Imaging of fracture zones in the Finnsjön area, central Sweden, using the seismic reflection method

Geophysics ◽  
1995 ◽  
Vol 60 (1) ◽  
pp. 66-75 ◽  
Author(s):  
Christopher Juhlin
Keyword(s):  
Fracture Zone ◽  
Seismic Profile ◽  
Decay Rates ◽  
Damping Factor ◽  
Fracture Zones ◽  
Borehole Data ◽  
Seismic Reflection Method ◽  
Adequate Quality ◽  
Initial Processing ◽  
Geophone Spacing

In 1987 the Swedish Nuclear Fuel and Waste Management Co. (SKB) funded the shooting of a 1.7-km long, high‐resolution seismic profile over the Finnsjön study site using a 60‐channel acquisition system with a shotpoint and geophone spacing of 10 m. The site is located about 140 km north of Stockholm and the host rocks are mainly granodioritic. The main objective of the profile was to image a known fracture zone with high hydraulic conductivity dipping gently to the west at depths of 100 to 400 m. The initial processing of the data failed to image this fracture zone. However, a steeply dipping reflector was imaged indicating the field data were of adequate quality and that the problem lay in the processing. These data have now been reprocessed and a clear image of the gently dipping zone has been obtained. In addition, several other reflectors were imaged in the reprocessed section, both gently and steeply dipping ones. Correlations with borehole data indicate that the origin of these reflections are also fracture zones. The improvement over the previous processing is caused mainly by (1) refraction statics, (2) choice of frequency band, (3) F-K filtering, and (4) velocity analyses. In addition to reprocessing the data, some further analyses were done including simulation of acquisition using only the near‐offset channels (channels 1–30) and the far‐offset channels (channels 31–60), and determining the damping factor Q in the upper few hundred meters based upon the amplitude decay of the first arrivals. The data acquisition simulation shows the far‐offset contribution to be significant even for shallow reflectors in this area, contrary to what may be expected. A Q value of 10, determined from observed amplitude decay rates, agrees well with theoretical ones assuming plane wave propagation in an attenuating medium.

3-D structure below Ävrö Island from high‐resolution reflection seismic studies, southeastern Sweden

Geophysics ◽  
1999 ◽  
Vol 64 (3) ◽  
pp. 662-667 ◽  
Author(s):  
Christopher Juhlin ◽  
Hans Palm
Keyword(s):  
Seismic Reflection ◽  
Fracture Zones ◽  
The South ◽  
North West ◽  
Reflection Seismic ◽  
Site Investigations ◽  
Seismic Reflection Method ◽  
Shot Point ◽  
Crystalline Crust ◽  
A Charge

Two 1-km-long perpendicular seismic reflection lines were acquired on Ävrö Island, southeast Sweden, in October 1996 in order to (1) test the seismic reflection method for future site investigations, (2) map known fracture zones, and (3) add to the Swedish database of reflection seismic studies of the shallow crystalline crust. An east‐west line was shot with 5-m geophone and shot point spacing, and a north‐south line was shot with 10-m geophone and shotpoint spacing. An explosive source with a charge size of 100 g was used along both lines. The data clearly image three major dipping reflectors and one subhorizontal one in the upper 200 ms (600 m). The dipping reflectors (to the south, east, and north‐west) intersect or project to the surface at or close to where surface‐mapped fracture zones exist. The south‐dipping reflector correlates with the top of a heavily fractured interval observed in a borehole (KAV01) at about 400 m. The subhorizontal zone at about 100–200 m correlates with a known fracture zone in the same borehole (KAV01). 3-D effects are apparent in the data, and only where the profiles cross can the true orientation of the reflecting events be determined. To properly orient and locate all events observed on the lines requires acquisition of 3-D data.

scholarly journals Reflection seismic studies over the end-glacial Burträsk fault, Skellefteå, Sweden

2010 ◽  
Vol 2 (2) ◽  
pp. 307-329 ◽  
Author(s):  
C. Juhlin ◽  
B. Lund
Keyword(s):  
Fracture Zone ◽  
Seismic Data ◽  
Mafic Rock ◽  
Seismic Profile ◽  
Large Displacements ◽  
Fault Geometry ◽  
Seismic Section ◽  
Glacial Ice ◽  
Crustal Rocks ◽  
Reflection Seismic

Abstract. Reflection seismic data were acquired along a ca. 22 km long profile over the end-glacial Burträsk Fault with a nominal receiver and source spacing of 20 m. A steeply dipping reflection can be correlated to the Burträsk Fault, indicating that the fault dips at about 55° to the southeast near the surface. The reflection from the fault is rather poorly imaged, probably due to a jump in the fault and the crookedness of the seismic profile in the vicinity of the fault. A more pronounced steeply dipping reflection is observed about 4 km southeast of the Burträsk Fault. Based on its correlation with a topographic low at the surface this reflection is interpreted to originate from a fracture zone. There are no signs of large displacements along this fault as the glacial ice receded, but it may be active today. Other reflections on the processed seismic section may originate from changes in lithological variations in the supra-crustal rocks or from intrusions of more mafic rock. Constraints on the fault geometry provided by the reflection seismic data will help determine what stresses were required to activate the fault when the major rupture along it occurred.

scholarly journals Shallow seismic reflection survey for imaging deep-seated coal layer - Case study from Muara Enim coal

2021 ◽  
Vol 24 (1) ◽  
pp. 15-29
Author(s):  
Muhammad Ramdhani ◽  
◽  
Muhammad Ibrahim ◽  
Hans Siregar ◽  
Tony Rahadinata ◽  
...  
Keyword(s):  
Seismic Reflection ◽  
Coalbed Methane ◽  
Coal Gasification ◽  
Reflection Method ◽  
Borehole Data ◽  
Seismic Section ◽  
Shallow Seismic ◽  
Coal Layer ◽  
Seismic Reflection Method ◽  
Amplitude Characteristics

Indonesia has a great potential for deep-seated coal resources. To assist and support the deep-seated coal exploration, a shallow seismic reflection method is applicable for this purpose. This study has conducted a shallow seismic reflection method in Musi Banyuasin Regency, South Sumatera Province. The Muara Enim coal target varies from 100 to 500 meters from the surface. The thickness of the coal layer varies from 2 to 10.65 meters. This study uses 48 channels with 14 Hz single geophone and MiniSosie as the energy source. The receiver and source interval is 15 meters. This study uses a fixed receiver and moving source configuration. From the interpreted seismic section, this study identified a deep-seated coal layer target. These layers are Mangus, Burung, Benuang, Kebon and Benakat layers. A simple interpretation is analyzed by combining the seismic amplitude characteristics and the thickness of the coal layer from the borehole data. From the interpreted seismic section, deep-seated coal layer targets have strong amplitude characteristics and are continuous from southwest to the northeast with a down-dip of around 20-30°. This study helps to inform the operator companies who develop the utilization of deep-seated coal (coalbed methane, underground coal gasification and underground coal mining) about the effective and proper geophysical method for imaging deep-seated coal layer.

scholarly journals Remarks on the Messinian evaporites of Zakynthos Island (Io- nian Sea, Eastern Mediterranean).

2016 ◽  
Vol 47 (1) ◽  
pp. 146 ◽  
Author(s):  
V. Karakitsios ◽  
M. Roveri ◽  
S. Lugli ◽  
V. Manzi ◽  
R. Gennari ◽  
...  
Keyword(s):  
Shallow Water ◽  
Eastern Mediterranean ◽  
Thickness Distribution ◽  
Planktonic Foraminifera ◽  
Water Environment ◽  
Foreland Basin ◽  
Seismic Profile ◽  
Depositional Environments ◽  
Borehole Data ◽  
Neogene Sediments

Detailed mapping of the Neogene deposits on Zakynthos Island shows that the Messinian primary evaporite basins, formed over Ionian basement, are delimited by the westernmost outcrop of the Triassic evaporitic diapirs, located west of the Kalamaki-Argasi Messinian gypsum unit. The post-Miocene external Ionian thrust is emplaced west of the Triassic diapirs. Planktonic foraminifera biostratigraphy indicates that primary evaporite accumulation took place probably during the first stage of the Messinian salinity crisis (5.96-5.60 Ma), in shallower parts of a foreland basin, formed over the Pre-Apulian and the Ionian zone basement. Establishment of these depositional environments, before the Ionian thrust emplacement, was probably due to the particularities of the foreland basin, which extended from the external Ionian to the internal Pre-Apulian zone. Field observations, borehole data and an onshore seismic profile show that the Neogene sediments over the Pre-Apulian  basement correspond to the foredeep through forebulge domain of the foreland basin, as it is documented from their spatial thickness distribution. In contrast, the Neogene sediments over the Ionian basement correspond to the wedge top of the foreland basin, which was less subsiding, as it is deduced by their reduced thickness. This lower subsidence rate was the result of the concurrent diapiric movements of the Ionian Triassic evaporites. In Agios Sostis area, located over Pre-Apulian basement, the Neogene sequence is intercalated by decametre-thick resedimented blocks consisting of shallow water selenite. To the southeast, this mass-wasting Messinian gypsum passes to mainly gypsum turbidite. In Kalamaki-Argasi area, located over Ionian basement, the shallow water environment led to the deposition of the observed primary gypsum. Erosion of the primary gypsum of both forebulge and wedge top supplied the foreland basin’s depocenter with gypsum turbidites.

scholarly journals Transformation and Upwelling of Bottom Water in Fracture Zone Valleys

2020 ◽  
Vol 50 (3) ◽  
pp. 715-726 ◽  
Author(s):  
A. M. Thurnherr ◽  
L. Clément ◽  
L. St. Laurent ◽  
R. Ferrari ◽  
T. Ijichi
Keyword(s):  
Fracture Zone ◽  
Bottom Water ◽  
Vertical Structure ◽  
Buoyancy Flux ◽  
Global Ocean ◽  
Fracture Zones ◽  
Flux Divergence ◽  
Flux Boundary Condition ◽  
Midocean Ridges ◽  
Slow Spreading

AbstractClosing the overturning circulation of bottom water requires abyssal transformation to lighter densities and upwelling. Where and how buoyancy is gained and water is transported upward remain topics of debate, not least because the available observations generally show downward-increasing turbulence levels in the abyss, apparently implying mean vertical turbulent buoyancy-flux divergence (densification). Here, we synthesize available observations indicating that bottom water is made less dense and upwelled in fracture zone valleys on the flanks of slow-spreading midocean ridges, which cover more than one-half of the seafloor area in some regions. The fracture zones are filled almost completely with water flowing up-valley and gaining buoyancy. Locally, valley water is transformed to lighter densities both in thin boundary layers that are in contact with the seafloor, where the buoyancy flux must vanish to match the no-flux boundary condition, and in thicker layers associated with downward-decreasing turbulence levels below interior maxima associated with hydraulic overflows and critical-layer interactions. Integrated across the valley, the turbulent buoyancy fluxes show maxima near the sidewall crests, consistent with net convergence below, with little sensitivity of this pattern to the vertical structure of the turbulence profiles, which implies that buoyancy flux convergence in the layers with downward-decreasing turbulence levels dominates over the divergence elsewhere, accounting for the net transformation to lighter densities in fracture zone valleys. We conclude that fracture zone topography likely exerts a controlling influence on the transformation and upwelling of bottom water in many areas of the global ocean.

Amplitudes of oceanic magnetic anomalies and the chemistry of oceanic crust: Synthesis and review of 'magnetic telechemistry'

1979 ◽  
Vol 16 (12) ◽  
pp. 2236-2262 ◽  
Author(s):  
P. R. Vogt
Keyword(s):  
Oceanic Crust ◽  
Fracture Zone ◽  
Hot Spots ◽  
Hot Spot ◽  
Magnetic Anomalies ◽  
High Amplitude ◽  
Fracture Zones ◽  
Juan De Fuca ◽  
Sea Floor Spreading ◽  
Ocean Ridge

A growing body of evidence suggests that certain areas of high-amplitude (H) sea-floor spreading-type magnetic anomalies reflect FeTi-enriched basalts of high remanent magnetization. A worldwide tabulation of these 'H-zones' is presented, together with a review of pertinent geochemical, rock magnetic, and deep-tow data relevant to the hypothesis of magnetic telechemistry.' H-zones are found in two tectonic settings: (1) along 102–103 km long sections of spreading axis close to hot spots; and (2) in narrow bands extending a few hundred kilometres along the edges of some fracture zones. Amplitudes in both provinces are 1.5 to 5, typically 2 to 3 times normal, and the hot spot H-zones are known from spreading half-rates of 0.6 to 3.7 cm yr−1 The highest amplitudes, magnetizations, and FeTi enrichment (up to 15–18% FeOT and 2–3% TiO2) seem to occur where both provinces overlap, i.e., where fracture zones occur near hot spots, for example along the Blanco Fracture Zone south of the Juan de Fuca hot spot and along the Inca Fracture Zone east of the Galapagos hot spot. The FeTi enrichment appears to reflect shallow-depth crystal fractionation (plagioclase, augite, and olivine), which is more extensive near hot spots, and more generally for fast-spreading ridges. H-zones presently affect at least 2.6 × 103 km, or 6.5% of the Mid-Ocean Ridge axis. However, the total known H-area of 8.5 × 105 km2 represents only 0.3% of oceanic crust. This suggests that older H-zones remain to be discovered, or/and that conditions favoring the formation of FeTi basalt and H-anomalies are more prevalent now than they have been on the average for the last 108 years. Evidence for the latter is provided by the known expansion of the magnetically well surveyed Juan de Fuca, Galapagos, and Yermak (Arctic) H-zones in the last 5 million years.

A new slant on seismic imaging: Migration and integral geometry

Geophysics ◽  
1987 ◽  
Vol 52 (7) ◽  
pp. 943-964 ◽  
Author(s):  
D. Miller ◽  
M. Oristaglio ◽  
G. Beylkin
Keyword(s):  
Wave Equation ◽  
Radon Transform ◽  
Inversion Formula ◽  
Acoustic Scattering ◽  
Seismic Profile ◽  
Seismic Inversion ◽  
Borehole Data ◽  
Velocity Models ◽  
Generalized Radon Transform ◽  
Zero Offset

A new approach to seismic migration formalizes the classical diffraction (or common‐tangent) stack by relating it to linearized seismic inversion and the generalized Radon transform. This approach recasts migration as the problem of reconstructing the earth’s acoustic scattering potential from its integrals over isochron surfaces. The theory rests on a solution of the wave equation with the geometrical‐optics Green function and an approximate inversion formula for the generalized Radon transform. The method can handle both complex velocity models and (nearly) arbitrary configurations of sources and receivers. In this general case, the method can be implemented as a weighted diffraction stack, with the weights determined by tracing rays from image points to the experiment’s sources and receivers. When tested on a finite‐difference simulation of a deviated‐well vertical seismic profile (a hybrid experiment which is difficult to treat with conventional wave‐equation methods), the algorithm accurately reconstructed faulted‐earth models. Analytical reconstruction formulas are derived from the general formula for zero‐offset and fixed‐offset surface experiments in which the background velocity is constant. The zero‐offset inversion formula resembles standard Kirchhoff migration. Our analysis provides a direct connection between the experimental setup (source and receiver positions, source wavelet, background velocity) and the spatial resolution of the reconstruction. Synthetic examples illustrate that the lateral resolution in seismic images is described well by the theory and is improved greatly by combining surface data and borehole data. The best resolution is obtained from a zero‐offset experiment that surrounds the region to be imaged.

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