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21st EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 06 Apr 2008 - 10 Apr 2008
- Location: Philadelphia, USA
- Published: 06 April 2008
101 - 120 of 136 results
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Bias Noise Correction In Multifrequency Fixed Small Loop Electromagnetic System
Authors Yuji Mitsuhata, Takehiko Imasato and Toshihiro UchidaRecently, the use of fixed small loop electromagnetic surveys has become popular for environmental and engineering investigations of relatively shallow subsurfaces. The fixed small loop
EM method is effective for the detection of anomalous zones by mapping and profiling of a large area. However, zero level adjustment is crucial for further inverse analysis in order to estimate subsurface resistivity structures. A method is proposed for on-site measurement and correction of bias error, and two application studies are demonstrated; a high-salinity groundwater investigation and a levee assessment survey.
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Design And Installation Of A Permanent Three-Component Geophone String At An Aquifer Storage And Recovery Site; Beenyup Western Australia
Authors Brett D. Harris, Milovan Urosevic, Anton Kepic and Michael SykesSeismic reflection is taking on a more prominent role in the world of hydrogeology. Seismic techniques such as Vertical Seismic Profiling (VSP), now common in the petroleum industry, must be suitably adapted for new shallower applications (i.e. Hydrogeology). Managed aquifer recharge (MAR) and aquifer storage and recharge (ASR) have become important tools for groundwater management and require a higher standard of investigation, especially in the near well setting. For relatively shallow injection zones the maximum rate of injection is dependent on a number of factors including; depth to the injection zone; distribution of hydraulic conductivity and the integrity/strength of layering above the injection zone. Vertical Seismic Profiling (VSP) has potential to recover detailed information about near well hydrostratigraphy. We have designed and installed a permanent three-component (3C) geophone string proximal to a high volume injection bore at the Beenyup trial aquifer storage and recover site; Perth Western Australia. The deepest geophone is 280 m below ground level. The string consists of ten vertical axis geophones and six tri-axial geophones. The geophone array spans up to five distinct lithological units including major aquatard zones (e.g. the Pinjar Member of the Leederville Formation) and the main aquifer zone (i.e. the Wanneroo Member of the Leederville Formation). In combination with a 3-D seismic survey the VSP data will be used to recover detailed hydrostratigraphy for input to
hydraulic flow and reactive transport models for the site. The geophone string will form permanent research infrastructure for a number of 3-D and 4-D VSP experiments to be conducted during the Injection trial at Beenyup. We detail the processes used to design and install the Beenyup geophone string and provide examples of the 3C - VSP data obtained.
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Application Of Surface-Wave Method To The Evaluation Of Local Site Effect Of 2007 Noto Earthquake
Authors Koichi Hayashi, Masahito Tamura, Tsutomu Hirade, Kosh Nath Adhikari and Yu ShizhouA surface-wave method and micro-tremor array measurements have been carried out at Central Anamizu, Ishikawa Prefecture, Japan in order to investigate local site effect of earthquake in terms of near surface S-wave velocity model. Many houses were collapsed or severely damaged by the 2007 Noto Earthquake at the Central Anamizu. Collapsed or damaged houses were concentrated around the K-NET Anamizu strong ground motion observation station. It seems that the near surface heterogeneity may strongly affect surface ground motion during the earthquake. The investigation result shows that S-wave velocity down to the depth of 20m has strong heterogeneity at the site. Low velocity layer, in which S-wave velocity is 60m/s, beneath the K-NET Anamizu disappears at the other side of the park only 50m away from the K-NET. Swedish ram soundings were also carried out round the K-NET and the results agreed with one of surface-wave method very well. Two-dimensional site amplification was calculated by finite-difference method using the S-wave velocity model obtained through the investigation. The calculation shows that near-surface heterogeneity strongly affects amplification of the site.
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Rayleigh-Wave Dispersive Energy Imaging By Highresolution Linear Radon Transform
Authors Yinhe Luo, Jianghai Xia, Richard D. Miller, Jiangping Liu, Yixian Xu and Qingsheng LiuMultichannel Analysis of Surface Waves (MASW) analysis is an efficient tool to obtain the vertical shear-wave profile. One of the key steps in the MASW method is to generate an image of
dispersive energy in the frequency-velocity domain, so dispersion curves can be determined by picking peaks of dispersion energy. In this paper, we propose to image Rayleigh-wave dispersive energy by high-resolution linear Radon transform (LRT). The shot gather is first transformed along the time direction to the frequency domain and then the Rayleigh-wave dispersive energy can be imaged by highresolution LRT. Results of synthetic and real-world examples demonstrate that, compared with slant stacking algorithm, high-resolution LRT can improve the resolution of images of dispersion energy by more than 50%.
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Use Of Resistivity And Ip As A Potential Screening Tool For Ydrocarbon Delineation In Highly Conductive Environments
More LessThe USEPA TRIAD approach was used at a former refinery complex to expedite the evaluation and subsequent redevelopment of the 13-acre brownfield property. The Geoprobe System soil conductivity/membrane interface probe (SC/MIP) and electrical resistivity imaging were selected as rapid screening tools to confirm the site conceptual model. These tools were combined with advanced data analysis techniques to develop a targeted soil and ground water sampling program and evaluate feasibility of various remedial strategies. A resistivity survey identified lithologic changes, bedrock surfaces, and provided a graphical three-dimensional representation of the subsurface geology which was critical to the design of the SC/MIP subsurface investigation. Resistivity and induced polarization (IP) data were collected with a 48-electrode Syscal Pro Switch using a modified offset wenner array designed for collection of high
signal-to-noise measurements in conductive environments. Resistivity and IP data from the former refinery process areas indicated several anomalous areas that were consistent
with the location of apparent underground refinery structures and associated hydrocarbon plumes. These results were confirmed with SC/MIP and subsequent soil and ground
water sampling programs. Results from this study corroborate the hypothesis that resistivity and IP data can be used to expedite subsurface investigations and reduce costs
associated with traditional investigation techniques.
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Hydraulic Conductivity Estimation Using Tomographic Ground Penetrating Radar Data Within A Sampling-Based Bayesian Inversion Approach
More LessTomographic ground penetrating radar (GPR) traveltime data has been frequently used for hydrologic parameter estimation in shallow subsurface. However, more efforts are needed to deal with the following issues: 1) to improve the accuracy and efficiency of GPR forward models that are used to compute GPR traveltimes; 2) to develop robust and efficient inversion approaches which can well quantify the uncertainty associated with parameter estimates. In this study, a finite-difference method that solves the eikonal equation in the celerity domain is adopted to compute GPR first arrival traveltimes. It combines a local traveltime computation scheme, where not only transmitted waves, but also diffracted and head waves are considered. The method uses a global fast sweeping method in the time domain to take all possible wave propagations from different directions into account. A samplingbased Bayesian inversion approach is proposed and evaluated. The approach directly estimates hydraulic conductivity based on GPR traveltime information, in the form of probability distributions. The corresponding statistics can be easily obtained through the distributions, providing a complete understanding of the uncertainty in the parameter estimates. The inversion framework is tested using synthetic GPR first-arrival traveltime data collected at different times during infiltration experiments. Results show that the inversion approach can well identify the hydraulic conductivity distributions and
therefore provides a good means of coupling different yet complementary information for hydraulic parameter estimation.
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Results Of Airborne Gradient Demonstration At Kirtland AFB
Authors T. Jeffrey Gamey, Jacob Sheehan, William Doll, Les P. Beard and Jeannie NortonIn April 2007, Battelle demonstrated two new airborne gradiometer systems for ESTCP at Kirtland AFB. The primary benefit of vertical gradient over total field is that it reduces helicopter noise, improving signal-to-noise by a factor of 5. The VG22 is designed for high resolution detection of small ordnance under good field conditions, while the VG16 is designed with a wider swath for better production rates or where conditions require slightly higher altitudes. The site conditions were well known from previous surveys. A magnetically quiet area was used for seeded items to test detection capabilities for small ordnance, and a known bombing target was used to test discrimination against background clutter. The seeded area included rounds ranging from 155mm to 40mm. The bombing target ordnance was primarily M38s, although there were too few within the survey block for meaningful analysis. This paper focuses on the analysis of the seed items. The VG16 system detected 67% of all seeded targets with a 1.5m search radius. The number of false positives is unknown since no follow-up was done on any of the anomalies, although the number of picks per acre was relatively low at approximately 12 picks/acre. The higher resolution VG22 system detected 90% of the seeded targets, including the 40mm projectiles, with a 1.0m search radius and roughly the same number of picks as the VG16. Reducing the picking threshold for the VG22 from 2.5nT/m to 2.0nT/m doubled the number of picks and increased the detection rate to 98%. The other significant benefit of the vertical gradient system is that it better defines closely spaced targets. To measure this effect, pairs of 60mm mortars were also seeded in this area at separation distances between 1m and 4m. This demonstration shows that the resolution of the VG22 system is such that the individual items within these pairs were detected at exactly the same probability as the more widely spaced 60mm seed items.
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Magnetic Surveys Over Burning And Remediated Coal Seam Fires In Western North Dakota
Authors Rob Sternberg, Andrew Sparks and Oscar KnudtsonBurning coal seams are increasingly recognized as an environmental hazard. These fires alter overlying soils and sedimentary rocks, changing their magnetic properties and causing magnetic
anomalies. We conducted magnetic surveys at four sites in an area of recently burning coal seams in the Little Missouri National Grassland, McKenzie County, western North Dakota. The maximum anomaly amplitude was over 1000nT at the site where the seam is still burning; the anomalies may be due to formation of highly magnetic clinker, and/or the reduction of magnetic minerals in the soil. Smaller anomalies were identified at one remediated site, and at a site not previously identified as having burning coal. The fourth site did not have significant anomalies, because the fire was extinguished and the profiles were probably too far away from the burn area. These kinds of anomalies could be useful in remediation efforts for delineation of subsurface burns and for simple depth estimates to coal seam fires.
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Magnetometer Survey For Buried Steamboats Near The Mouth Of The Missouri River
Authors Timothy H. Larson and F. Terry NorrisDuring the early- to mid-eighteenth century hey-day of steamship navigation on the Mississippi and Missouri Rivers literally hundreds of ships were lost due to snags, explosions and collisions. Shifting river channels, especially prior to channel stabilization in the early twentieth century, has left many of the wrecks below corn fields instead of water. We conducted a reconnaissance magnetometer survey near the mouth of the Missouri River to search for four reported shipwreck sites. The magnetometer and DGPS location and guidance system were mounted on an aluminum backpack with the GPS antenna at 2 m and the magnetometer sensor at 2.3 m above ground. Approximately 370 acres were searched with a 20 m transect spacing in June, 2007, locating one large magnetic anomaly within 100 m of one reported shipwreck location. A more detailed assessment of this anomaly using 5 m transect spacing revealed it to be 50 m in diameter and 102 nT maximum amplitude. This magnetic anomaly is consistent with an 8 to 10 ton iron object buried 13 m below ground. Further investigation of the site of this magnetic anomaly is recommended.
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Characterizing Groundwater Using Controlled Source Audio Frequency Domain Magnetics
Authors Jerry Montgomery and Val O. KofoedTraditional methods for determining the potential flow paths of subsurface pollution require extensive drilling a time-consuming process which results in significant expense and environmental trauma. The minimally invasive imaging procedure detailed in this presentation generates accurate groundwater maps and models but requires significantly less drilling. As a result, this method entails fewer costs in terms of time, money and ecological disruption. Initial case studies indicate that this procedure may be particularly well suited to the task of environmental remediation. This procedure relies on the principles of Controlled Source - Audio Frequency Domain Magnetics (CS-AFDM). First, electrodes charge the groundwater in question with a low voltage electrical current. As the current passes through the water between the electrodes, it emits a magnetic field whose size, shape, magnitude and direction are characteristic of the surrounding aqueous system (Biot-Savart Law). This field is then read at the surface by a specially tuned receiver, ultimately resulting in maps and models that indicate the attributes of the subsurface water network, including potential flow paths.
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Identification Of Unexploded Ordnance From Clutter Using Neural Neworks
Authors Anna Szidarovszky, Mary Poulton and Scott C. MacInnesThe largest costs associated with subsurface Unexploded Ordnance (UXO) remediation are associated with removing non-UXO. Discrimination between UXO and non-UXO is important for both
cost and safety reasons. A neural network was developed to distinguish between UXO and non-UXO clutter using TEM data. There are two stages for the learning process of neural network, training and validation. A synthetic dataset was created using actual acquisition configurations, with varying amounts of random noise. This dataset included 934 UXO targets representing 7 different UXO types, and 789 clutter objects based on four templates with varying size and random asymmetry. The results show 97% accuracy for correctly classifying clutter, and 97% accuracy for correctly classifying UXO. The level of success for classification is based on the classification Receiver Operating Characteristic (ROC) curves. The ROC curve represents the relationship between UXO classified correctly (Hit rate) versus clutter miss classified (False alarm).
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Sv-Wave And P-Wave High Resolution Seismic Reflection Using Vertical Impacting And Vibrating Sources
Authors André J.-M. Pugin, Susan E. Pullan and James A. HunterShear wave seismic reflection profiles are generally acquired using sources and receivers horizontally oriented to avoid interferences of the signal with unwanted energies like converted- or P-waves. With various tests using a sledge hammer impulsive source, we have defined a window of acquisition for SV-wave recording. The SV reflected signal can be differentiated from the surface waves in the frequency domain. Our tests have been conducted on the Champlain Sea sediments in the vicinity of Ottawa, Ontario, Canada. These Sediments have already proven to be ideal for P-wave and SH-wave high-resolution seismic imaging. SV sections using a vibrating source otbained over such environment display the highest resolution we have yet observed in any land-based seismic reflection survey. Tests over more compacted sediments have been less conclusive but it is hypothesized that this new SV high resolution seismic reflection method may be used in sedimentary environments that are already kwown to give good results with the SH-wave reflection method. The greatest benefit of the method is that with the same raw shot records it is possible to process a P and an SV high resolution seismic section.
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Considerations For Interpreting Surface Wave Data In Sites With Shallow Bedrock
Authors Daniel W. Casto, Barbara Luke, Carlos Calderón-Macías and Ronald KaufmannThe inversion of dispersive surface wave data has been shown to be successful in providing reliable estimated shear-wave velocities within unconsolidated materials in the near-surface. However, when this method is applied to a site consisting of unconsolidated material over hard bedrock (i.e. shearwave velocities >>800 m/sec), inverting the fundamental mode of Rayleigh waves results in shear-wave velocities within the rock that are less than half of the expected values. This phenomenon is illustrated using the multi-channel analysis of surface waves (MASW) method at a site consisting of clay residuum overlying basalt bedrock. In addition, some data processing techniques normally reserved for the spectral analysis of surface waves (SASW) method are applied to the MASW data set for comparisons. Forward modeling shows that for sites with a high impedance contrast (i.e. high velocity layer beneath low velocity layer), the fundamental-mode Rayleigh-wave dispersion curve is hardly sensitive to bedrock velocity perturbations over a practical range of wavelengths. Use of higher modes, additional geophysical data and/or a priori information of the site might be needed to converge to a more meaningful shear-wave velocity profile.
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Water Reservoirs As A Record Of Watershed And Reservoir Processes: A Case Example Lake Abilene, Tx
Authors Heidi Estep and John DunbarThis study investigates a different approach for identifying water reservoir sediment source regions within watersheds. The method uses physical and chemical analysis of targeted vibracores augmented with multi-frequency acoustic profiling to examine the sediment in Lake Abilene, Texas. The acoustic survey was conducted using a sub-bottom profiling system with central frequencies at 200, 125, 50, 24 and 12 KHz. Vibracores were taken in 4 locations to verify the acoustic interpretations and perform sediment analysis. Fluctuations in the Cs137 concentration were used as date markers to establish sedimentation rates. The C/N was used to discriminate between sources of organic material to indicate whether the sediment was deposited under base flow or flood conditions. Thin sections were examined to characterize the contact between sediment layers and the deposition pattern in each layer. XRD analysis was preformed to correlate the mineralogy of the layers in the sediment to sources in the watershed. Results show that the sediment cores are characterized by dark grey, cm-scale lamination with high C/N that consists of graded silt beds that fine upward in thin section. Approximately 35% of the sediment is derived from within the watershed, while the rest is eroded from the lakeshore.
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Levee Evaluation Using Masw: Preliminary Findings From The Citrus Lakefront Levee, New Orleans, Louisiana
The utility of the multi-channel analysis of surface waves (MASW) seismic method for noninvasive assessment of earthen levees was evaluated for a section of the Citrus Lakefront Levee, New
Orleans, Louisiana. This test was conducted after the New Orleans’ area levee system had been stressed by Hurricane Katrina in 2005. The MASW data were acquired in a seismically noisy, urban environment using an accelerated weight-drop seismic source and a towed seismic land streamer. Much of the seismic data were contaminated with higher-order mode guided-waves, requiring application of muting filtering techniques to improve interpretability of the dispersion curves. Comparison of shear-wave velocity sections with boring logs suggests the existence of four distinct horizontal layers within and beneath the levee: (1) the levee core, (2) the levee basal layer of fat clay, (3) a sublevel layer of silty sand, and (4) underlying Pleistocene deposits of sandy lean clay. Along the surveyed section of levee, lateral variations in shearwave velocity are interpreted as changes in material rigidity, suggestive of construction or geologic heterogeneity, or possibly, that dynamic processes (such as differential settlement) are affecting discrete levee areas. The results of this study suggest that the MASW method is a geophysical tool with significant potential for non-invasive characterization of vertical and horizontal variations in levee material shear strength. Additional work, however, is needed to fully understand and address the complex seismic wave propagation in levee structures.
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Electrical Resistivity As A Tool For Identifying Geologic Heterogeneities Which Control Seepage At Mirror Lake, Nh
The results of electrical resistivity surveys using cables both surface-towed and stationary (lakebottom) at Mirror Lake, NH suggest that resistivity surveying can be useful for characterizing geologic heterogeneities that control groundwater-surface water interaction, as well as for imaging road-salt contamination. In-situ measurements of seepage coincident with resistivity surveys suggested a relationship between resistivity values and seepage rates. Specifically, we observed that seepage rates were low (averaging -22 cm/day) at Mirror Lake where resistivity values were greater than or equal 3000 Ω-m and where they were less than or equal to 100 Ω- m. Low (100 Ω- m) resistivity values were indicative of organic matter deposits. High (3000 Ω-m) resistivity values were indicative of low porosity, poorly sorted till. Intermediate (~ 1500 Ωm) resistivity values were observed in the regions where seepage rates were highest (averaging -92 cm/day). Core, modeling, and slug test data suggest that these intermediate resistivity values reflect more well-sorted, higher-porosity drift. Resistivity surveys of the suspected region of salt contamination revealed a plume-shaped feature of low resistivity. Low resistivity and high chloride content were confirmed by laboratory analysis of pore fluid. We conclude that the rapidly acquired towed-cable survey can guide placement of higher-resolution, more time-consuming stationary cable surveys. The use of stationary cable resistivity surveys in the very nearshore
environments (<2 m from shore), which are generally inaccessible with a towed-cable survey, can guide seepage meter placement.
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A 3D Seismic Survey For Shallow Aquifer Mapping
Authors Don C. Lawton, Robert R. Stewart and Malcolm B. BertramA high-resolution 3D seismic survey was undertaken to map shallow aquifers near Calgary, Alberta. The survey was 500m x 300m in area, with shot and receiver lines in an orthogonal
geometry using 50m line separation. Shots and geophones were spaced at 10m intervals along source and receiver lines, respectively. The surface source used was an 18,000lb EnviroVibe
sweeping over a 10Hz to 180Hz range. The survey yielded excellent reflections with a dominant frequency of 50Hz. One high-amplitude west-dipping reflection, occurring between depths of
250m and 450 m, was mapped over the survey area. The event is correlative to a Tertiary sandstone which may be a regional aquifer.
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Application Of Magnetic Resonance Soundings And Other Surface Geophysical Methods To Enhance Subsurface Ananlysis Of A Ground-Water Availability Model – A Pilot Study
Authors Sachin D. Shah, Wade H. Kress and Anatoly LegchenkoAn integrated surface geophysical pilot study at the Texas A&M University Brazos River Hydrologic Field Research Site (BRHFRS), College Station, Texas, was done to determine the
effectiveness of methods for defining the hydrostratigraphic framework and hydrogeologic properties for a ground-water availability model. Time-domain electromagnetic (TDEM) soundings and directcurrent (2D–DC) resistivity imaging were used to define the lateral and vertical extent of the Brazos River alluvium aquifer, the Ships clay within the aquifer, and the Yegua Formation underlying the aquifer at the BRHFRS. Magnetic resonance soundings (MRS) were used to derive estimates of hydrogeologic properties including percentage water content, transmissivity, and hydraulic conductivity. Stratigraphically, the principal finding of this study was the relation between electrical resistivity and the depth and thickness of the subsurface geologic units at the site. Not only could thicknesses and extents of these units be defined to a greater level than previously interpreted, but lateral variations in resistivity within the alluvium aquifer also could be detected. MRS data have added supporting data to the 2D–DC resistivity and TDEM data allowing for improved understanding of the hydrostratigraphic framework. Hydrostratigraphically, individual hydraulic conductivity values derived from MRS were in close agreement with previously conducted aquifer tes+H99ts. Average hydraulic conductivity values from the aquifer test are about 61 to 80 m/d, whereas, the MRS-derived hydraulic conductivity values are about 27 to 97 m/d. Results from the geophysics study demonstrated the usefulness of combined TDEM, 2D–DC resistivity, and MRS methods to reduce the need for additional boreholes and to create more accurate ground-water availability models using the acquired data.
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Humanitarian Geophysics
Authors Catherine Skokan and Fred BoaduSeventy percent of the earth’s surface is covered by ocean and approximately 97% of the earth’s water is saline. Two percent of the earth’s water is contained in ice sheets and glaciers. There remains one percent of the water that is available as a resource that we can consume. More than one billion people do not have access to potable water. Underground water is often safer and more reliable that surface sources. Clean water is essential to the general health of any population. Although geophysicists are often employed in the arena of extraction of energy and materials resources, many geophysicists have dedicated their work to humanitarian causes. Such humanitarian causes include either mapping of clean groundwater, or making sure that groundwater resources available are clean for human consumption. This presentation will summarize some of the most of the recent the geophysical work that has been beneficial to the poor and the underserved in the search for groundwater, in mapping saline/fresh water interfaces, and in determining water quality.
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An Experimental Investigation Of Surface Wave Scattering Over A Site With Shallowly Buried Heterogeneities
Authors Barbara Luke and Carlos Calderón-MacíasA two-component (vertical and radial) seismic array with a hammer source was deployed over two shallowly-buried drums to study near surface scattering at a controlled site where background properties are known. Hodograms demonstrate Rayleigh wave energy in the response and possible nearsurface effects. For each shot location, gathers are processed through a frequency-phase velocity transform. The resulting maps are stacked across frequency and velocity to produce two-dimensional images useful for analyzing energy distribution along the array. The analysis might be used to identify portions of the dataset that should be filtered out for background mapping, or, alternatively, to illuminate buried features.
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