Geophysical Challenges Research Articles

Sustainable Agriculture for Alaska and the Circumpolar North: Part II. Environmental, Geophysical, Biological and Socioeconomic Challenges

Local agriculture, food security and food supply are limited in Alaska, as well as in much of the circumpolar North. These limitations stem from a suite of challenges that have never been well characterized, categorized, or wholly defined. We identify these challenges as being environmental, geophysical, biological, or socioeconomic in nature, noting that some challenges are interrelated. Additionally, Alaska is expansive, and growing conditions are highly variable across different regions and microclimates of the state. Environmental challenges to Alaskan agriculture are generally linked to high latitude and include strong seasonality, a short growing season, cold temperatures, and unpredictable frosts. Geophysical challenges are characterized by a high percentage of soils that are wet and cold or low in natural fertility. Biological challenges include cultivar adaptability and selection; the control of various pests, weeds, and diseases; and decreased microbial activity in cold soils, which can allow pesticides to linger and slow mineralization of organic fertilizers. Socioeconomic challenges to farming in Alaska are especially limiting and may categorically represent the strongest hindrances to agriculture. They often overlap or interact with many of the identified agro-ecological and biogeographic challenges. Major socioeconomic issues can be a relatively low financial incentive or reward for farmers; inconsistent or limited markets; the high cost of land, infrastructure, and inputs; zoning challenges; a lack of cooperatives; and for rural farmers, time conflicts with more traditional means of subsistence food acquisition. These challenges collectively represent factors that limit agriculture in Alaska, and they provide a basis and justification for developing more sustainable solutions. agriculture; Alaska; challenges; climate; circumpolar; farming; soils; subarctic; sustainable; socioeconomic

Characterization of geothermally relevant structures at the top of crystalline basement in Switzerland by filters and gravity forward modelling

Some of the major geothermal anomalies in central Europe are linked to tectonic structures within the top of crystalline basement, which modify strongly the top of this basement. Their assessment is a major challenge in exploration geophysics. Gravity has been proven to be suitable for the detection of mainly large scale lithological and structural inhomogeneities. Indeed, it is well known and proven by different wells that, for example, in northern Switzerland extended negative anomalies are linked to such structures. Due to depth limitation of wells, there vertical extension is often unknown. In this study, we have investigated the potential of gravity for the geometrical characterization of such basement structures. Our approach consists in the combination of the series of Butterworth filters, geological modelling and best-fitting between observed and computed residual anomalies. In this respect, filters of variable wavelength are applied to observed and computed gravity data. The geological model is discretized into a finite element mesh. Near-surface anomalies and the effect of the sedimentary cover were eliminated using cut-off wavelength of 10km and geological and seismic information.We analysed the potential of preferential Butterworth filtering in a sensitivity study and applied the above mentioned approach to part of the Swiss molasses basin. Sensitivity analyses reveal that such sets of residual anomalies represents a pseudo-tomography revealing the distribution of different structures with depth. This finding allows for interpreting negative anomalies in terms of 3-D volumes. Best-fitting then permits determination of the most likely 3-D geometries of such basement structures. Our model fits both, geological observations and gravity: among 10 deep boreholes in the studied area, six reach the respective units and confirm our distribution of the negative (and positive) anomalies.

SEAM Update

The SEG Advanced Modeling (SEAM) initiative was launched in 2007 with the mission to “advance the science and technology of applied geophysics by organizing industry research consortia to develop industrial-strength earth models and synthetic data sets to address current high-impact geophysical challenges.” Since then SEAM Phase I Deepwater Subsalt Imaging (2007–2013) was completed with 24 participating companies and a budget supplemented with a $2.7 million research grant from RPSEA (a research organization affiliated with the U. S. Department of Energy). SEAM Phase II Land Seismic Challenges was launched in 2011 with 22 participating companies and is making excellent progress tackling three challenges: unconventional reservoir (shale) delineation, arid near surface (deserts), and foothills topography.

The Stybarrow Field - a 4D Case Study

The Stybarrow oil field was discovered in 2003 in the Exmouth Sub Basin, offshore Western Australia. Production started at Stybarrow in November 2007. A 4D seismic modelling study conducted early in the field's life indicated that Stybarrow would be a good candidate for reservoir monitoring using the 4D seismic technique. The modelling indicated that changes in reservoir pressure produced by the water injectors and changes in water saturation due to reservoir depletion should be observable on 4D seismic data. The first monitor survey at Stybarrow was recorded in November 2008, 12 months after the start of production and a second monitor in May, 2011. There were a number of geophysical challenges that needed to be overcome for the 4D seismic technique to be successful at Stybarrow. These included very strong azimuthal anisotropy, orthogonal acquisition directions and strong currents. Azimuthal anisotropy produced large artifacts in the data and degraded the strength of the 4D signal. Two methods were used to overcome the effect of azimuthal anisotropy. The first was a conditioning step based on azimuthal NMO prior to prestack time migration and the second method was orthorhombic prestack depth migration. Both methods produced good results. The results of the surveys were in agreement with the 4D modelling and a development well was drilled on the basis of the first monitor survey. The 4D surveys have proven to be an important tool at Stybarrow for facilitating optimal reservoir management.

GlobalPwave tomography of Earth's lowermost mantle from partition modeling

Determining the scale‐length, magnitude, and distribution of heterogeneity in the lowermost mantle is crucial to understanding whole mantle dynamics, and yet it remains a much debated and ongoing challenge in geophysics. Common shortcomings of current seismically derived lowermost mantle models are incomplete raypath coverage, arbitrary model parameterization, inaccurate uncertainty estimates, and an ad hoc definition of the misfit function in the optimization framework. In response, we present a new approach to global tomography. Apart from improving the existing raypath coverage using only high‐quality cross‐correlated waveforms, the problem is addressed within a Bayesian framework where explicit regularization of model parameters is not required. We obtain high‐resolution images, complete with uncertainty estimates, of the lowermost mantlePwave velocity structure using a hand‐picked data set ofPKPab‐df,PKPbc‐df, andPcP‐Pdifferential travel times. Most importantly, our results demonstrate that the root mean square of thePwave velocity variations in the lowermost mantle is approximately 0.87%, which is 3 times larger than previous global‐scale estimates.

Solving 3D anisotropic elastic wave equations on parallel GPU devices

Efficiently modeling seismic data sets in complex 3D anisotropic media by solving the 3D elastic wave equation is an important challenge in computational geophysics. Using a stress-stiffness formulation on a regular grid, we tested a 3D finite-difference time-domain solver using a second-order temporal and eighth-order spatial accuracy stencil that leverages the massively parallel architecture of graphics processing units (GPUs) to accelerate the computation of key kernels. The relatively small memory of an individual GPU limits the model domain sizes that can be computed on a single device. To circumvent this constraint and move toward modeling industry-sized 3D anisotropic elastic data sets, we parallelized computation across multiple GPU devices by using domain decomposition and, for each time step, employing an interdevice communication protocol to exchange data values falling within interior boundaries of each subdomain. For two or more GPU devices within a single compute node, we use direct peer-to-peer (i.e., GPU-to-GPU) communication, whereas for networked nodes we employed message-passing interface directives to route data over the network. Our 2D GPU-based anisotropic elastic modeling tests achieved a [Formula: see text] speedup relative to an OpenMP CPU implementation run on an eight-core machine, whereas our 3D tests using dual-GPU devices produced up to a [Formula: see text] speedup. The performance boost afforded by the GPU architecture allowed us to model seismic data for 3D anisotropic elastic models at lower hardware cost and in less time than has been previously possible.

Multiphysics borehole geophysical measurements, formation evaluation, petrophysics, and rock physics — Introduction

This special section showcases some emerging trends in the acquisition, processing, and interpretation of borehole geophysical measurements, with emphasis on multiphysics and multiscale applications that integrate geologic information, petrophysical properties, rock physics, and core measurements. Borehole geophysical measurements are routinely acquired to appraise, produce, and monitor subsurface resources, including fossil fuels, minerals, and water. They also are used in shallow reconnaissance projects, for civil-engineering studies, and for soil characterization and remediation, to cite a few salient applications. Wells usually contain the most dense and diverse types of information concerning geology, fluid flow, and rock physics, when assessing subsurface resources. During the last decade, borehole geophysical measurements have experienced significant developments in their acquisition, processing, and interpretation; they no longer can be viewed in isolation but rather as representing an important component of the quantitative description of petrophysical and rock physical properties under a geologic framework. Relatively recent entries to this endeavor are the petrophysical and geophysical appraisal/monitoring of subsurface CO 2 sequestration and the petrophysical/rock-physics characterization of hydrocarbon-bearing shale. It is expected that future interpretation challenges in borehole geophysics will consistently require the solid quantitative integration of measurements across disciplines and physical scales in formation-evaluation and rock-physics studies. The papers included in this issue consider topics as diverse as new multiphysics methods to combine electromagnetic and seismic amplitude measurements to estimate petrophysical properties between wells and the basis of Archie’s laws. Several of these contributions formally and rigorously integrate the physics of fluid flow in porous media with borehole geophysical measurements to estimate static and dynamic petrophysical properties. New rock-physics methods also are introduced to estimate elastic properties …

An integrated groundwater study for Chasnigua, Honduras: Measurements and thoughts

This paper is an update on a project initiated last year with a grant from SEG's Geoscientists Without Borders program. The project benefited from prior established relationships with water and sanitation administrators in the Municipality of Villanueva and with faculty and students from the Universidad Autonoma de Honduras, campus Valle Sula. The project rationale was the substantial need for a clean, reliable source of potable water for people living in the village of Chasnigua and the interesting geophysical challenges of an area complicated with volcanic/limestone terrain. Because of the geological complexity of this region, a suite of magnetic, magnetic gradient, direct current (dc)resistivity, and frequency-domain electromagnetic studies were performed. Soil and water sampling and testing, and topographic mapping of the terrain augmented this geophysical study. Through this effort, a site was identified that holds substantial promise for a sustainable subterranean water source. However, challenges have now moved from the technical to the social context as the person responsible for land on which most of our geophysical studies have focused, is now reluctant to allow water well drilling and completion on that land. In addition to the technical aspects of this project, this paper includes a discussion of approaches that may provide a more successful outcome with regard to social contextual challenges in which these type projects are inevitably immersed.

Seismic acquisition design for the SEG Advanced Modeling (SEAM) project

Our industry—like so many others globally—contains the efforts of many specialists working independently toward the same goal. The SEG Advanced Modeling (SEAM) project is a cooperative industry effort focused on the construction of subsurface models and generation of synthetic data sets. These data sets will be used to investigate geophysical challenges important to the resource extraction industry; they also are expected to be useful to acquisition geophysicists, general seismic processors, imaging specialists, and interest groups involved in reservoir characterization and rock property inversion.

SEG Applied Research Workshop: Geophysical challenges in southeast Asia exploration

The 2008 SEG Applied Research Workshop, in Kuala Lumpur, Malaysia on 7–10 April 2008, was the first such event to focus on regional challenges rather than on a specific technical topic. This is a premier example of how SEG is exploring for new ways to reach out and encourage more interaction with its global membership base. The workshop attracted 64 attendees, with regional representation from Malaysia, Thailand, Indonesia, and India; attendees from outside the region represented North America, Europe, and Australia. The participants covered the full spectrum of business interests: international oil companies, national oil companies, independent oil companies, research institutes, and geophysical service providers. The workshop was sponsored by Hess Corporation, ExxonMobil, and National GeoConsultant of Indonesia.

Q&A with Sami Iskander

Oil prices, profits, and capital spending are on the rise, and future hydrocarbon demand appears robust. Do you expect the industry to enjoy a sustained boom? All indicators point to a sustained period of strong activity. Two things are key. First, the industry must add production capacity to meet increasing demand. Second, it must also replace production lost to the decline of the aging production base. With E&P spending increasing to record levels, with only a thin margin existing of excess supply over demand, and with oilfield services' human and equipment resources stretched, we see no immediate drop in activity. How have the sustained high oil prices affected Schlumberger's business and operations? Last year set new records for Schlumberger activity as the price of oil allowed the E&P industry to increase spending. This year looks even stronger as more companies return to exploration-type work, and this is of particular significance to us because we have a wide range of technologies well suited to exploration, such as the Scanner family. What is the significance of the Scanner family of wireline measurements introduced late last year? The Scanner family is a step change in technology and in the benefits it brings to how our clients populate reservoir models with their data. What kind of field test results have you had thus far? Results have been great in a wide range of applications for each technology worldwide. Client acceptance has been very encouraging, and the number of case studies is growing. Where do you see its main applications? Scanner applications are numerous throughout all the oil and gas upstream processes. The Rt Scanner runs with our Platform Express service in standard induction applications, with its major advantages apparent in thin beds, low-resistivity pay, and high-formation-dip environments. The Sonic Scanner is a service applicable to nearly every oilfield discipline: meeting geophysical challenges, detecting anisotropy accurately as low as 2%, delivering a clear understanding of formation-integrity issues likely to happen during production, and allowing completion engineers to make the best decisions on their completion and fracture designs. MR Scanner provides nuclear-magnetic-resonance measurements to help resolve petrophysical challenges in a variety of environments, including unknown or varying salinities, low-resistivity or low-contrast pay zones, and/or thin beds. This is made possible using a single pass with multiple depths of investigation. MR Scanner provides detailed formation evaluation of the near-wellbore region, fluid identification, and in-situ hydrocarbon characterization. In Middle East carbonate reservoirs, which are normally difficult to interpret using conventional resistivity techniques, the MR Scanner service has clearly distinguished hydrocarbon-bearing zones from the water zones by use of patented interpretation techniques. What other technologies are you working on that will address key production issues facing the industry? To address the pressing production issues, we are launching two new cased-hole Scanner technologies: the Isolation Scanner and Flow Scanner. The next-generation zonal-isolation evaluation service is the Isolation Scanner for the latest lightweight and foam cements as well as traditional and heavy weight cements. It measures radially and deeper toward the formation, confirming zonal isolation, pinpointing channels in the cement, and ensuring confident squeeze or no-squeeze decisions. Because of its increased depth of investigation, the Isolation Scanner also provides detailed information about casing and casing-within-casing eccentralization, allowing for better identification of whipstock locations for sidetracking, while its internal-diameter and thickness measurements identify corrosion and drilling-induced wear.

The use of Borehole Radar for the delineation of thin tabular orebodies ahead of mining

A significant portion of South Africa?s current gold resources come from the Ventersdorp Contact Reef (VCR). The VCR is a tabular orebody, generally less than 1.2m thick. Slopes, terraces and faults impact on VCR geometry. Gold is generally concentrated in palaeochannels, corresponding to terrace elevations. Conversely, slopes are generally associated with lower gold grades. To be able to site support pillars, such that they correspond with lower gold grade slope areas, has significant financial benefit. The geophysical challenge is, thus, to model the reef geometry prior to mining in order to facilitate optimal ore extraction. Borehole radar has proven to be applicable for mapping topography on the VCR. In this paper a case study is presented where radargrams and 3D visualization from three boreholes show continuity of geometrical features on the VCR target horizon. The improved confidence in the geological model has an immediate impact on resource estimation and mine planning with immediate financial benefits.

Oil and gas in the United States: The geophysical challenge

For more than a century, success in the oil and gas industry has relied on new technology; bright, innovative people, and access to geologic areas that hold promise of hydrocarbons. In the next century, that will not change. However, the challenge to develop that technology, find the people, and secure access will be greater than ever.

Geophysical investigation of buried Pleistocene subglacial valleys in Northern Germany

Buried Pleistocene subglacial valleys are extensively used as groundwater reservoirs by waterworks in northern Germany, although little is known about the locations and size of these valleys and the internal structure of the sediment fill. This lack of knowledge about important groundwater reservoirs is a challenge for geophysics. This paper summarizes the geophysical investigation of two buried Pleistocene subglacial valleys in northern Germany—the Ellerbeker Rinne and the Bremerhaven–Cuxhavener Rinne—including seismic, gravity, and airborne electromagnetic (AEM) surveys. Seismic sections show the detailed structure of the paleovalleys. The reliability of interpretation is enhanced by vertical seismic profiles in wells. The maximum depths of the Ellerbeker Rinne and the Bremerhaven–Cuxhavener Rinne were found to be 360 and about 400 m, respectively. Gravity survey revealed Bouguer anomalies above the sediment fill of both buried valleys. The Ellerbeker Rinne produces a negative residual anomaly of −0.5 mGal, whereas the sediments of the Bremerhaven–Cuxhavener Rinne produce a positive anomaly. The latter one is superimposed by negative gravity anomalies due to near-surface structures. The Bremerhaven–Cuxhavener Rinne can be mapped by airborne electromagnetics at locations without saltwater intrusion, which would affect the measurements. The electrical conductivity of the clay layer at the top of the valley fill differs significantly from that of the surrounding sand. The combined use of these three geophysical methods, which measure different physical parameters, leads to a better understanding of the subsurface geology and the hydrogeology of the Pleistocene subglacial valleys.

Geophysical Challenges facing Santos

This presentation covers some of the key geophysical technical challenges facing Santos in the Cooper Basin, Otway Basin, and NW Shelf of Australia.In the Cooper Basin, Santos' immediate need is to...

An introduction—Carbonate geophysics

Carbonates are strange rocks to most exploration geophysicists although they hold more than half the world's petroleum reserves. The geophysicist's exposure to geology in colleges and graduate shools was concentrated on basin analysis, structure, stratigraphy of clastic sedimentary rocks. This estrangement to carbonate sedimentary rocks was further perpetuated in the past two decades, primarily because of the industry's focus on deeper water exploration and development of turbidite sands. Owing to the advancement of geophysical technology, favorable rock physics and superior seismic data quality, geophysics played a critical role in finding many elusive oil and gas traps deep in the ocean. Geophysics was indispensable in aiding the development of the discovered deepwater fields by reducing the risk and cost of expensive deepwater wells. Geophysical applications in carbonate reservoirs are less mature and abundant than those associated with clastic reservoir. This lack of maturity is primarily the result of lower business priorities of carbonate reservoirs in the last two decades. Additionally, carbonate reservoirs are notoriously more difficult to characterize than siliciclastic reservoirs. Compared to siliciclastic reservoirs, carbonate reservoirs offer unique geophysical challenges with respect to reservoir characterization. These include: (1) tight rock fabric resulting in problematic and not widely accepted rock physics models; (2) greater heterogeneity due to rapid vertical and lateral facies variation; (3) lower seismic resolution due to higher velocities; (4) physical and chemical alterations causing fracturing and diagenesis; and (5) mostly land and shallow water seismic data, meaning relatively lower data quality over most carbonate fields. Recent business opportunities in the former Soviet Union and Middle East have triggered renewed interest in carbonate reservoirs and carbonate specific exploration, development, and production …

Energetics of mantle convection and the fate of fossil heat

Reconstructing the thermal history of the Earth, consistent with the low concentration of heat‐producing elements in convecting mantle as well as with modest secular cooling required by geological records, has been a major challenge in geophysics and geochemistry. By developing the self‐consistent energetics of plate‐tectonic mantle convection, we show that the low Urey ratio of convecting mantle can yield a geologically reasonable solution in the thermal evolution model of the Earth. The effect of dehydration on mantle rheology during plate formation with mantle melting results in more sluggish plate tectonics (i.e., lower heat flow) for hotter mantle. This inverse relationship between mantle temperature and surface heat flux leads to the efficient storage of fossil heat, preventing the drastic secular cooling of the Earth.

Geophysical challenges and opportunities of the deepwater Angolan offshore

Recent giant field discoveries on the western edge of the Lower Congo Basin have triggered new opportunities for Tertiary plays offshore Angola. These discoveries have also ratified the impact of 3-D seismic in deepwater prospect evaluation and field development, especially now that drilling and production in water depths of 500 m and more are no longer an obstacle. This paper highlights some of the key challenges and opportunities that would increase the use of geophysics.

Geophysical Challenges in Using Large-Scale Ocean-Generated EM Fields to Determine the Ocean Flow

For some time, oceanographers have used measurements of the small electromagnetic fields generated by the flow of the electrically conductive oceans through the Earth's main magnetic field to infer values for the ocean flow velocities. An overview of the process of motional induction is given including a description of the global electromagnetic fields generated by a global model ocean. We describe how electromagnetic methods are currently used in oceanography and outline the most important challenges presently faced.

Geophysical challenges after the Brundtland year

Geophysical challenges after the Brundtland year