jgg     2013 33 (4):  39-43    ISSN: 0412-1961  CN: 21-1139/TG OBSERVATIONAL DATA ANALYSIS OF VERTICAL GEOELECTRIC  FIELD IN TIANZHU AREA, GANSU PROVINCE Institute of Crustal Dynamics, CEA, Beijing 100085 Received: 2012-12-29  Revised: null  Online: 2013-08-16 Supporting info