Subhashis Mallick

Analysis of multipath sound propagation in the ocean near 49° N, 128° W

Downslope sound propagation data collected off the west coast of Vancouver Island, British Columbia for a shallow source and a deep receiver are examined. The approach used in our analysis was, first, to identify ray paths from the source to the receiver and then to model the variation in measured pressure amplitude with range. The bottom interacting rays, bottom penetrating rays, and channel rays with bottom interaction were identified by the travel time differences between different bottom‐bounce paths. Travel times were computed by ray tracing with an algorithm in which the time step along the ray is the independent parameter. Different propagation paths could be identified in different bathymetric regions viz., continental shelf, continental slope, and deep ocean. A ridge at a range of 79 km shadowed some of the propagating rays; ray tracing from the apex of the ridge was used to calculate travel time curves for some of the diffracted arrivals; however, these diffracted arrivals were difficult to iden...

Time-lapse Monitoring Carbon Sequestrated Brine Aquifers- a Feasibility Study

Substantial research efforts are now underway on injecting (sequestrating) carbon dioxide (CO2) into deep saline aquifers. These sequestration efforts require remote monitoring using available geophysical tools to ensure that the sequestrated CO2 is in place and does not disturb the geological integrity of the surrounding rocks. Since seismic method is the only accepted geophysical tool that can potentially image detailed subsurface information to large depths, here we develop a monitoring strategy using seismic data alone. Fluid substitution at different saturations of CO2 in a brine filled aquifer and comparing its elastic properties with the original indicates that the formation density will play the key role in successful monitoring of carbon-sequestrated aquifers. As multicomponent seismic data are more sensitive to subsurface density variations than vertical (P-wave) component data, we believe that multicomponent seismic data are necessary for obtaining an accurate subsurface presequestration model. Multicomponent data are however more expensive than conventional (P-wave) data. Therefore, acquiring multicomponent data both for baseline and for successive monitoring surveys is not cost-effective. Since above-normal pore pressure due to sequestration may fracture the overlying formations, we investigate if microseismic events generated from these fractures could be used for monitoring. Inducing microseismic events with different fault-plane source mechanisms and computing passive seismic responses from them, we find that these computed responses are sensitive to the fracture fault plane geometry, and passive seismic data could be a potential monitoring tool. We conclude that if multicomponent seismic data are acquired prior to sequestration as a baseline survey and inverted for an accurate presequestration elastic earth model, we can then use passive seismic data for subsequent monitoring. This strategy, in turn, may provide a cost-effective way to monitor carbon sequestrated deep saline aquifers.

Prestack Waveform Inversion- the Present State And the Road Ahead

Prestack waveform inversion (PWI) is one of the emerging technologies for reservoir characterization. inversion uses the full wavefield information in modeling engine allowing the models to be sub-wavelength resolutions. Consequently, PWI much more accurate inversion result compared to the conventional amplitude-variation-with-offset (AVO) based inversions. PWI have been successfully applied in the recent past in a variety of applications such as obtaining an accurate velocity of the water-column for predicting ocean temperatures (Padhi et al, 2010), characterization of hydrocarbon reservoirs (Mallick, 1999; Sen among others), and predicting drilling hazards (Mallick and Dutta, 2002). Prestack waveform inversion (P-wave) and multicomponent seismic data are available (Mallick, 2000).

Genetic Algorithm with Applications in Geophysics

Much of what we know about the Earth’s subsurface has been derived from indirect measurements. Three-dimensional images of the Earth’s interior have been derived from recordings of earthquakes caused by tectonic forces existing inside and on the surface of the earth. Variations in the gravity, magnetic, electric and electromagnetic fields recorded with advanced instruments placed on the surface, boreholes and airplanes, have also been used for this purpose. In seismic exploration, we make use of the principles of and lessons learnt from earthquakes to determine finer scale-subsurface features to identify zones that are favorable for the accumulation of hydrocarbons. Similarly monitoring of fluid movement during hydrocarbon production is also carried out by repeat seismic measurements (called time-lapse seismic experiments).

A New Ray-based Offset-to-angle Transform For Anisotropic Medium And Its Implication In the Prestack Waveform Inversion of Seismic Data

Summary We present a ray-based approach to convert offset domain prestack seismic data into angles. From any given time sample, we ray-trace through the medium to the corresponding source and receiver locations and compute the ray-path length, travel time, offset, and the angle mute for a given angle. We then search for the offset traces that are within the angle mute, locally fit a polynomial to obtain the times corresponding to those offsets and partially stack those offset trace samples to obtain the corresponding zerooffset angle domain response. It is straightforward to compute both primary (P-P) and converted wave (P-S) angle gathers using this approach. Although we present our method for a horizontally stratified VTI medium, it can be easily extended to arbitrarily anisotropic dipping layers. Computing theoretical offset domain seismograms for a variety of models and converting them to angles, we verify that we are able to extract accurate response out to large angles. Our new ray-based transform will be useful for AVO/AVA analysis. In addition, it will also be useful for computing accurate angle gathers from observed seismic data and iteratively matching them with synthetic angle gathers in a prestack single or multicomponent waveform inversion for dipping anisotropic medium.