MAGNETIC ANOMALIES AT THE SITE OF THE PEACEFUL UNDERGROUND NUCLEAR EXPLOSION «CRYSTAL» (WESTERN YAKUTIA)

The relevance of the study is caused by the necessity to assess the current radiation and geo-ecological situation at the sites of peaceful underground nuclear explosions. In addition, this study will allow us to predict possible scenarios for development at such items located near settlements. The circumstantial research of the structure of the geotechnogenic system «center of an underground nuclear explosion – the host geological environment – the surface of the Earth» involves magnetic study. The land magnetic survey allow us to separate geoelectric anomalies caused by buried iron objects from other items that have a different nature. The aim of the research is to detect and localize the buried iron objects on the site of the peaceful underground nuclear explosions «Crystal» based on the data of land magnetic survey. Object. The area of the emplacement hole of the peaceful underground nuclear explosion «Crystal» was chosen for study. The capacity of the peaceful underground nuclear explosions «Crystal» was of 1,7 kt in TNT equivalent. It was made on October 2, 1974 at a depth of 98 m in frozen rocks at a distance of 6,7 km away from Udachny town (Western Yakutia). The debri mound (or sarcophagus) was built on the mouth of the emplacement hole in two steps – in 1992 and 2007. The diameter of the mound is 260 m, the maximal height of it is 20 m. Methods. The magnetic survey was carried out on the profiles with a proton magnetometer MMPOS-1 with a reference frequency of 3 s. Georeferencing of points was performed using the Garmin 64st GPS receiver. Variations of the Earth's natural magnetic field were registered by a GSM-19 quantum magnetometer with a frequency of 10 s in a forest area near «Crystal» sarcophagus. Software complexes of statistical and spectral-correlation analysis of geophysical data «KOSKAD 3D» (author A.A. Nikitin, A.V. Petrov, A.S. Aleksashin) and of numerical modelling PRISMA (author A.S. Dolgal) are used for the data analysis. Results. 5323 readings of magnetic field on the area 430×330 m around the emplacement hole of the peaceful underground nuclear explosion «Crystal» were taken and systemized. The anomalous magnetic field ΔТа of the site of the peaceful underground nuclear explosion «Crystal» was mapped based on these data. The local magnetic anomalies formed by buried iron objects were revealed. The widest and most intense anomaly with a diameter of 46 m and an amplitude of up to 8000 nT is located above the emplacement hole at the center of the sarcophagus. The sequential energy filtering of measured magnetic data allowed separating this magnetic anomaly into parts associated with different fragments of the emplacement hole casing. The results of modeling magnetic anomalies caused by the hole casing fragments are consistent with the measured magnetic data. So, the additive character of the magnetic anomaly is revealed. The other magnetic anomalies are presumably caused by casing pipes of research and technical wells that existed before the explosion at the site and were randomly removed by the explosion.