Nalin B. Trivedi

Wave characteristics of daytime and nighttime Pi 2 pulsations at the equatorial and low latitudes

Peculiarities of daytime and nighttime Pi 2 pulsations at the dip equator are examined by using multipoint measurements from the 210° magnetic meridian (MM) magnetometer network. We found that during daytime the amplitude of Pi 2 pulsations at the dip equator is enhanced, and the phase lags ∼ 34° behind those at low-latitude (magnetic latitude Φ = 19.5-46.2°) stations. On the other hand, during nighttime the amplitude of Pi 2 pulsations at the dip equator is depressed, and the phase lags ∼ 18° behind those at the lower latitudes. Because the zonal ionospheric conductivity at the dip equator is much higher than that at the off-dip equator region, Pi 2 signals are expected to be distorted more effectively at the dip equator. The observations imply that the daytime and nighttime Pi 2 pulsations in the equatorial and low-latitude regions can be explained by invoking an instantaneous penetration of electric field variations from the nightside polar ionosphere to the dayside equatorial ionosphere, and a direct incidence of compressional oscillations from the nightside inner magnetosphere, respectively.

Wave characteristics of geomagnetic pulsations across the dip equator

In order to clarify the wave characteristics of Pi2 and Pc 4–5 magnetic pulsations around the dip equator, we analyzed magnetic data from the latitudinally dense magnetometer array in Brazil. We found that the phase difference between Pi2 pulsations observed at globally separated low-latitude stations is small, whereas Pi2 pulsations observed within the dayside dip equator region of ±2° latitude show phase lags of about 30° ∼ 50° behind those in the off-dip equator region. Pc 4–5 magnetic pulsations at the dip equator also show the same phase character. Pi2 amplitudes are enhanced in the equatorial region, where the phase lags of pulsations must be associated with the enhancement of ionospheric conductivity. The equatorial phase lags can be explained by invoking the induction effect of the equatorial enhanced ionospheric current above the good conductor Earth.

Upper crustal structure of the northeast Paraná Basin, Brazil, determined from integrated magnetotelluric and gravity measurements

Eight magnetotelluric (MT) soundings were performed as a continuation of an earlier audiomagnetotelluric (AMT) survey conducted in the northeastern border of the Parana basin, a large intracratonic basin located in central eastern South America and constituted by Silurian to Jurassic sedimentary rocks with Lower Cretaceous sill-type magmatic intrusions and overlying volcanics. Two of the MT stations were carried out near two deep petroleum wells. The remaining MT soundings were done on a profile traversing two important gravity features: a positive anomaly near the border of the states of Sao Paulo and Minas Gerais and a strong (trending NW-SE) linear gradient. Major findings of an integrated interpretation of the MT survey and of available gravity data are as follows: (1) inhomogeneities and/or strong magnetization of the superficial volcanics and diabase intrusions in the Paleozoic sedimentary rocks appearing to distort the MT results; and (2) identification of important structural discontinuities, including a possible different crustal structure beneath the Parana basin compared to the region on the north, a thickening of the crust toward the NE, and the probable existence of a trough (graben?) within the basin, characterized by a thick accumulation of sediments and basalts.

Geophysical constraints on tectonic models of the Taubaté Basin, southeastern Brazil

Abstract Magnetotelluric, gravity and geothermal flux data are analysed to constrain the tectonic evolution of the Taubate Basin, a SW-NE 20 km wide and 170 km long rift-like Tertiary basin in southeastern Brazil. The basin is characterized by half-grabens and contains up to 1000 m of sediments cut by normal faults. The magnetotelluric measurements show the presence of two layers: an upper highly conductive one (less than 10 Ωm from 1-D inversions), associated with Tertiary sediments, and an extremely resistive (more than 10000 Ωm) lower layer, related to the crystalline basement. Gravity anomalies in the region are ascribed to both near-surface sources, sediments and granites, and a deep-seated source associated with crustal thinning towards the continental margin. Available geothermal flux values within the basin are anomalously high; however, they have probably been affected by convective transport because they were determined in water boreholes. These results suggest a stable thermal regime at present and, viewed together with the absence of magmatism and thermal subsidence, indicate that the mechanism related to the basin formation did not originate a deep-seated thermal anomaly in the region. The event can be seen as a shallow one, involving a small area in the upper part of the crust. The region can be characterized typically as a strike-slip mobile zone, with horizontal displacements between juxtaposed blocks. A transtensional model is suggested for the Taubate Basin to explain some of its principal geological features.

Effects of the equatorial electrojet on aeromagnetic data acquisition

In recent years, considerable advances have taken place in aeromagnetic surveying. These improvements involved data acquisition (instruments and survey design), processing, and interpretation. In addition to improved spatial resolution, the high‐resolution aeromagnetics, as applied to oil exploration, attempts to resolve very low amplitude (1 nT or even subnanotesla) magnetic features (Paterson and Reeves, 1985). These features are caused by weak intra‐sedimentary magnetic sources of magnetite and pyrrhotite, which could have been formed as a result of hydrocarbon seepage (e.g., Reynolds et al., 1990, 1991). For such small spatial variations to be meaningful, it is required that similar temporal and spatial variations due to external sources be corrected accordingly.