engleski

Geomagnetic survey, Quiet-Time Level determination and reduction on Palagruža repeat station

The first geomagnetic survey using the onsite variometer in Croatia was realized within the bilateral project ‘Joint Croatian-Hungarian Geomagentic Repeat Station survey and Joint Geomagnetic Field model‘ on the repeat stations Krbavsko polje (KRBP), Sinjsko polje (SINP) and Palagruža (PALA) in the period 19-31 July 2010. The results presented in this paper rely on the survey on PALA repeat station performed in the period 27-31 July 2010. In addition to usual equipment used for declination D and inclination I determination i.e. nonmagnetic fluxgate theodolite Zeiss 020A with DMI D&I electronic unit, the magnetic field during the survey was observed and monitored with onsite dIdD variometer. The time variability of the geomagnetic field elements differences between the variometer station (VAR) and repeat station (RS) (i.e. baselines) noticed on KRBP location were more pronounced on SINP and PALA repeat station locations. One of possible explanations of such a temporal changes can be the significant conductivity contrast between the Adriatic sea and the mainland that can result in spatial field variation in small spatial and temporal scales, even in large distances. Moreover, the temporal change of total intensity (F) gradient was detected by two PPM sensors observing on the same vertical (one above the other). Despite of noticed temporal change, the dIdD calibration parameters have been determined enabling the determination of the baselines and subsequent spatial reduction from VAR station to the repeat station. In such conditions, the repeat station was operating as a temporary geomagnetic observatory. The quiet-time difference between the geomagnetic elements on each repeat station and THY reference observatory was determined from 60 minutes long sliding windows provided the STDEV of the difference was < 0.3 nT in X, Y, Z components and F as well. In such a way determined quiet- time level differences have shown significant fluctuations, so they were determined considering several geomagnetic observatories surrounding Croatian territory (CTS, FUR, GCK, PAG, THY). Consequently, the quiet-time differences from 60 minutes sliding windows simultaneously determined with data from all surrounding observatories have provided more reliable results. Absolute set observations collected at the repeat station were reduced to reference observatories using reduction methods based on the assumption that transient (including diurnal) variations of the magnetic field are identical at both repeat station and chosen reference observatory (without and with consideration of the secular variation difference between the repeat station and reference observatory). Definitive geomagnetic element reduced values were given as weighted average (based on STDEV and SCATTER) taking into consideration all surrounding reference observatories. The comparison of different data reduction method results has shown that reduction to a quiet-time level provides better accuracy estimations. In order to provide reliable quiet-time level determinations, it is desirable that the repeat station is surrounded by several reference observatories at acceptable distances and that the survey is carried out during quiet external field conditions. Further investigations should be taken in order to clarify the impact of the high conductivity contrast to the temporal change of the geomagnetic field spatial gradients.

repeat station; dIdD variometer; baselines; data reduction; Quiet-Time Level

nije evidentirano