Ana Osella

Demographic conditions necessary to colonize new spaces : the case for early human dispersal in the Americas

Abstract During the last decade or so, several authors have discussed human dispersal using mathematical simulations in the Americas (Anderson and Gillam 2000; Aoki 1993; Steele 2002; Steele et al. 1995, 1998; Surovell 2000, 2003), as well as other parts of the world (Gkiasta et al. 2003; Hughes et al. 2007; Mithen and Reed 2002; Pope and Terrell 2008; Ray 2003; Young 2002; Young and Bettinger 1992, 1995). In this journal, Steele et al. (1998) presented a demographic model based on the notion of dispersal affected by environmental variability through time and space. We modified the mathematical formula to make the process stochastic. This allows the exploration of different demographic conditions under which humans could successfully colonize the Americas. The simulations demonstrate two human dispersal scenarios: 1) a single dispersal process with two major tempos or 2) two different dispersal processes, one from Beringia to the Isthmus of Panama and the other for South America.

Currents induced by geomagnetic storms on buried pipelines as a cause of corrosion

Protection of buried pipelines from corrosion constitutes an important objective to be taken into account in the design of pipelines networks. Most of the destructive processes which affect metals are of an electrochemical nature, mainly associated with the characteristics of the soils where they are buried. Protection methods usually involve the application of a non-corrosive coating over the pipes together with cathodic protection, which prevent the flow of ionic currents from the pipe to its surrounding soil. Nevertheless, these methods do not always compensate for the pipeline corrosion effects. In particular, during days of high geomagnetic activity and depending on the electrical conductivity of the host soil, excessive induced currents are channeled along the pipes which can override the protection measures. Within this framework, we present a study of electromagnetic induction in buried pipelines, assuming that geomagnetic storms are the external source responsible for the erratic currents. Our aim is to quantify the induction as a function of the characteristic of the soils and of the frequency and intensity of the inducing field in order to contribute to better understand and improve the pipeline corrosion protective methods. The results of our study indicate that geomagnetic effects can introduce significant disturbances on buried pipelines, increasing their risk of corrosion and potentially reducing their safe useful life.

3D electrical imaging of an archaeological site using electrical and electromagnetic methods

Floridablanca is an 18th century archaeological site located in southern Argentina. Archaeological investigations at the site began in 1998, and in 2000 we started a project to perform geophysical studies there. In this paper, we report the implementation of electrical and electromagnetic (EM) methods in a sector of the site that corresponds to the settlers’ houses. The objective of the project was to characterize the zone and the buried archaeological structures (adobe walls, tiles from a collapsed roof) with 2D and 3D electrical and EM techniques. We first applied an EM induction method covering a 600-m 2 area with a frequency ranging from 1000 to 19 000 Hz. A 3D visualization of the in-phase and quadrature components gave an initial description of anomalies possibly associated with buried structures. We then performed dipole-dipole profiles and inverted the data to obtain the corresponding 2D and 3D electrical images. Finally, after correlating the information obtained from the analysis of both EM and electrical data, we performed a more localized 3D dipole-dipole mesh (25 m 2 ) to achieve the final electrical image of the most representative buried structure. The combination of both techniques allowed us to map two entire houses and to identify three types of walls: main, separating, and inner. These results have been confirmed by an archaeological excavation.

Geoelectrical modeling of shallow structures using parallel and perpendicular arrays

In this article we analyze the sensitivity of a geoelectrical modeling technique to image 2D shallow structures. Firstly, we extend a previously developed 2D method based on Rayleigh-Fourier expansions, in order to allow arbitrary locations for the electrodes and also 3D earth models. This method is an alternative to finite element and finite difference techniques and is especially suitable to model multilayered structures, with smooth irregular boundaries. Then, for simple 2D models we build up two synthetic pseudosections, one for electrode deployments parallel to a profile perpendicular to the strike, and other for deployments perpendicular to it. We analyze the advantages in using both pseudosections to model these structures. We also compare geoelectric results with the corresponding audiomagnetotelluric transverse electric and transverse magnetic responses. Finally, we perform a geoelectrical survey to image a shallow buried structure and show the goodness of the model fit obtained considering both pseudosections. For the examples studied here, we conclude that considering both pseudosections leads to a more accurate description of the structures. When a 2D anomaly is present, its effect on the perpendicular component is more focused, both in width and depth, than in the parallel component. Hence the perpendicular component helps to constrain the localization of the inhomogeneity. In addition, we find similarities between the geoelectric parallel and perpendicular responses and the corresponding audiomagnetotelluric transverse magnetic and transverse electric results, respectively. When inverting audiomagnetotelluric data using 2D codes, better resolution in the electrical imaging is obtained when both modes are considered; then it is expected that 2D imaging of geoelectric data including both arrays should lead to an optimization of the inversion process. Even more, if results of these inversions could be used in correlation with AMT results, it is clear that this kind of joint inversion should contribute to remove uncertainties allowing an improvement in the description of the actual structures.

Solar quiet geomagnetic variations and E-region neutral winds at equatorial latitudes

Abstract A model of the ionospheric current system valid at zones close to the geomagnetic equator, taking into account the contribution of neutral winds, is proposed. From this, the external magnetic field at ground is calculated. Also, ground records of the geomagnetic field variations at the Peruvian equatorial zone were separated into their external and internal contributions. Using an iterative process a local particular fitting was found by comparing the separated external field to the one calculated with the proposed model.