Detailed Subsurface Information Research Articles

Understanding Shallow Basaltic Aquifer System Near West Coast of Maharashtra, India

Two-Dimensional (2D) Electrical Resistivity Tomography (ERT) and geophysical logging and injected tritium tracer studies with hydrogeological tests were carried out at selected location in an area of 5 km2 near the coastal region of Tarapur, Thane district, Maharashtra. The area is in basaltic terrain, overlain by thin cap of alluvium formation and receives an annual rainfall of about 2000 mm. An integrated investigation method was adopted to delineate the subsurface lithological variations, understand the existing aquifer system up to 25 m depth, evaluate aquifer properties, recharge potential and also to monitor the groundwater flow characteristics. The investigation results showed a variable thickness of weathered zone of 1-3 m, existence of two basalt flows, low vertical natural recharge, high transmissivity, high hydraulic conductivity and high groundwater flow rates. The results also depicted that a combination of hydrogeological tests along with resistivity and tracer investigation is an effective tool in mapping and characterizing the shallow potential groundwater aquifer zone in Deccan traps. The integrated study carried out in the coastal area provided detailed subsurface information useful for planning specific water conservation strategies for sustainable groundwater supply. Keywords: Deccan Traps, ERT, Tritium Tracer, Recharge, Groundwater Flow

Characterization of a Shallow Coastal Aquifer in the Framework of a Subsurface Storage and Soil Aquifer Treatment Project Using Electrical Resistivity Tomography (Port de la Selva, Spain)

Water percolation through infiltration ponds is creating significant synergies for the broad adoption of water reuse as an additional non-conventional water supply. Despite the apparent simplicity of the soil aquifer treatment (SAT) approaches, the complexity of site-specific hydrogeological conditions and the processes occurring at various scales require an exhaustive understanding of the system’s response. The non-saturated zone and underlying aquifers cannot be considered as a black box, nor accept its characterization from few boreholes not well distributed over the area to be investigated. Electrical resistivity tomography (ERT) is a non-invasive technology, highly responsive to geological heterogeneities that has demonstrated useful to provide the detailed subsurface information required for groundwater modeling. The relationships between the electrical resistivity of the alluvial sediments and the bedrock and the difference in salinity of groundwater highlight the potential of geophysical methods over other more costly subsurface exploration techniques. The results of our research show that ERT coupled with implicit modeling tools provides information that can significantly help to identify aquifer geometry and characterize the saltwater intrusion of shallow alluvial aquifers. The proposed approaches could improve the reliability of groundwater models and the commitment of stakeholders to the benefits of SAT procedures.

On Including Near-surface Zone Anisotropy for Static Corrections Computation—Polish Carpathians 3D Seismic Processing Case Study

Obtaining the most accurate and detailed subsurface information from seismic surveys is one of the main challenges for seismic data processing, especially in the context of complex geological conditions (e.g., mountainous areas). The correct calculation of static corrections allows for the reliable processing of seismic data. This, in turn, leads to better geological interpretation. A seismic signal passing through a near-surface zone (NSZ) is adversely affected by the high heterogeneity of this zone. As a result of this, observed travel times often show anisotropy. The application of refractive waves and the time delay solution without taking into account the effects caused by the complex anisotropy of an NSZ does not meet the standards of modern seismic surveys. The construction of the NSZ model in mountain regions with the use of refraction may be extremely difficult, as the vertical layers can be observed very close to the surface. It is not sufficient to apply regular isotropic refractive solutions in such conditions. The presented studies show the results of taking into account the anisotropy of an NSZ in the calculations of static corrections. The presented results show that this step is critical for the detailed processing of three-dimensional (3D) seismic data collected in the difficult region of the Carpathians in Southern Poland.

Edge Detection on Images of Pseudoimpedance Section Supported by Context and Adaptive Transformation Model Images

Abstract Most of underground hydrocarbon storage are located in depleted natural gas reservoirs. Seismic survey is the most economical source of detailed subsurface information. The inversion of seismic section for obtaining pseudoacoustic impedance section gives the possibility to extract detailed subsurface information. The seismic wavelet parameters and noise briefly influence the resolution. Low signal parameters, especially long signal duration time and the presence of noise decrease pseudoimpedance resolution. Drawing out from measurement or modelled seismic data approximation of distribution of acoustic pseuoimpedance leads us to visualisation and images useful to stratum homogeneity identification goal. In this paper, the improvement of geologic section image resolution by use of minimum entropy deconvolution method before inversion is applied. The author proposes context and adaptive transformation of images and edge detection methods as a way to increase the effectiveness of correct interpretation of simulated images. In the paper, the edge detection algorithms using Sobel, Prewitt, Robert, Canny operators as well as Laplacian of Gaussian method are emphasised. Wiener filtering of image transformation improving rock section structure interpretation pseudoimpedance matrix on proper acoustic pseudoimpedance value, corresponding to selected geologic stratum. The goal of the study is to develop applications of image transformation tools to inhomogeneity detection in salt deposits.

Techbits: Reservoir Testing's Benefits Reviewed in Malaysia

Techbits Recognizing that step changes in oil and gas recovery can only be achieved through analogous step changes in reservoir understanding, a recent SPE Applied Technology Workshop aimed to explore the limits and new promises in reservoir testing. The May 2009 workshop, titled "Reservoir Testing for Maximizing Understanding and Recovery," gathered more than 60 participants from 18 countries to Kota Kinabalu, Sabah, Malaysia to review different reservoir-testing and fluid-sampling technologies aimed at providing more detailed subsurface information which will ultimately lead to improved recovery. The participants represented both service and operating companies and had an average of 15 years experience, a point which Tuan Haji Akbar Tajudin Abdul Wahab, senior general manager, Petroleum Engineering Department, Petronas Carigali, acknowledged in his keynote address. He said that collectively there were nearly 1,000 years of wisdom in the room. Wahab used his keynote to challenge the use of the term "well testing," suggesting that it fails to capture the diversity of testing available to the industry today. He proposed that it is time to expand the scope of reservoir testing to cover new technologies, and that new ways of working should be developed to allow an optimal use of new technology with the resources. He also emphasized the need to properly integrate reservoir test results into reservoir models, which can be assisted by proper test planning and design. Not only can the value of the reservoir information be better assessed with proper planning, but it may help avoid unnecessary tests and bring innovation back into reservoir testing. New Technologies, Improved Reservoir Characterization In the session titled "Applications of New Technologies," discussion leaders (DLs) reviewed new methods to collect data cost-effectively and ways to maximize the utility of the information collected. The DLs discussed alternative ways of analyzing large data sets from permanent sensors and the long-term reliability of hardware, such as logging-while-drilling tools, in providing fluid samples that could accurately characterize formation properties. The last DL examined smart-well technologies, demonstrating how these could be useful additions to not only production and recovery optimization, but also in providing greater flexibility in testing at all stages of a well's life.

Enhancing seismic data resolution using the prestack blueing technique: An example from the Ketzin C O2 injection site, Germany

Restricted resolution imposes limits on the detection of subsurface geologic features using surface seismic data. Despite great improvements in acquisition and processing techniques in recent years, seismic data still suffer from limited resolution. Detailed subsurface information on the geologic conditions at depth cannot always be attained from conventional surface seismic data. For the area of this study, the carbon dioxide [Formula: see text] storage site at Ketzin, Germany, high seismic resolution is required for mapping the internal structure of the main target reservoir. Processing of the 3D surface seismic data using conventional methods did not provide optimum resolution. The spectral blueing technique attempts to enhance the surface seismic data resolution. Well-log data generally show a blue spectrum with higher-amplitudes at the higher frequencies than the commonly assumed white reflection series. By designing and applying one or several operators to poststack data, it has been possible to better match the reflectivity series and improve resolution. Applying the blueing operator to prestack data was investigated to possibly improve seismic resolution, produce more consistent results, and cause fewer ringing artifacts than when applied to poststack data. Prestack blueing, poststack blueing, and no blueing of the Ketzin 3D seismic data were compared with zero-offset vertical seismic profile (VSP) and synthetic seismograms. The comparison shows that prestack spectral blueing can indeed enhance seismic resolution with fewer artifacts associated with it than the poststack technique. The prestack and poststack spectral blueing approaches improve the well to seismic tie.

Deep faulting and structural reactivation beneath the southern Illinois basin

The investigation of deep fault structure and seismogenesis within “stable” continental interiors has been hindered by the paucity of detailed subsurface information and by low levels of seismicity. Outstanding seismotectonic questions for these areas include whether pre-existing structures govern the release of seismic energy as earthquakes, can reactivation of such structures be recognized, and to what extent have Precambrian basement structures exerted long-lived controls on the development of overlying Phanerozoic features. The southern portion of the Illinois basin provides a premier area in which to study the relation between contemporary seismicity and pre-existing structures due to the frequency of seismic events, the concentration of available geophysical data, and the wealth of borehole information. We have integrated the study of this information in order to create a 2.5-dimensional picture of the earth for local seismogenic depths (0–15 km) for a study area of moderate 20th century earthquake activity. The area is located along the western flanks of two of the major structures within the Illinois basin, the Wabash Valley fault system (WVFS) and the La Salle anticlinal belt (LSA). The results of reprocessing seismic reflection profiles, combined with earthquake hypocenter parameters, suggest three distinct seismotectonic environments in the upper crust. First, we have delineated a fault pattern that appears to correspond to the steep nodal plane of a strike-slip mechanism event (1974.04.03; m b = 4.7). The fault pattern is interpreted to be a deeply buried rift zone or zone of intense normal faulting underpinning a major Paleozoic depocenter of the Illinois basin (Fairfield basin). Second, a similar event (1987.06.10; m b = 5.2) and its well-located aftershocks define a narrow zone of deformation that occurs along and parallel to the frontal thrust of the LSA. Third, the hypocenter of the largest event in the study area (1968.11.09; m b = 5.5) may be spatially associated with a prominent zone of dipping middle crustal reflections, just west of the WVFS, which have been interpreted as a deeply buried blind thrust. The proposed correlation of pre-existing structures with earthquakes having consistently oriented structural parameters supports the reactivation of old deformation zones by contemporary stresses as previously proposed by earlier workers. However, the degree to which deformation has propagated upward from Precambrian basement into the Paleozoic rocks varied significantly even over a small study area. The societal value of associating an earthquake with a specific pre-existing deformation zone in the seismogenic crust is to improve the assessment of seismic hazard or to assess the integrity of a stratigraphic formation, being considered as a target for natural gas storage or carbon sequestration.

Depth Estimation from Gravity Data Using the Maximum Entropy Method (MEM) and the Multi Taper Method (MTM)

The depth determination from the gravity data in frequency domain is carried out using the classical fast Fourier transform (FFT) method utilizing scaling properties of ensemble of anomalous source. The problem of calculating power spectrum from the FFT is well described in the literature. Here, the application of other high-resolution methods of power spectrum calculation, such as maximum entropy method (MEM) and multi-taper method (MTM) are explored to estimate depth to anomalous sources. At the outset, the FFT, the MEM and the MTM are tested on synthetic gravity data, generated for different types of synthetic models and then all these methods are applied to the field gravity data of the Bengal basin. The MTM with scaling is found to be superior for providing the detailed subsurface information rather than the MEM and the FFT methods in the case of synthetic as well as field examples.

Using Geophysics to locate burials and other cultural features, Isle of the Dead, Port Arthur, Tasmania

Multi-technique geophysical investigations have been conducted on the convict-period cemetery on the Isle of the Dead, Port Arthur, Tasmania. With the exception of historic photographs and some limited historical research, very little is known about the layout of burials or physical characteristics of the subsurface. Approximately 1100 burials took place on the island and less than 10% of these were formally marked. Apparent conductivity and magnetic surveys were conducted across the accessible portion of the island to locate subsurface artefact that may be associated with individual burials. The results show gradual variations in soil conditions over the site, some surface cultural features such as pathways, and numerous near-surface unidentified ferrous objects. Although some of these features showed apparent linear patterning, this did not correlate with individual areas of disturbance visible in the ground penetrating radar (GPR) profiles. 500 MHz and 250 MHz GPR surveys provided detailed subsurface information and was used to define the lateral extent of individual anomalous responses and areas containing complex multiple reflectors. The former were classified into various types, according to amplitude, shape and continuity of the reflectors. They were interpreted as possible or probable graves and located on a cultural sensitivity plan as zones of high archaeological potential. Of the techniques used so far, GPR appears to be the most effective method for detecting disturbances associated with graves on the Isle of the Dead.

Sedimentary architecture and post-glacial evolution of Cheekye fan, southwestern British Columbia, Canada

Abstract The purpose of this paper is to examine the internal architecture and post-glacial evolution of Cheekye fan, British Columbia, Canada. Analysis of a large database of ground penetrating radar (GPR) profiles has allowed the identification of ten reflection configurations that characterize this high-energy environment. GPR profiles augmented with test-pit, well-log and radiocarbon data provided detailed subsurface information and revealed the large-scale internal architecture and Holocene sedimentation history of Cheekye fan. Based on a shift in reflection configuration with depth, GPR data appears to record a change in the mode of deposition through time. This data suggests that Cheekye fan is a paraglacial fan, largely a product of the geological past. This fact should be taken into consideration when making future hazard estimates.

An inverse‐scattering series method for attenuating multiples in seismic reflection data

We present a multidimensional multiple‐attenuation method that does not require any subsurface information for either surface or internal multiples. To derive these algorithms, we start with a scattering theory description of seismic data. We then introduce and develop several new theoretical concepts concerning the fundamental nature of and the relationship between forward and inverse scattering. These include (1) the idea that the inversion process can be viewed as a series of steps, each with a specific task; (2) the realization that the inverse‐scattering series provides an opportunity for separating out subseries with specific and useful tasks; (3) the recognition that these task‐specific subseries can have different (and more favorable) data requirements, convergence, and stability conditions than does the original complete inverse series; and, most importantly, (4) the development of the first method for physically interpreting the contribution that individual terms (and pieces of terms) in the inverse series make toward these tasks in the inversion process, which realizes the selection of task‐specific subseries. To date, two task‐specific subseries have been identified: a series for eliminating free‐surface multiples and a series for attenuating internal multiples. These series result in distinct algorithms for free‐surface and internal multiples, and neither requires a model of the subsurface reflectors that generate the multiples. The method attenuates multiples while preserving primaries at all offsets; hence, these methods are equally well suited for subsequent poststack structural mapping or prestack amplitude analysis. The method has demonstrated its usefulness and added value for free‐surface multiples when (1) the overburden has significant lateral variation, (2) reflectors are curved or dipping, (3) events are interfering, (4) multiples are difficult to identify, and (5) the geology is complex. The internal‐multiple algorithm has been tested with good results on band‐limited synthetic data; field data tests are planned. This procedure provides an approach for attenuating a significant class of heretofore inaccessible and troublesome multiples. There has been a recent rejuvenation of interest in multiple attenuation technology resulting from current exploration challenges, e.g., in deep water with a variable water bottom or in subsalt plays. These cases are representative of circumstances where 1-D assumptions are often violated and reliable detailed subsurface information is not available typically. The inverse scattering multiple attenuation methods are specifically designed to address these challenging problems. To date it is the only multidimensional multiple attenuation method that does not require 1-D assumptions, moveout differences, or ocean‐bottom or other subsurface velocity or structural information for either free‐surface or internal multiples. These algorithms require knowledge of the source signature and near‐source traces. We describe several current approaches, e.g., energy minimization and trace extrapolation, for satisfying these prerequisites in a stable and reliable manner.

Tectonic geomorphology of the Ash Hill fault, Panamint Valley, California

ABSTRACTPanamint Valley, in eastern California, is an extensional basin currently bounded by active, dextral‐normal oblique‐slip faults. There is considerable debate over the tectonic and topographic evolution of the valley. The least‐studied structure, the Ash Hill fault, runs for some 50 km along the valley’s western edge, and active strands of the fault continue south into the neighbouring Slate Range. Vertical displacement on the fault is valley‐side up, creating topography that conflicts with the gross morphology of the valley itself. We use this topography, along with kinematic and geological markers, to constrain the Quaternary slip rate and orientation of the Ash Hill fault. The fault offsets all but the active channel deposits in the valley, and slickenlines indicate a strike‐slip to dip‐slip ratio of 3.5:1. An offset volcanic unit dated at 4 Ma provides a minimum slip rate of 0.3±0.1 mm yr−1, and a long‐term strike‐slip to dip‐slip ratio of 5.2:1. Slip on the fault has warped a palaeolake shoreline within the valley. Simple elastic dislocation modelling of the vertical deformation of the shoreline suggests total fault slip of ≈60 m, valley‐side up. The shoreline probably dates to 120–150 ka, implying a late Quaternary slip rate of 0.4–0.5 mm yr−1. We suggest two possible mechanisms for the apparently anomalous slip behaviour of the Ash Hill fault. The fault may be a listric structure related to the proposed low‐angle fault underlying Panamint Valley. Alternatively, the Ash Hill fault is a high‐angle fault, implying that the valley is currently bounded by high‐angle dextral‐slip faults. Lack of detailed subsurface information precludes any knowledge of the true relationships between the presently active faults.

The Origin of Oil and Gas Accumulations in the Western Sector of the South Caspian Sea: ABSTRACT

The western sector of the South Caspian Sea is the most studied offshore area within the intermontane South Caspian Depression. The base of the sedimentary section occurs at depths of 15-18 km and the thickness of Pliocene-Quaternary section reaches 10 km. Hundreds of drill wells, some exceeding 6 km, have been drilled in the study area and adjacent onshore providing detailed subsurface information for evaluating the petroleum system. Molecular characterization of Miocene-Pliocene reservoired hydrocarbons indicate most of the oils have been sourced from the same or similar source facies at temperatures near or less than peak generation. Examination of Neogene and Paleogene source rock organic richness and molecular character indicates the hydrocarbons are not syngenetic in origin but most likely sourced from Lower Miocene or older shales. Maturation modeling based on well and seismic data indicates temperatures sufficient to generate large volumes of hydrocarbons does occur at depths from 6.5 to 12 km, 2 to 3 times greater than their present reservoir depths. A multi-stage model of hydrocarbon emplacement for evolving structural and stratigraphic traps has been postulated based on the distribution and type of hydrocarbons, tectonic-stratigraphic development, hydrocarbon generation models, and probable migration pathways.

Seismic reflection profiling and continental tectonics

The application of seismic reflection profiling to problems of continental evolution and tectonics represents a major advance in our ability to address questions about fundamental geologic processes with detailed subsurface information. Among geophysical techniques, reflection profiling has unparalleled potential for spatial resolution of subsurface structures, both in basement rocks and their sedimentary cover. Despite the rapid development and refinement of the seismic reflection method for hydrocarbon exploration and the widespread use of reflection profiling in marine investigations, the geologic community has been slow to take advantage of the method for studies of the continental crust in the United States. In the past 4 years, however, reflection profiling by the Consortium for Continental Reflection Profiling (Cocorp) project administered by Cornell University and by the U.S. Geological Survey has yielded remarkable results in several regions (see figure). It is, perhaps, easiest to illustrate the potential contribution of reflection profiling by briefly outlining the results of several recent studies.