Step 4: Design the AEM survey
Recommendations: Identifying Data Gaps and AEM Survey Design
The design of the AEM survey includes selecting the AEM system and the data acquisition parameters, and determining the location, orientation, and density of the flight lines. The survey should be designed so as to best address the questions posed by the agency and to add spatial coverage of critical regions of the subsurface. One of the first steps in survey design is to select the general flight areas so as to avoid populated areas, powerlines, and other infrastructure likely to interfere with the data acquisition. Knowledge of the local geology, obtained by a review of existing data and reports, assists with the general survey design. More detailed information about the variation in lithology and electrical properties of the subsurface, also obtained from existing borehole, geophysical, and geological data, is needed in order to simulate, through forward modeling and inversion, the acquisition and interpretation of AEM data in the area. This makes it possible to predict the quality, spatial resolution, and depth of reliable imaging that can be obtained. Such an assessment is important both for survey design and for setting realistic expectations about what questions can be addressed, in the study area, through the use of the AEM method.
The recommended approach is to use existing lithology logs and geophysical data to generate a lithology model of the subsurface, with corresponding resistivity values, and then simulate the AEM systems to recover the subsurface model. The various AEM systems available are considered for the simulations so that the appropriate system is selected given the survey objectives. Current systems (of which we are aware) that are likely to be available in California are the CGG HeliTEM, MultiPulse, Resolve and Tempest; Geotech VTEM ET and VTEM Max; SkyTEM 304 and 312. This analysis of the various systems is so crucial for a successful survey that any potential vendors need to provide reliable and comprehensive information about their systems. It is key to these simulations that realistic data noise levels, flight speeds, and flight altitudes are taken into consideration. Likewise, the full system transfer function needs to be modelled, and if bias in the data is expected, this must be considered as an additional source of data uncertainty. The analysis is carried out by simulation of how well the given model is recovered given all these parameters and how well the resistivities and thicknesses are determined by the model sensitivity analysis. Through repeating the numerical exercise, it is possible to estimate the spatial resolution of layers, the vertical resolution of shallow layers, and the maximum depth of reliable imaging (depth of investigation) that would be obtained with the different systems in one or more parts of the survey area, representative of different lithologic variation. Through this process the various elements of the survey design are assessed and determined.
A high level of data accuracy is crucial for the quality of the resulting images derived from the AEM data. Any system needs to be able to deliver data that are not systematically biased and it needs to be able to output not only a mean value for a given data point, but also the statistical uncertainty for that point. A pre-qualification of the potential systems should be designed so they all fly a test line of 1 to 10 km length for assessment of the ability of the system to deliver accurate data, the system noise level, the ability to resolve the near-surface layer, and the depth of investigation. Determination of the bias level is done by comparison to calibrated ground-based measurements made along the test line.
In the pilot study, we explored the acquisition of AEM data directly over the wells from which the high-quality lithology data were obtained. The co-location of lithology and resistivity significantly improves the accuracy of the transform linking resistivity to lithology. As a result, we recommend acquiring data as close as possible to selected wells, while staying the required separation distance from the powerlines supplying the wells with power. The required separation distance is determined by the electrical properties of the subsurface, both the background resistivity and that in the top ~6 m, and the grounding of the powerline. While it is typically assumed that one should stay at least 150 to 200 m from a powerline, the highly resistive nature of the top ~6 m in parts of California has been found to result in no powerline effect in the AEM data even when flight lines are within 100 m of a powerline.
Upon completion of the survey design, contracting for a geophysical vendor is then pursued based on cost, proposed system characteristics, and demonstrated ability to successfully complete the survey. Often this is done by requesting an RFP or quotes for cost and timing to complete the work and deliver data. In addition to the elements of the survey design discussed above, the final contract needs to include specified flight height limits, specified flight speed limits, and specified tilt angles (X, Y) limits.