jgg     2014 34 (6):  32-36    ISSN: 0412-1961  CN: 21-1139/TG STUDY ON NORMAL GRAVITY FIELD CORRECTION METHODS College of Marine Geosciences，Ocean University of China，Qingdao 266100 Received: 2014-03-04  Revised: null  Online: 2014-12-09 Supporting info