A seismic reflection survey that was conducted in downtown Boise, Idaho, to help city planners site a new well for injection of spent geothermal water illustrates some methods to safely and successfully employ a seismic reflection survey in an urban setting. The objective of the seismic survey was to estimate the depth and continuity of a basalt and rhyolite volcanic sequence. Well siting was based on geothermal aquifer depth, location of interpreted faults, projected thermal impact of injection on existing wells, surface pipe extension costs, and public land availability. Seismic acquisition tests and careful processing were used to ensure high-quality data while minimizing the potential for damage along city streets. A video camera placed in a sewer and a blast vibration monitor were used to confirm that energy from the seismic source (a 75-in3 land air gun) did not damage nearby buildings, street surfaces, or buried utilities along the survey lines. Walkaway seismic tests were also used to compare signal quality of the air-gun source to an explosive source for imaging targets up to 800 m depth. These tests show less signal bandwidth from the air-gun source compared to the buried explosive source, but the air-gun signal quality was adequate to meet imaging objectives.
Seismic reflection results show that the top of this rhyolite/basalt sequence dips (~8-11°) southwest away from the Boise foothills at depths of 200 to 800 m. Seismic methods enabled interpretation of aquifer depths along the profiles and located fault zones where injected water may encounter fracture permeability and optimally benefit the existing producing system. The acquisition and processing techniques used to locate the Boise injection well may succeed for other hydrogeologic and environmental studies in urban settings.
This document was originally published in Geophysics by Society of Exploration Geophysicists. Copyright restrictions may apply. doi: 10.1190/1.1444430
Liberty, Lee. (1998). "Seismic Reflection Imaging of a Geothermal Aquifer in an Urban Setting". Geophysics, 63(4), 1285-1294.