Showing papers on "Magnetic dip published in 1996"

Theory of 2-D complex seismic trace analysis

Abstract: The ideas of 1-D complex seismic trace analysis extend readily to two dimensions. Two-dimensional instantaneous amplitude and phase are scalars, and 2-D instantaneous frequency and bandwidth are vectors perpendicular to local wavefronts, each defined by a magnitude and a dip angle. The two independent measures of instantaneous dip correspond to instantaneous apparent phase velocity and group velocity. Instantaneous phase dips are aliased for steep reflection dips following the same rule that governs the aliasing of 2-D sinusoids in f-k space. Two-dimensional frequency and bandwidth are appropriate for migrated data, whereas 1-D frequency and bandwidth are appropriate for unmigrated data. The 2-D Hilbert transform and 2-D complex trace attributes can be efficiently computed with little more effort than their 1-D counterparts. In three dimensions, amplitude and phase remain scalars, but frequency and bandwidth are 3-D vectors with magnitude, dip angle, and azimuth.

Equatorial plasma depletion precursor signatures and onset observed at 11° south of the magnetic equator

Abstract: Coordinated radio and optical measurements of the structure and dynamics of the postsunset equatorial ionosphere were conducted on October 1, 1994, from Agua Verde, Chile (11.3°S magnetic latitude (MLat)). The measurements clearly show a north-south aligned undulation or ripple on the bottomside of the F layer at 2000 LT, appearing as an eastward propagating decrease in the 630.0-nm airglow, resembling a traveling ionospheric disturbance in the digital portable ionosonde measurements and causing a total electron content decrease in the Global Positioning System (GPS) satellite measurements. The initial development of this feature, toward the east and away from the magnetic equator, took place in an otherwise smooth, unstructured ionosphere. Spread F began to develop in the ionograms at 2020 LT, and, at this same time, local onset of satellite signal scintillation was detected using the multiple ray paths throughout the sky available from the GPS satellite constellation transmitting at L band frequencies. UHF scintillation measurements from Ancon, Peru, along the same magnetic field line, show that intense scintillation and ionospheric irregularities had developed over the magnetic equator almost 60 min prior to their development at 11°S MLat. The observations suggest that the east-west electric field expected to be present within the earlier developed depletion and scintillation region at the magnetic equator mapped along magnetic field lines to lower altitudes and higher latitudes, resulting in an undulation or dome-shaped structure, before evolving into a fully developed depletion (with associated ionospheric irregularities) all along the magnetic flux tube.

Spread F and the structure of equatorial ionization depletions in the southern anomaly region

Abstract: Combined optical and radio sensors provide a unique characterization of the structure of equatorial emission depletion regions connected to rising bubbles over the magnetic equator. In Chile, as part of the MISETA campaign in fall 1994, a CCD-enhanced all-sky imaging photometer provided optical images of the postsunset appearance and motions of the depletion bands at a magnetic dip latitude of 11°S. Concurrently, a Digisonde collocated with the photometer monitored the appearance of spread F. In between the ionograms, the sounder operated as a Doppler interferometer identifying the locations of F layer irregularities associated with the spread F. They were found to lie inside the emission depletion regions. The HF sounder, requiring orthogonality with the field-aligned F layer irregularities to generate the spread structure, tracked these irregularities inside the emission depletion bands as they drifted eastward. Ray tracing simulations show that the radio waves become trapped within the depletion regions when the depletions are within 300 km of the sounder site. Model calculations indicate that the sounder rays encounter orthogonality with the Earth's magnetic field within the depletion bubble southward from the site, consistent with the local dip angle. The combination of optical images with HF radio sounding demonstrated that radio imaging in the equatorial ionosphere can be done with a digital ionosonde that operates as a Doppler interferometer. The Digisonde measurements and ray tracing show for the first time that the spread F signatures on ionograms are the result of coherent scatter from irregularities primarily within the walls of the depletion.

Determination of horizontal resistivity of formations utilizing induction-type logging measurements in deviated borehole

Abstract: A method for determining horizontal resistivity of an anisotropic formation and a formation anisotropy response factor with an induction logging tool is claimed. The method utilizes two differing induction type measurements to determine the horizontal resistivity for the formation without knowledge of a borehole dip angle. The differing induction measurements may be different types of measurements or the same type of measurement made under differing operating conditions. The method further discloses a means for determining the vertical resistivity of an anisotropic formation based on the dip angle, the horizontal resistivity and the formation anisotropy response factor.

Geomagnetic field inclinations for the past 400 kyr from the 1-km core of the Hawaii Scientific Drilling Project

Abstract: A volcanic record of geomagnetic field inclination for the past ∼400 kyr at Hilo, Hawaii, has been obtained from the 941.5 m of core recovered by the Hawaii Scientific Drilling Project. The analysis of 195 lava flows reveals six instances of near-zero inclination and two instances of fully negative inclination (reverse polarity) within an otherwise normal-polarity core. In particular, flow unit 23 (∼178 m depth) records a horizontal inclination and may be associated with the Laschamp event; flow units 40 and 42 (∼260 m depth) record negative inclinations and are close in age to the Blake event; and flow unit 55 (∼320 m depth) records a negative inclination with a relative declination change of ∼75° with respect to the overlying flow and is probably the Jamaica/Biwa I/Pringle Falls event. The five instances of shallow inclination found below 400 m depth appear to have resulted from long-term secular variation as they are part of inclination swings between ∼0° and ∼60° with a periodicity of ∼10–50 kyr. In contrast, the inclination shifts at ∼178 m and ∼320 m depths significantly deviate from long-term trends, suggesting the existence of at least two independent processes producing time variations of the geomagnetic field. The secular variation has a mean of 30.9° (α_(95) = 2.27°), which is significantly shallower than the expected dipole mean of 36°. The dispersion (σ = 12.5°) agrees with global paleosecular variation data for 0–5 Ma and secular variation models.

Paleomagnetic study along the southeastern edge of the Altiplano ‐ Puna Plateau: Neogene tectonic rotations

Abstract: In northwestern Argentina the transition zone between the Puna and the Sierras Pampeanas was deformed in Late Miocene and Pliocene time. Structural observations suggest that local clockwise block rotations might be observed associated with dextral transpression. This study contributes new paleomagnetic results from four sites in Cretaceous rocks and 19 sites in Neogene sedimentary sequences. When compared with reference paleomagnetic data for stable continental South America, the results show an average inclination shallowing error of 10.5° and a pattern of clockwise rotations up to 29°. Study of the anisotropy of magnetic susceptibility (AMS) on 278 specimens provided, for most sites, AMS tensors with an oblate shape controlled by sedimentation and compaction, explaining the observed inclination shallowing. A slight AMS lineation was measured and we argue that this magnetic lineation reflects Late Miocene-Pliocene compression. A correlation observed between magnetic lineations, deduced from the AMS analysis, and rotation estimates from remanent magnetizations, demonstrates that the inferred shortening directions have also been rotated clockwise.

The Position of the North Magnetic Dip Pole in 1994.

Abstract: In the spring of 1994, a survey was carried out to redetermine the position of the North Magnetic Pole (NMP). Observations of declination, inclination, and total intensity were made at eight sites around the expected location of the pole. All observations were corrected for transient variations using data from a variometer set up in the survey area, and data from magnetic observatories at Mould Bay and Resolute Bay. The NMP position was determined by performing a spherical cap harmonic analysis of the corrected data. The computed position of 78.3°N, 104.0°W for 1994.0 was 150 km northwest of the position previously determined in 1984, and showed that the NMP was moving more rapidly than anticipated. The motion of the NMP has changed from a uniform drift of about 9 km/yr, prior to mid-1971, to a uniform acceleration of approximately 0.34 km/yr2, after mid-1971. This change may be related to the widely-reported geomagnetic jerk that occurred in 1969/70.

Convenient formulas for determining dip-slip fault parameters from geophysical observables

Abstract: Simple equations are presented for estimating slip magnitude, fault dip angle, width of the zone of compression, and other co-seismic parameters from geologic, seismic, and geodetic observations made in subduction zone and continental dip-slip environments. These equations are easy to memorize and use without computational aids.

Suppression of sea-floor-scattered energy using a dip-moveout approach; application to the mid-ocean ridge environment

Abstract: Multichannel seismic (MCS) images are often contaminated with in- and out-of-plane scattering from the sea floor. This problem is especially acute in the mid-ocean ridge environment where sea-floor roughness is pronounced. Energy shed from the unsedimented basaltic sea floor can obscure primary reflections such as Moho, and scattering off of elongated sea-floor features like abyssal hills and fault scarps can produce linear events in the seismic data that could be misinterpreted as subsurface reflections. Moreover, stacking at normal subsurface velocities may enhance these water-borne events, whose stacking velocity depends on azimuth and generally increases with time, making them indistinguishable from subsurface arrivals. To suppress scattered energy in deep water settings, we propose a processing scheme that invokes the application of dip moveout (DMO) to deliberately increase the differential moveout between sea-floor-scattered and subsurface events, thereby facilitating the removal of unwanted energy in the stacked section. After application of DMO, all sea-floor scatterers stack at the water velocity, while subsurface reflections like Moho still stack at their original velocity. The application of DMO in this manner is contrary to the intended use that reduces the differential moveout between dipping events and allows a single stacking velocity to be used. Unlike previous approaches to suppress scattered energy, dip filtering is applied in the common-midpoint (CMP) domain after DMO. Moveover, our DMO-based approach suppresses out-of-plane scattering, and therefore is not limited to removal of in-plane scattering as is the case with shot and receiver dip filtering techniques. The success of our DMO-based suppression scheme is limited to deep water (a few kilometers of water depth for conventional offsets), where the traveltime moveout of energy scattered from the sea floor has a hyperbolic moveout with a stacking velocity that depends on the cosine of the scatterer steering angle in a manner analogous to how the moveout of a dipping reflector depends on the dip angle. The application of DMO-based suppression to synthetics and MCS data collected along the southern East Pacific Rise demonstrates the effectiveness of our approach. Cleaner images of primary reflectors such as Moho are produced, even though present shot coverage along the East Pacific Rise is unduly sparse, resulting in a limited effective spatial bandwidth.

Magnetic fabrics and sources of magnetic susceptibility in lower crustal and upper mantle rocks from Hess Deep

Abstract: We present a magnetic fabric study of mafic and ultramafic rocks recovered at Ocean Drilling Program Sites 894 and 895 from Hess Deep, a tectonic rift in the equatorial East Pacific (2°15'N, 101°30'W). We demonstrate, using thermomagnetic curves and high-field (up to 1.5 T) susceptibility measurements, that the magnetite contributes >95% of magnetic susceptibility of these rocks. Bulk susceptibilities for the Site 894 gabbroic rocks are on average 1.72 × I0"2 and for the Site 895 peridotites 4.29 × 10~2 (S1 volume units). The anisotropy of magnetic susceptibility is moderate to high and average kmw/kmjn is 1.15 ikmax kint kmin are the principal susceptibilities) at Site 894 and 1.11 at Site 895. Because of the pervasive serpentinization and the formation of randomly distributed secondary magnetite, the magnetic fabrics of the Site 895 peridotites show no apparent relationship to structural features. In the foliated gabbroic rocks from Site 894, we observe a close relationship between magmatic flow fabrics (defined by the preferred orientation of Plagioclase) and magnetic fabrics: kmin is perpendicular to the macroscopic foliation and kmax dips steeply within the foliation plane and is parallel to the magmatic mineral lineation. Macroscopically isotropic gabbros yield the same principal susceptibility directions as the foliated gabbros. We interpret magnetic fabrics in these rocks to represent a weakly developed rock fabrics. We argue that the process for the development of the AMS is a distribution anisotropy which is caused by the growth of equant, irregular, or skeletal magnetite grains into a preferredly oriented (by magmatic flow) Plagioclase "template." Magnetic fabric data record a mineral preferred orientation with a north-south, East Pacific Rise parallel strike and a near-vertical inclination, and can be interpreted to record the upward flow of melt at the top of an axial magma chamber into the base of an overlying sheeted dike complex.

The influence of sunspot canopies on magnetic inclination measurements in solar plages

Abstract: Sunspots are known to have large, low-lying magnetic canopies, i.e. horizontal magnetic fields overlying a field-free medium, that cover substantial fractions of active region plage. In this paper we consider the influence of such canopies on the inclination of plage magnetic fields. We find that for observations in spectral lines like 5250.2A the neglect of a sunspot canopy when determining magnetic inclination angles of plage fields can introduce errors exceeding 5–10°. This is particularly true if the observations do not have high spatial resolution. Thus this effect may explain some of the measurements of substantially inclined fields in solar plages. Furthermore we find that the Fe I 15648 A line is far superior in giving correct flux-tube inclinations in the presence of a sunspot magnetic canopy. Finally, the inversion of full Stokes profiles is shown to produce more reliable results than results obtained by considering only ratios of individual Stokes profile parameters.

21. paleomagnetism and rock magnetic properties of gabbro from hole 894g, hess deep1

Abstract: Results of paleomagnetic and rock magnetic measurements are presented from gabbroic samples recovered during Ocean Drilling Program Leg 147 at the Hess Deep. Paleomagnetic measurements indicate that samples acquired up to two small components of secondary remanent magnetization. Stable magnetic inclinations determined after alternating-field and thermal demagnetization reveal a mean stable magnetic direction (38°) that is significantly steeper than that predicted for this equatorial site (<5°). Thus, it is likely that remanent magnetization was acquired before tectonic uplift. The mean intensity of natural remanent magnetization for the recovered gabbros is 2.3 A/m, and the Koenigsberger ratio indicates that the in situ magnetization is dominated by remanent, rather than induced, magnetization. Measurements of hysteresis loop parameters indicate that the effective magnetic grain size of the gabbro samples falls within the pseudo-single domain region. Although recovered from a fast-spreading ridge, the paleomagnetic and rock magnetic properties of the gabbros from Hole 894G are very similar to those of gabbros recovered from slow-spreading ridges.

Magnetic and Gravity Field Analyses of Izu-Ogasawara (Bonin) Arc and Their Tectonic Implications.

Abstract: The magnetic and gravity anomaly maps covering the Izu-Ogasawara arc are compiled using the MGD77 data measured by the Hydrographic Department of Japan (JHD). A magnetic basement map is also obtained by applying the three-dimensional inversion method. Characteristic features of the crust of the Izu-Ogasawara arc are made apparent from the view point of the magnetic, gravity and topographic features. Especially, the distribution of the magnetic bodies with low ρ/J ratio, which belong to the granitic rock of magnetite series, becomes apparent in the forearc area of the northern part of the Izu-Ogasawara arc. The magnetic anomalies of the Nisi-Sitito ridge are well explained by the normal magnetization of topographic highs. This feature suggests the N-S arrangement of the arc when it was formed. The bulk magnetization per unit area (km2) along the Quaternary volcanic front of the southern arc is about 2.4 times as large as that of the northern arc. These features can be explained by the narrow and stable location of the volcanic front in the southern arc, which is related with the steep gradient of the subducting slab. The magnetic edge effect of the Moho boundary can be recognized in the western margin of the northern arc. However, this feature becomes vague in the southern arc. A tectonic model to explain the origin of the different characteristics between the northern and southern arcs is presented by assuming the long and stable state of the steep dip angle of the subducting slab in the southern arc under the N-S arrangement of the arc. The plausible origin of NE- and NNE-trending en-echelon ridge and trough morphology is also given in consistent with the tectonic framework mentioned above.

17. magnetic fabric analysis of fine-grained sediments, iberia abyssal plain 1

Abstract: The fabric of fine-grained sediments, cored during Ocean Drilling Program Leg 149 to the Iberia Abyssal Plain, was analyzed using the method of anisotropy of magnetic susceptibility. The purposes of this study were to investigate downcore changes in magnetic fabric, and to determine paleocurrent direction. The general magnetic fabric analysis reveals an oblate shape, with Kmin axes perpendicular to the bedding plane, which is a typical sedimentary fabric. The magnetic lineation (L = Kmax/Kint) remains fairly constant throughout the section. However, the magnetic foliation structure (F = Kint/Kmin) shows distinctive variation. Several cases show sharp changes at lithostratigraphic boundaries. Poorly foliated intervals at Sites 897 and 898, where inclination of Kmin is not parallel to the bedding plane, probably resulted from postdepositional disturbance. For selected data from lithostratigraphic Unit I at Sites 897, 898, and 900, the magnetic lineation represented by the Kmax axes shows clustering indicative of paleocurrent direction. These data suggest that turbidity currents originated from the eastern continental margin of Iberia during the middle Miocene to late Pleistocene.

Method for determining validity of seismic reflections below lateral velocity variations

Abstract: A method for determining the reliability of time processed seismic reflection data near a significant velocity differential overhang includes displaying the seismic reflection data as a seismic section showing impedance differential interfaces (12). The position of an edge of the velocity differential overhang is located and acoustic velocity values are interpreted (14). A set of curves representing boundaries of reliability for predetermined dip values is determined (16). The set of curves representing boundaries of reliability may be displayed (18). The display of the set of curves representing boundaries of reliability may be overlaid (20) on the seismic section representing the seismic reflection data. The dip angle for an interface of interest is approximated (22). The interface of interest is compared (24) with the curve representing boundaries of reliability for the dip angle for the interface of interest to determine the extent of reliability of the data used to map the end of the interface under the overhang.

Incorporating geological dip into geophysical inversions

Abstract: A new model objective function is proposed that allows the input of dip information about geology in geophysical inversion. A rotation matrix is applied to the spatial derivatives defined along the principal axes of model discretization so that the derivative of the model in an arbitrary direction is obtamed. A model objective function consisting of the squares of the rotated derivatives then allows the inversion to construct models that have elongated features of specified dip angle. Two-dimensional DC resistivity inversions are used as examples to illustrate the utility of the objective function.

Study on the sources of some deep focus earthquakes in Northwest Pacific region

Abstract: The sources for thirteen deep focus earthquakes ofm b≥5.5 andh>400 km in Northwest Pacific region were studied using waveform fitting and shear fracture source model. The source parameters were obtained as follows: focal depth, faulting plane, slip direction, rupture velocity, rupture length, rupture direction and scalar moment tensor. It was found that all these earthquake sources can be interpreted as shear faulting and have simple source time functions. The strike direction of faulting plane for most deep focus earthquakes coincides with that of the subduction zone, especially in the deep part of the subduction zone, it results in the tendency of reducing the dip angle of the subduction zone. The multiple point source model was also used to study the source process. The waveform fitting is better than the shear fracture model, but the general rupture direction which coincides with that from unilateral shear source model can not be obtained from the multiple point source model.