IDENTIFICATION OF SUBSURFACE ROCKS IN GEOTHERMAL AREA OF NORTH TAPANULI USING HORIZONTAL TO VERTICAL SPECTRAL RATIO METHOD. einstein e-j. [Internet]. 2026 Feb. 25 [cited 2026 Apr. 24];14(1):47-55. Available from: https://jurnal.unimed.ac.id/2012/index.php/einsten/article/view/64966