GEOPHYSICAL EVALUATION OF GROUND CONDUCTIVITY OF A SUB-SURFACE IRRIGATION SITE USING GEM-2 ELECTROMAGNETIC SURVEYS, POWDER RIVER BASIN, WYOMING

A sub-surface irrigation system (SDI) has been developed and used to provide an effective and beneficial disposal method for coal bed natural gas (CBNG) produced water. The system is being monitored by The National Energy Technology Laboratory (NETL) and the United States Geological Survey (USGS) through the use of geophysical applications. The Headgate Draw study area is located at the confluence of Crazy Woman Creek and the Powder River, approximately 17 km south of Arvada, Wyoming. The site includes six agricultural fields and covers approximately 1.2 square kilometers of area. The SDI system is designed to assist in irrigation and reduce environmental impacts associated with the disposal of produced water. Pre-treated produced water is pumped through perforated tubing at a depth of approximately 1 meter. Geophysical surveys were performed via air, ground, and borehole methods prior to the SDI installation. Since installation, ground geophysical surveys have been conducted approximately 3 times per year beginning in the fall of 2008 using GEM-2, a handheld electromagnetic sensor. The focus of this presentation is field 6 due to its initially low and uniform conductivity values. Ground surveys completed in March and August of 2009 indicate that near surface conductivity values have increased in the SDI area compared to data collected outside the irrigated area. Future surveys will be performed to help improve the understanding of SDI systems that use produced water, and to assess change in near surface conductivity over time.