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- 1. Special Section Advances in time-lapse geophysics — Introduction David Lumley1 , Martin Landrø2 , Ivan Vasconcelos3 , Leo Eisner4 , Paul Hatchell5 , Yaoguo Li6 , Matthew Saul7 , and Mark Thompson8 Time-lapse geophysics is becoming an increasingly important and powerful method to measure, monitor, verify, and predict com- plex time-varying processes in the earth. Applications include re- source management (hydrocarbons, groundwater, geothermal: : : ), geohazard risk assessment (natural and induced seismicity, overpres- sure zones : : : ), environmental issues (CO2 sequestration, ground- water contamination and remediation : : : ), geotechnical engineering (dams, roads, bridges : : : ), and fundamental science questions (subsurface flow of fluids, stress and heat, time-variant rock properties, fault dynamics and fracturing, geophysical source mechanisms, near-surface variations in vadose and permafrost zones : : : ). Advances in time-lapse geophysics are being driven by spectacu- lar innovations in theory, data acquisition, and quantitative data analysis. Theoretical innovations include new math and physics de- velopments to properly incorporate 4D space-time variations in modeling, imaging, and inversion methods, and to correctly account for full wavefield or potential field representations of time-lapse geophysical phenomena. Data acquisition innovations include new developments to improve 4D signal/noise levels, and repeat surveys more frequently or continuously, with new source and receiver instrumentation, (semi) permanent arrays, and others. New developments in time-lapse imaging and inversion are allowing us to extract more detailed (and often surprising) infor- mation, increasingly in near real-time, to help better understand time-varying processes in the earth’s subsurface. Recent develop- ments in quantitative time-lapse data analysis and interpretation are providing new knowledge to help improve our dynamic earth models of subsurface processes, and more accurately predict future behavior. The intent of this special section of GEOPHYSICS is to present a set of technical articles that samples the current state-of-the-art in time- lapse geophysics, and highlights emerging concepts for future re- search, across a wide range of applications including 4D seismic, EM, gravity and magnetics, and across a wide range of theory, data acquisition, and quantitative data analysis. We hope this special sec- tion will convey new developments and capabilities to the broader geophysics community and stimulate new ideas and research to ad- dress the remaining unsolved challenges. Mercier et al. apply passive seismic body-wave traveltime tomography to a data set of mining-induced microseismic events to image the distribution and evolution of the P-wave velocity dur- ing block caving. The results show that time-lapse P-wave velocity models can be used to better understand the rock mass response to mining during development and production in a block caving context. Zong et al. extend elastic inverse scattering theory for fluid dis- crimination using time-lapse seismic data. A linearized approxima- tion of the reflectivity variation is derived in terms of the changes of fluid factor, shear modulus, and density based on perturbation theory, and a Bayesian prestack inversion approach is presented to estimate these physical property changes directly from the time-lapse seismic data. Blanchard and Delommot measure time-lapse changes in seis- mic attenuation in a reservoir undergoing depletion, gas exsolution and water injection, and show two examples of how to utilize this Published online 10 March 2015. 1 University of Western Australia, Perth, Australia. E-mail: david.lumley@uwa.edu.au. 2 Norwegian University of Science and Technology, Trondheim, Norway. E-mail: martin.landro@ntnu.no. 3 Schlumberger Cambridge Research, Cambridge, UK. E-mail: ivasconcelos2@slb.com. 4 Institute of Rock Structure and Mechanics, Czech Academy of Sciences, Prague, Czech Republic. E-mail: leo@irsm.cas.cz. 5 Shell International E&P, Houston, Texas, USA. E-mail: paul.hatchell@shell.com. 6 Colorado School of Mines, Golden, Colorado, USA. E-mail: ygli@mines.edu. 7 Chevron Australia, Perth, Australia. E-mail: matthewsaul@chevron.com. 8 Statoil ASA, Trondheim, Norway. E-mail: math@statoil.com. © 2015 Society of Exploration Geophysicists. All rights reserved. WAi GEOPHYSICS, VOL. 80, NO. 2 (MARCH-APRIL 2015); P. WAi–WAii. 10.1190/2015-0120-SPSEINTRO.1 Downloaded06/05/15to129.241.221.254.RedistributionsubjecttoSEGlicenseorcopyright;seeTermsofUseathttp://library.seg.org/
- 2. information. First, the authors show how the measurements can be used for time-lapse Q-correction to improve 4D inversion and in- terpretation in an underlying reservoir, and second, they attempt to integrate the measured attenuation changes with changes in travel- time to try and separate gas and water saturations in the reservoir. He et al. present a field trial of time-lapse continuous electromag- netic profile for monitoring gas reservoir production. The estab- lished data acquisition procedure, data processing algorithm, and the inversion-based interpretation have general applicability and open new avenues for time-lapse electromagnetic monitoring of oil and gas production. Krahenbuhl et al. present a feasibility study on the application of time-lapse gravity as a monitoring tool for a proposed CO2 se- questration test site. The approach integrates the reservoir property model and seismic imaging data with surface and borehole gravity data for improved recovery of the injected CO2. Roach et al. apply time-lapse processing to two vintages of seis- mic data acquired prior to CO2 injection at the Aquistore CO2 stor- age site using a sparse permanent land array. Time-lapse analysis combined with fluid substitution modeling indicates excellent re- peatability between surveys, adequacy in imaging of the subsurface, and that the data should provide the required sensitivity for mon- itoring CO2 in the reservoir. Vanorio shows experimental evidence of time-lapse changes in the transport and elastic properties of the rock frame due to the chemo-mechanical interaction between fluids injected into the rock and the rock matrix itself. Capriotti and Li use time-lapse gravity data to invert for the permeability distribution of a reservoir. By directly linking the equations for fluid flow in porous medium to the time-lapse gravity response, the authors are able to recover meaningful distributions of permeability. Reitz et al. present a feasibility study of monitoring steam- assisted gravity drainage (SAGD) reservoirs using time-lapse gra- vimetry and gravity gradiometry because advances in these technol- ogies have made them viable monitoring tools. The results indicate that under certain conditions SAGD production should produce a detectable anomaly using both methods, but the level of detail that can be recovered through inversion is site dependent. Saul and Lumley present a new nonelastic method to describe the pressure sensitivity of rock properties, including changes in grain contact cement, and apply the method to a 4D seismic data example from offshore Australia. The authors show that high-pres- sure water injection may mechanically weaken the poorly consoli- dated reservoir sands in a nonelastic manner, allowing them to explain observed 4D seismic signals that are larger than can be pre- dicted by purely elastic rock-physics theory. White et al. use repeated 3D seismic surveys acquired with a sparse permanent array of buried geophones to assess the level of data repeatability. Signal-to-noise ratio and overall repeatability are enhanced by the permanent array. Young and Lumley discuss the recent development that highly accurate seafloor gravity data can detect small density changes in subsurface hydrocarbon reservoirs by precisely repositioning the gravimeters on the seafloor. The authors use this method to assess the feasibility of time-lapse seafloor gravity monitoring for the giant gas fields in Australia’s premier hydrocarbon province and find that several of these producing gas reservoirs can result in readily de- tectable gravity signals (>5 μGal) within just a year or so of gas production. Doetsch et al. monitor geochemical changes induced by injected CO2 in a shallow aquifer using time-domain spectral induced polarization. The time-lapse full-decay induced polarization inver- sions image the CO2 plume as a decrease in resistivity and an in- crease in normalized chargeability, and the imaged plume agrees well with electrical conductivity and aluminum concentration mea- sured on water samples. WAii Advances in time-lapse geophysics — Introduction Downloaded06/05/15to129.241.221.254.RedistributionsubjecttoSEGlicenseorcopyright;seeTermsofUseathttp://library.seg.org/