Near-Surface Geophysics General Contributions II Posters
Presiding: R Knight, Stanford University; K Singha, Stanford University
NS23B-01 1330h
Study of the Crossborder Geothermal Field in the Sarandoporos- Konitsa Area With Integrated Geophysical and Geochemical Methods.
The studied area lies north of the city of Konitsa in Greece and includes the Albanian village of Koukes. Specifically, it lies between the geographic latitude (ö) 40° 02' up to 40° 07' and in geographic longitude (‰) 20° 37' up to 20° 45'. The geothermic field in Sarandaporos -Konitsa lies in the cross-border area between Albania and Greece. The field has several surface manifestations and extended geological investigations, including tectonic geochemical and geomorfological studies have been carried out. This work presents the conduct and interpretation of vertical electrical soundings (V.E.S.) in both sides of the borders. It is aiming to study the structure down to the depth of 1000m, in a relative large area, where information about the deep structure would have an enormous cost if acquired by a network of boreholes. Thermal springs known in the Greek part of the region are the thermal springs of Kavassila - Piksaria where the water temperature reaches up to 31o C. In the Albanian side the existence of hot springs (Koukes) is also well known, where the water supply is provided by the hot springs situated in the Skordili bridge. Two main geoelectrical formations revealed which coincide with the flysch and the limestone basement. The area appears faulted in the NW-SE and NE-SW directions and a few concealed graben and horst structures exist. Low resistivity values were observed above basement uplifts and major faults. These values were attributed to hot fluid circulation.
NS23B-02 1330h
AVO Analysis of GPR Data over Thinly Laminated Strata
Identification of multiple closely spaced thin layers (< 1/8 of GPR wavelength) in GPR records can be difficult as these sequences generate interference patterns. No distinct correspondence between the reflection events and the reflecting horizons can be made in such cases. As thin layers are common features in most sedimentary environments, any information related to their presence made available from GPR data would be valuable for stratigraphic purposes. FDTD (finite-difference-time-domain) models for different thin layer patterns show that detectable GPR responses can be produced at frequencies of 100MHz, 200 MHz and 500 MHz. In general GPR response from laminated zones shows a spectral shift toward higher frequencies. Here we examine the effects of thinly laminated zones on GPR trace amplitudes and frequencies as a function of antenna offset (AVO) with 2D FDTD models. Model results are compared against GPR common-midpoint gathers over magnetite-rich beach laminations on Waites Island, South Carolina.
NS23B-03 1330h
Development of a Seismic Physical Modeling System for Surface Wave Experiment
Imaging the near-surface objects in environment studies or even in globe seismology, surface waves play a curious role in seismic methods. For fundamental understanding of surface wave generation and data acquisition, we developed a physical modeling system with LabVIEW (a graphic programming language) to facilitate the surface wave modeling and simulation. The hardware of the system was configured with dedicated motion control board, source excitation, data acquisition board, serial ports card, angle beam transducers and wedges. The main program of the system consists of signal generation, data acquisition and acquisition automation. It is feasible to collect surface wave data in desire arrays that can be analysis simultaneously by surface wave methods, as well as seismic reflection. The system features a flexible geometry capability, a simple user interface, and transducer position real time display. It also include some embedded data analysis functions such as vertical stack, bytes swap, first-arrival auto pick, fast Fourier transform, time-gated spectrum analysis, shot gather display, and data annotation.
NS23B-04 1330h
Analyzing Layers of Soil Colluvia for Reconstruction of Soil Erosion and Holocene Landscape Genesis With Ground Penetrating Radar
Since the GPR method is suitable to differentiate soil layers with different water content based on the dielectric contrast, we apply it to solve landscape genetic and geomorphological questions. Historical and recent soil erosion events, caused by surface runoff, are documented in sequences of soil colluvia. These depositional areas called geoarchives often contain dateable objects, such as artifacts (potsherd or bricks) and charcoal. Geoarchives, e.g. colluvial fans and trench in-fills, are used as a source of information about past environmental conditions and for determination of land use impacts caused by human activities. Large exposures are common to characterize soil colluvia stratigraphy, and additional drillings are needed to correlate the layers and horizons found in different exposures. Often, soil colluvia sequences are characterized by a well defined layering and consecutive layers show different grain size. These layers have different saturation-suction relationships (pF-curve) and varied moisture contents. Our research focuses on radar mapping and characterizing these layers of soil colluvia in consideration of different moisture distributions. We present measurements with 200 MHz and 400 MHz antennas determined in a catchment area in northern Germany. Common offset measurements were used to map the distribution of accumulated sediments. GPR travel times were depth migrated to correlate them with the exposure survey. The velocity distribution with depth was determined with multi offset measurements and analysis of reflections of a metal rod in a known depth. TDR measurements in different layers within the exposure are used to verify the moisture distribution with depth. We mapped the boundary between soil colluvium and the underlying parent material (weichselian till, glaciofluviatil sand) and differentiated layers within the soil colluvia. Consequently a more detailed balancing of erosion and accumulation rates to quantify historical soil losses is possible. GPR measurements in soils are due to the fast and nearly non-destructive application and the additional detailed spatial information an excellent supplement of landscape genetic investigations.
NS23B-05 1330h
Method for Enhancing the Depth and Spatial Resolution of One and Two Dimensional Residual Surfaces Derived from Scalar Potential Data
An improved method is reported for collecting or assembling scalar potential data measurements that are to be subsequently prepared as a surface representation for residual analysis via frequency domain transform filters. Measurements are made over a geographic reference region which extends in all cardinal directions from the center of some previously determined primary region. The reference dimensions must contain the primary region and must be plural multiples of the dimensions and the depths of the primary region to be considered in analyzing the contributions to the measurements. An iterative filtering approach is described which permits maximum advantage to be realized from the additional information provided by the extended baseline used for the field measurements. Traditional statistical techniques may used to construct an histogram from the set of values comprising a surface representation. This histogram constitutes a Spatially Correlated Potential Spectrum for the surface, and provides additional information regarding specific spatial and physical parameters. The improved resolution of geological structures over the depths and spatial extents under consideration is demonstrated and discussed.
NS23B-06 1330h
Resistivity Imaging of Spring Valley, Nevada Using the Audiomagnetotelluric Method
Audiomagnetotelluric (AMT) sounding data collected in Spring Valley, NV show significant two-dimensional (2D) structure within the upper kilometer of the valley and help define the shallow basement surface. We collected AMT data along two profiles in the southern part of Spring Valley in the Fall 2004, using the Geometrics StrataGem EH4 system, a four channel, natural and controlled-source tensor system recording in the range of 10 to 92,000 Hz. To augment the low signal in the natural field a transmitter of two horizontal-magnetic dipoles was used from 1,000 to 70,000 Hz. Profile A extends 12.6 km from the Fortification Range on the west across southern Spring Valley to the Limestone Hills in the east with soundings recorded every 200 m. Profile B is a 2-km long, roughly E-W trending line located at the northern margin of the Fortification Range, roughly parallel to and 12 km NW of Profile A, with sounding spacings of 200-400 m. Data were recorded with the electrical field (E) parallel and perpendicular to the regional geological strike direction. We computed our preferred two-dimensional, inverse models from the E perpendicular mode data using the conjugate gradient, finite-difference method of Rodi and Mackie (2001) and a 100 ohm-m half-space, starting model. Inverse models were also computed using both modes and an equivalent model resulted, with a slightly higher RMS fit, indicating the two-dimensionality of the structure. Various starting models were used to test the depth of investigation. The model along Profile A shows detailed structure within the alluvial basin. Preliminary interpretation shows a clear transition between unsaturated (200-500 ohm-m) and saturated alluvium/volcanic rocks (20-50 ohm-m) at roughly 100 m depth. Highly-resistive (>1000 ohm-m) carbonate rocks are clearly defined at the east end of Profile A, and the locations and dips of several range-front and inter-basin faults, which lack surface expression, are delineated throughout the upper 1 km of section. In addition, our results define the shape of and the depth to the basement surface, which correlates well with depth to basement estimates derived from the inversion of gravity data. The model along Profile B shows conductive features within carbonate rocks that crop out nearby. The saturated alluvial valley fill and volcanic rocks are well defined along with the location of a range-front fault. As these results show, the AMT technique is a valuable tool for defining subsurface structure and stratigraphy within Spring Valley to roughly 1 km depth.
NS23B-07 1330h
Geophysical Exploration of the Red Rocks Canyon Landfill in Colorado Springs, Colorado
Our introductory geophysics class conducted a survey of the Red Rocks Canyon landfill to determine its boundaries, depth, type of fill, and groundwater runoff patterns. In the 1970s and 1980s the canyon was filled with domestic waste, and has recently been acquired by the city to extend an existing park. Our results in general portray a heterogenous subsurface and reveal that the landfill likely contains many metallic objects. More specifically we found the following: A negative Bouguer anomaly across the landfill matches a model for a ~25 m thick fill. Resistivity is much lower on the landfill than off, but we could not confirm our hypothesis that the landfill drains water through the north end of the canyon. Our seismic data are inconclusive, we think that our assumption of a planar bottom and seismically homogenous fill is violated. The magnetic data show on average higher total values, large (>1,000 nT) variations and high magnetic gradients on the landfill; because this method is fast and very clearly maps the landfill boundaries we propose a magnetic survey to detect possible unregistered dumps outside the mapped boundaries of the landfill. Measuring ground conductivity utilizing an electromagnetic survey produces significantly higher values on the landfill than off. Additionally, directional anisotropies match the direction of the strike of the geological formations adjacent to the landfill and are more randomly oriented on the landfill.
NS23B-08 1330h
Multi-method Geophysical Surveys for the Study of Holocene Landslide Deposits
This paper integrates the results of a geophysical investigation of two Holocene landslides in the Cypress Hills, Alberta, Canada. Seismic refraction, ground penetrating radar (GPR) and electrical resistivity imaging (ERI) were used to retrieve information about the type and morphometry of the landslide surfaces and boundaries. The earlier event occurred about 9,400 years ago and produced the East Slump Block (ESB); the West Slump Block (WSB) dates from mid- to late Holocene. The debris surfaces were encountered in boreholes at 6.5 m on the ESB and at 1.5 m on the WSB. The geological and geomorphological settings suggest a simplified 3-layer model: soil and post-landslide deposits of varying thickness, landslide mass, and bedrock or channel fill depending on elevation. Previous research suggests that the landslides evolved in a conglomerates formation. GPR profiling was used to characterize the first few meters of the subsurface, as the depth of penetration was limited due to highly attenuating lithologies. Stratigraphic elements were inferred from changes in the propagation velocity of the ground wave, the presence of diffraction events and weak, discontinuous reflectors from depths greater than 10 m. The envelopes of strong diffractions at a depth of 6-8 meters can be interpreted as the top of the landslide. Seismic refraction complements the GPR measurements, as it can generally provide data from greater depth. Sites at the toe of the landslides produced an interface at depths less than 8 meters for the ESB and less than 2 m for the WSB. For sites located nearer to the scarp, the characteristic element is an interface at average depth of 15 m. ERI is in principle the most appropriate method for clay-rich deposits. A number of 2D sections were obtained parallel to main seismic and GPR profiles. Electrical images provide information about the structure of the upper 30 meters of the subsurface and generally reinforce the results from other methods. GPR, seismic refraction and ERI were found suitable to analyze the shallow internal structure of the landslide mass. These geophysical techniques could therefore become standard tools for the characterization of ancient landslide deposits.
NS23B-09 1330h
Near surface geophysics on Magallanes-Fagnano Fault System. Tierra del Fuego. Argentina
A main portion of the South America-Scotia plate boundary related to the Magallanes-Fagnano Fault System (MFS) is occupied by the lago Fagnano. Just at the eastern end of lago Fagnano a sag pond area -which is the surface expression of one basin associated with the MFS- is flanking the Hewhoepen monzodioritic intrusion. The MFS overprints the fold-and-thrust-belt of the Fuegian Orogene and is associated with basin formation along its length. These basins display the greater sedimentary thickness against the main fault in the major displacement zone. Both Paleogene and Quaternary outcrops show transtensional structures associated with MFS. An electric resistivity survey was carried on at the eastern lago Fagnano with the purpose of constraining the subsurface continuation of W-E faults related to MFS and the unexposed portion of the Hewhoepen body. A Syscal R1+Switch 48 resistivity-meter system was employed to perform the 1.6 km-long, N-S, two-dimensional electrical imaging/tomography survey. The roll all on method was applied with a Wenner array using 48 electrodes (with 10 m of constant spacement) connected to a 480 m-long multi-core cable, with four sections and 48 channels. The pseudo section and the obtained 2D inversion model allow inferring two lithologies in the subsurface which display at least two contrasted resistivities, i.e. the uppermost peat blanket and lower sandstones. Both litologies show a network of vertical and horizontal discontinuities defining small fragments dipping southwards. A high resistivity body was also recognized in correspondence with the magnetic anomaly of the Hewhoepen intrusive.
NS23B-10 1330h
Subsurface Investigation of the Internal Architecture of Massawepie Esker-Mire Complex, Gale, New York
The retreat of the Laurentide Ice Sheet from the Adirondack Highlands of northern New York approximately 12,500 yr. B.P. led to the development of a landscape marked by glaciofluvial landforms such as kames, outwash, kettles and eskers. The Gale, New York study site (44o 15'N, 74o38'W) contains a prominent esker approximately 30 m high that trends northeast/southwest for 12 km. A 364 ha ombrotrophic peatland underlain by outwash lies adjacent to the esker along a portion of its length. Ground Penetrating Radar (GPR) surveys were conducted on the esker and peatland areas, as well as at the contact between these landforms in an attempt to establish possible linkages between their environmental, developmental and topographic settings. Using 50 and 100 MHz antennas, the esker profiles yielded a signal penetration of approximately 20 m through primarily sand. Interpreted facies showed reflector beds of variable continuity indicative of both turbulent and laminar flow, chute and pool structures, erosional unconformities, random boulder distribution and diagenetic failure planes. On the peatland, the surveys were successful in mapping internal structure and development of the peat, and delineation of a relatively flat basal outwash unit dated at 5,600 yr. B.P. at approximately 3 m depth. The basal unit consists of medium to coarse grained sand, with discrete portions of the bed raised 2 m above the high amplitude basal reflectors suggesting possible channelization. The preliminary interpretation of a channelized base suggests that there is a link between the esker and peatland. The sinuous form of the infilled subglacial network suggests that the rise in the peatland base may be a remnant of an esker branch. This smaller remnant branch bounded by shallow stagnant ice depressions could create a landscape that would encourage peatland development. The peatland currently has three different floral zones showing different levels of wetland maturation possibly reflective of the basal topography.
NS23B-11 1330h
Adaptive Power Spectrum Method for Processing Full Waveform Logs
The power spectra of borehole mode waves like compressional and shear head waves and Stoneley waves etc., are very important information from full wave acoustic logging data analysis. Conventional power spectrum analysis based on Fourier transform cannot derive such detailed power spectra because of none resolution in time domain. We have developed a new power spectrum analysis technique for full waveform acoustic array logs. Based on local cosine packet analysis and K-L transformation, this technique can diagonalize the correlation matrix of the acoustic signal. Meanwhile for the local stationary signal, the local cosine base is the best candidate of K-L transformation base. After the correlation matrix is diagonalized, the elements at diagonal represent the power spectrum of the signal. By arranging this procedure into an optimization procedure, we can thus determine the time window for each mode adaptively while calculating each individual power spectrum at the same time. The first step of the data processing algorithm is to decompose the whole signal with local cosine packet. And then the power spectrum of each component is calculated. Finally, we will find the optimum base with the local cosine packet tree. The objective function of the optimization is composed of the norm-2 of the power under certain base. Because the full waveform of acoustic logging is not an exact local stationary signal, the compressional and shear head waves are correlated. Therefore the STC method is employed to get the arrival time of compressional and shear head waves from the array waveform data, and then the shear arrival time can be taken as a constraint for this optimization. We have applied this method to a set of field full waveform logs. Our results have shown that the individual borehole modes are separated correctly and the power spectra for each mode agree with the existing theoretical predictions.
NS23B-12 1330h
Use of Aeromagnetics in Mineral/Structural Exploration in Nigeria.
Aeromagnetics surveys have been employed in discovering a number of oil fields.In this study, various graphical methods of aeromagnetic interpretation was employed in determining the depth to anomalous structure. The anomalies on which these methods were tested were taken from the lower Benue trough of Nigeria. The parameter estimated in this study was the depth to the buried magnetic source or structure. It was discovered that the depth to the top of the structure or source lies between 0.55Km and 9.20Km. The magnetic anomalies over parts of this trough could be explained by the existence of the intrusive bodies and elavation of crystalline basement. This work reconfirms the usefulness of some of the graphical methods of aeromagnetic interpretations especially when applied to isolated and non-interfering anomalies. Depths estimates from these work can be used as input for more detailed investigation of the anomalous bodies using more quantitative approaches.
NS23B-13 1330h
ProcintLog: A GUI Academic Software For Open Hole Well-Logging Analysis
ProcintLog is a GUI academic software that runs under MATLAB and is useful for simplified conventional oil and gas well logs analysis. It works through two distinct modes: (1) direct petrophysical parameters inclusion from both field copies and digital files and (2) by reading and filling data from visual log zoning. The software load ASCII and LAS format files and identifies the logs available, supplying both a header file with additional information about the well and a matrix that depicts the depth in the first column and the logs in the remain ones. The last can be loaded by ProcintLog and checked out for unusual highly noisy and spurious data, which allows a complete data control. Furthermore, each one of the proposed processing modes fits well to any chosen processing sequence, allowing the log interpreter to direct your own necessities into log analysis. To provide a little of perspective, fundamental well-logging principles are stressed by informative icons that can be searched at any time the interpreter need. Calculations such as invasion-corrected resistivity, shale volume, Archie and Simandoux methods for water saturation computations and three types of water resistivity computations are viable. One complete example is performed on both synthetic and real data and results are checked against visual analysis to show the usefulness, rapidness and accuracy of ProcintLog in performing accurate logs analysis.
http://www.seg.org
NS23B-14 1330h
A New Method for Improving the Resolution of Shallow Sub-bottom Seismic Reflections in Deep Water
A new method of achieving very high resolution of seismic reflections near the sea floor in deep water has been developed and tested during several sea trials. The method employs a conventional energy source deployed on the sea surface and a single channel receiver deployed at few hundred meters depth. As much as possible, the source is towed directly above the receiver. The intention of this geometry is to record a far-field source signature that is essentially a vertically traveling plane wave. The principal restriction is that the receiver be located far enough above the sea floor that the direct arrival is not contaminated by the sea-floor reflection. This means that, if pneumatic sources are to be used, the water should be at least a hundred meters deeper than the receiver tow depth. During data acquisition, the procedure known as "over sampling" is practiced by digitizing the received signals at a rate on the order of 100,000 samples per second. This is sufficient to achieve good waveform fidelity in the 5-1 kHz range. Post processing then uses the source signature to collapse the waveforms of the reflected wavelets on a trace-by-trace basis. The result is that the reflected wavelets are transformed into waveforms that are generally symmetric about a central peak or trough, depending on the sign of the reflection coefficient. Self-calibration permits the magnitude of the central lobe to be used to estimate the value of reflection coefficients. Sea trials using 15 and 80 cu.in. waterguns were carried out in water depths ranging from 300m to 1600m. Results indicated that the thicknesses of layers up to 75ms below the sea floor were resolved to within a few tenths of a millisecond. In one trial across a well where a potential shallow flowing sand 400 ms below the sea floor had been encountered but controlled, the upper boundary of the sand body was resolved to less than a millisecond.
NS23B-15 1330h
Sinkhole Imaging With Multiple Geophysical Methods in Covered Karst Terrain
A suite of geophysical surveys was run at the Geopark at the University of South Florida campus in Tampa in attempt to determine the degree to which methods could image a collapsed sinkhole with a diameter of ~4m and maximum depth of ~2.5m. Geologically, the Geopark is part of a covered karst terrane, with collapsed sinkholes filled in by overlying unconsolidated sand separated from the weathered limestone beneath by a clayey sand layer. The sinkholes are hydrologically significant as they may serve as sites of concentrated recharge. The methods used during the study include: refraction seismics, resistivity, electromagnetics (TEM and EM), and ground penetrating radar (GPR). Geophysical data are compared against cores. The resistivity, GPR, and seismic refraction profiles yield remarkably consistent images of the clayey sand layer. EM-31 data revealed regional trends in subsurface geology, but could not delineate specific sinkhole features with the desired resolution.
NS23B-16 1330h
Deepwater Gulf of Mexico Shallow Hazards Studies Best Practices
ConocoPhillips (hConoco) has been involved in deepwater exploration in the Gulf of Mexico for the last 5 years using a dynamically positioned (DP) drillship. As part of the Federal (MMS) and State permitting process for deepwater exploration, ConocoPhillips (COPC) actively undertakes in securing seabed and shallow subsurface hazard surveys and analyses for every potential drillsite. COPC conducts seabed and shallow subsurface hazards surveys for at least two main reasons: To be a safe, efficient operator, seabed and shallow subsurface hazard surveys and analyses are necessary steps of the Exploration Work Flow to help ensure a safe well, and to fulfill MMS (or local government) regulatory requirements The purpose of shallow geohazards studies is to determine seafloor and sub-bottom conditions, inspect for possible chemosynthetic communities, and to provide a shallow hazards assessment in accordance with NTL 2003-G17. During the five years of deepwater exploration COPC has contracted Fugro Geoservices to perform hazards studies in over 30 offshore blocks. Deepwater Gulf of Mexico Shallow Hazards Studies Best Practices The results of the seabed and shallow geohazards are a critical part of the construction of all of our well plans and are dynamically used in all MDT's. The results of the seabed and shallow geohazards investigations have greatly improved our drilling efficiency by predicting and avoiding possible chemosynthetic communities, sea floor faults, shallow gas, and shallow water flow. CoP's outstanding safety record and environmental stewardship with regards to geohazards has helped us in accelerating certain Exploration Plans (within MMS guidelines). These types of efforts has saved money and kept the drilling schedule running smoothly. In the last two years, the MMS has given COPC approval to use existing 3D spec seismic volumes for Shallow Hazards Assessment at several locations where applicable. This type of effort has saved ConocoPhillips hundreds of thousands of dollars that would have been spent in either acquiring 2D high resolution seismic data or reprocessing an existing 3D data volume. Examples from Selected Prospects: Magnolia (Garden Banks 783/784); Voss (Keathley Canyon 347/391/435); Lorien (Green Canyon 199); Yorick (Green Canyon 391/435)