3.1.4 Legacy Gravity Data 34 3.1.5 Legacy Well Data 34 3.1.6 Legacy MT and TDEM Data Analysis 35 3.2 New MT and TDEM Data Acquisition 39 3.2.1 Survey Location, Permitting and Design 39 3.2.2 MT and TDEM Field Survey 40 3.3 MT and TDEM Interpretation Data Processing 42 3.3.1 Initial Processing 43 3.3.2 TDEM Inversion and MT Static Correction 44 3.3.3 MT Dimensional Assessment and Inversion 46 CHAPTER 4: Developments in 3D MT Resistivity Imaging 51 4.1 Interpretation Approach 51 4.1.1 3D Forward Modeling 52 4.1.2 3D Inversion 52 4.1.3 Advances in 3D MT Inversion during this Project 53 4.1.4 3D MT Inversion Resolution and Uncertainty 55 CHAPTER 5: Resistivity Interpretation of the Geothermal Resource 56 5.1 Confirmation of Clay-Oriented Resistivity Interpretation 56 5.1.1 Well 88A-28 56 5.1.2 Well 17A-6 60 5.2 MT Maps Illustrating Overall Clay Cap Geometry 61 5.2.1 Map of 3D MT Conductance to 600 m (1969 ft) Depth 62 5.2.2 Maps of Elevation of the Base of the Clay Cap 63 5.3 MT Resistivity Cross-Sections 66 5.3.1 MT Resistivity Cross-Sections Correlated with Wells 67 5.3.2 MT Resistivity Cross-Sections and Reservoir Geometry 71 5.4 MT Resistivity Maps 74 5.4.1 Interpreting Resistivity Maps at Different Elevations 74
The geothermal applications of TDEM are mapping the shallow geometry of the clay cap without dimensional distortion and correcting MT statics. For both of these applications, a central loop TDEM geometry has significant advantages over non-symmetrical TDEM geometries such as moving the detector outside the transmitter large loop. 2.4.2.1 Procedures TDEM survey operations are conducted by a crew of two to five workers, usually using only one vehicle. If the station site is not accessible by vehicle, TDEM equipment can be easily carried using backpacks. Depending on the size of loop used, it takes 15 minutes to several hours to lay out a station, 5 to 20 minutes to acquire the data, and 10 to 30 minutes to pick up the station and move on. Therefore, from 1 to 9 TDEM soundings per day can be acquired by a single crew, depending on loop size, with a 1000 m (1000 ft) diameter loop taking a full day or more and a 50 m (150 ft) diameter loop taking less than an hour. A large loop requires a large generator that usually cannot be moved far from road access. Depending on expected variations in the subsurface resistivity and the objectives of the survey, the TDEM loops may be laid out as individual stations separated by up to several km, along continuous profile lines where the edge of one station coincides with an edge of an adjacent station, or at locations collated with an MT station. The TDEM soundings released by Calpine in support of this project include both several hundred distributed stations using 300 m (1000 ft) loops and over 100 stations in profiles in which 300 m (1000 ft) diameter loops are spaced at 300 m (1000 ft) intervals so that one loop shares a common side with an adjacent loop. The budget submitted to the CEC for the TDEM survey conducted as part of this project includes supplementing the existing TDEM data with 40 more stations at 40 of the new MT stations for which static control from nearby existing TDEM stations might not be available. Figure 7: Sirotem TDEM Receiver/Transmitter and Crew