Acquisition of Airborne Electromagnetic Data in the Groundwater Basins of California: Initial Assessment of a Statewide Reconnaissance Project

1 Acquisition of Airborne Electromagnetic Data in the Groundwater Basins of California: Initial Assessment of a Statewide Reconnaissance Project Rosemary Knight (Stanford University) Paul Gosselin (Butte County Dept. of Water and Resource Conservation) Graham Fogg (UC Davis) 1 November, 2016 California has embarked on a historic journey to achieve groundwater sustainability through the Sustainable Groundwater Management Act (SGMA). Local agencies are vested with the responsibility for achieving sustainability. For most of the state, the basis for making decisions lacks sufficient understanding of the structure of groundwater basins. Traditional methods of characterizing aquifers are slow, expensive and insufficient. There is a well-established geophysical method, the airborne electromagnetic (AEM) method, which has the potential to make a significant contribution to the way we map and manage groundwater systems in California. The following describes how a statewide reconnaissance program using AEM could advance the goals of groundwater sustainability and increase the likelihood of successful SGMA implementation. The AEM method is deployed using a helicopter that moves geophysical instruments 30 meters (100 feet) above the land surface at a speed of about 80 km per hour (50 miles per hour), imaging to a depth of approximately 500 m (1500 ft). The result, after data processing and analysis, is a set of 2D slices displaying the detailed variation in the electrical resistivity of the subsurface. Through calibration with well data and geologic interpretation, this can be transformed to map out the distribution of sediment textures (sand, silt, clay), defining the architecture of the aquifer and non-aquifer materials continuously. An example of a data set acquired in Nebraska is shown in Figure 1. The 2D slices have been combined to display the project area as a fence diagram showing the geologic interpretation of the AEM data. In areas of saltwater intrusion, the sensitivity of the AEM method to electrical resistivity makes it possible to map out the extent of saltwater intrusion, due to the large contrast in electrical resistivity between saltwater and freshwater. The AEM method has been used worldwide for groundwater evaluation and management, with over 300,000 line-kilometers (186,000 line-miles) acquired in Australia, over 60,000 linekilometers (37,000 line-miles) of data acquired in Denmark, and over 28,000 line-kilometers (17,000 line-miles) recently acquired in Nebraska to support groundwater management efforts. Other countries where AEM has been deployed to characterize the groundwater systems include Germany, South Africa, Namibia, Botswana, Ireland, Italy, Canada, Brazil, Mexico, Thailand, India, France, Norway, Sweden, and Russia. Despite the successful deployment of this method in many locations worldwide, there have been only a few AEM data sets acquired in California. We suggest that this is due to the lack of familiarity with the method, as the value-added to groundwater management has been repeatedly demonstrated after ~15 years of use. 2 Adoption of the AEM method for mapping the groundwater basins of California would provide critical data needed to address requirements under the Sustainable Groundwater Management Act (SGMA). Some examples are given below: 1) A hydrogeologic conceptual model is required (GSP Regulations §354.14). The information obtained from the AEM data can fill in the large gaps that inevitably exist in well data, both in terms of lateral extent and depth, and will greatly reduce uncertainty when correlating between wells. One of the major determinants of groundwater model reliability is a well-defined hydrogeologic conceptual model. 2) The assessment and improvement of monitoring networks is required (GSP Regulations §354.38). The information obtained from the AEM data can be used to determine the optimal locations for monitoring wells – at $150,000 per well (or more) there is an obvious need for guidance in efficient well placement and monitoring network design. 3) Under Project and Management Actions (GSP Regulations §354.44) there is a need to have accurate predictive capabilities about the impact of any action (e.g. recharge or extractions). The information obtained from the AEM data can reduce the uncertainty in conceptual models and facilitate more accurate and representative groundwater flow modelling, thus improving predictive capabilities. In particular, because most California alluvial aquifers are confined, identifying gaps through confining beds is invaluable for determining optimal locations for recharging those confined aquifers. Statewide AEM Reconnaissance Project Proposal We suggest an approach that involves an investment by the state in the acquisition, processing, analysis, and interpretation of AEM data, at a reconnaissance scale, in all of the 135 groundwater basins designated by DWR as either high or medium priority under SGMA. Local agencies could then build on this framework and acquire additional AEM data in key areas of interest. We have designed a plan of flight lines for the reconnaissance-scale data acquisition; this is shown in Figure 2. While these lines are not intended to represent the final survey design, they are representative of the length and spacing necessary to yield a meaningful reconnaissance of each basin. To the extent possible, lines have been oriented along or across depositional and/or geologic trends, facilitating a more meaningful interpretation than would be derived if the lines were simply oriented to an arbitrary map grid. A total of 20,000 line-km (12,500 line-miles) are shown, which generally allows for coverage in every township within any given basin. Line spacing varies, due to factors related to basin geometry, from approximately 6.5 to 11 km (4 to 7 miles), with most areas having 8 to 9 km (5 to 5.5 miles) between lines. The primary urban areas are excluded, as AEM surveys may not be flown in these areas. The finalized survey design in each basin will likely vary from this initial framework, due to logistical considerations such as avoiding power lines, highways, etc. and will be configured to maximize flight efficiency. Data acquisition is followed by data processing, analysis and interpretation, incorporating all available information from wells and other surveys. The final product is typically in the form shown in Figure 1 – an image of the hydrostratigraphy of a region. 3 What would this cost? $10 million We have estimated the cost based on similar surveys elsewhere in the U.S. Data acquisition costs will be between $180 and $250 per line kilometer ($110 to 155 per line-mile). Using the upper estimate, and our estimated 20,000 line-km (12,500 line-miles), this yields a cost for data acquisition of $5 million. Data processing, analysis and interpretation is estimated to cost $250 per line kilometer ($155 per line-mile), for a cost of $5 million for the 20,000 line-km (12,500 line-miles). In contrast, the same amount funding would support the installation of about 60 monitoring wells. How long would this take? 16 months This estimate has been provided by a private company that has been involved with a similar effort in Nebraska over the past 12 years, where 28,000 line-km (17,000 line-miles) of data were acquired and interpreted and are now being used for groundwater management. Starting with a Pilot Study One way to initiate this type of work in California would be to select one high priority, critically overdrafted basin and complete the workflow from data acquisition through interpretation. One suggestion would be the Paso Robles Basin. Given our flight lines in this basin, we estimate a reconnaissance-level AEM survey of the Paso Robles Basin would require 600 line-km (375 line-miles). The total cost of this work would be $300,000; the time to complete would be approximately 5 months. 4 FIGURE 1. Fence diagram showing the geologic interpretation of AEM data acquired in Nebraska for the Eastern Nebraska Water Resources Assessment project. In the top of the diagram the following materials are shown: Coarse Aquifer Principal Aquifer Marginal Aquifer Non-Aquifer In the lower part of the diagram the following geological units are shown: Pierre Formation Niobara Formation Carlile Formation Dakota Group Pennsylvanian System Mississippian System 5 FIGURE 2. Suggested flight lines for the acquisition of AEM data at the reconnaissance scale in the high and medium priority groundwater basins in California.