contents, and an overlying seal that is too thin and permeable to effectively retain methane in the reservoir. • Clay content provides a primary control on the distribution and peak saturation of gas hydrate. • Gas hydrate was not observed to form in the coarse-grained, primary reservoir where the clay fraction exceeded ~0.3. • Overlying fine-grained sediment is a more effective low-permeability seal than the gas hydrate-saturated reservoir is. Imperfect seals in Area C can be linked with incomplete occupation of the coarse-grained sediment by gas hydrate.

Integration of mini-frac stress results with core data to identify ductile behavior of overpressured sandstone reservoir in deep-water offshore Krishna-Godavari basin, India

Insitu reservoir stress measurements are essential from geomechanics perspective to efficiently develop an oil and gas field. Wireline conveyed mini-frac tests gives accurate measurements of minimum horizontal stress. Wireline conveyed mini-frac test was performed in the clastic cretaceous syn-rift high pressure (9000 psi) gas bearing sandstone reservoir of an offshore deep-water well (water depth—1108 m) in Krishna-Godavari basin of East Coast off India in order to determine the insitu reservoir stresses. All the mini-frac tests attempted within the syn-rift reservoir were unsuccessful to achieve the formation breakdown pressure even with the applied pressures significantly higher than the vertical stress. This paper discusses the post-job investigation of all these unsuccessful mini-frac tests within the reservoir. Distinct large plastic deformation behaviors were identified in all these test. This investigation in conjunction with rock mechanics data derived from core samples suggest that strong relationship exist between pore fluid pressure and rock mechanical properties. High pore fluid pressure within the reservoir has made sandstone rocks mechanically weak and ductile in nature. The distinct large plastic deformation behaviors are the manifestation of this reservoir ductility. Awareness and evaluation of rock ductility in high overpressured reservoir condition is necessary during planning stages of mini-frac stress testing job. Additionally, paper shows that the rock mechanical properties plays significant role on rock deformation mode within reservoir with pressure depletion during production phase. Paper demonstrates the effective use of rock mechanical properties to predict the brittle–ductile (B–D) transition value in Q–P space (differential stress vs. mean effective stress), which will help in forecasting the reservoir deformation mode during field production stage. Furthermore, the paper highlights the importance of performing critical laboratory tests to aid significant value in field development program from geomechanics perspective.

Acoustic reflections in the water column of Krishna-Godavari offshore basin, Bay of Bengal.

Seismic oceanographic studies from various oceans worldwide have indicated that the acoustic reflections are mostly observed along thermal boundaries within the water column. However, the authors present a case study of seismic data from Krishna-Godavari Basin which shows that salinity variations also play an important role in the occurrence of water column reflections. The observed reflection is modeled using the reflectivity series derived from the salinity and temperature profiles from a nearby Conductivity-Temperature-Depth (CTD) location. Sensitivity analysis of temperature and salinity on soundspeed shows that the effect of salinity cannot be ignored for modeling acoustic reflections. The synthetic seismogram matches well with the observed reflection seismic data. Remarkable similarities between the reflection seismic and the salinity profile in the upper thermocline suggest the importance of salinity variations on the water column reflection. Furthermore, impedance inversion of the reflectivity data reveals several thermohaline structures in the water column. The origin of these thermohaline structures is largely unaddressed and may be attributed to the fresh water influx coming from Himalayan and Peninsular rivers or due to the presence of different water masses in the Indian Ocean which warrants a detailed study using concurrent seismic and CTD data.

Basement configuration of KG offshore basin from magnetic anomalies

Marine magnetic anomalies along three representative profiles falling between shelf break and continent–ocean boundary in the offshore Krishna–Godavari basin were quantitatively interpreted for understanding the nature and structure of the magnetic basement using inversion technique. The interpretation of the anomalies shows that the magnetic basement lies deeper than the base of the sediments, i.e., acoustic basement identified by the seismic studies. This interpretation also shows that the magnetic basement is faulted along the NW–SE direction with the upthrown side lying to the north of the anomaly trend of this region. The coincidence of magnetizations observed through the present interpretation with that of charnockites of neighbouring EGMB and onshore K–G basin areas indicates that EGMB geology (charnockites, granitic gneiss, etc.) extends up to COB in the offshore K–G basin.