Probabilistic groundwater salinity mapping using airborne electromagnetic data in California’s San Joaquin Valley
Speaker: Lyndsay Ball, United States Geological Survey
Summary: Growing water stress has led to emerging interest in protecting fresh and brackish groundwater as a potential supplement to water supplies and raised questions about factors that could affect the future quality of fresh and brackish aquifers. Limited well infrastructure, particularly in brackish regions where elevated salinity has led to limited historical groundwater development, hinders traditional salinity mapping through groundwater sampling. This talk presents a quantitative salinity mapping approach of the upper 300 m using high‐resolution, regionally comprehensive resistivity models derived from Bayesian inversion of an airborne electromagnetic survey adjacent to the Lost Hills and Belridge oil fields in the southwestern San Joaquin Valley of California. Using limited available local water quality observations as an interpretational foundation, a probabilistic approach yields categorical maps of fresh, saline, and brackish groundwater while quantifying joint uncertainty inherited from the geophysical data and interpretational relations. Saline and fresh regions are mapped with relatively high confidence in many locations, while areas of lower confidence, particularly at depth, can be mapped as their most probable salinity category while reflecting the relative uncertainty in the interpretation.
ZTEM Natural Field EM, Magnetic and Radiometric Results over the Rio Zarza Au-Ag epithermal Project Zamora Province, Ecuador
Speaker: Jean Legault, Geotech Ltd.
Summary: The Zamora-Chinchipe Province in the south east of Ecuador, bordering with Peru, is considered some of the most prospective ground in the country since the discovery of Fruta del Norte Au-Ag deposit in 2006, with an indicated mineral resource of 23.8 million tonnes at 9.61 g/t gold (Lundin Gold Technical Report, 2015). The Fruta del Norte deposit is an intermediate sulphidation epithermal gold–silver deposit measuring approximately 1,670 m along strike, 700 m down dip, and between 150 m and 300 m wide (www.lundingold.com). Fruta del Norte is a blind deposit covered by approximately 200 m of post mineralization rocks. Tempus Resource Ltd.’s Rio Zarza Project is located 850 m west of the Fruta del Norte deposit and exhibits strikingly similar geological, structural and alteration characteristics.
Previous exploration at Rio Zarza between 2008 until 2012 included surface magnetics, induced polarization (IP), gravity surveying, soil/stream sediment sampling, mapping, and limited diamond drilling. However, all drilling at Rio Zarza was undertaken prior to a new geological interpretation and was ineffectual in reaching the target depth. Under current geological interpretation, it is thought that the Misahualli volcanics have been dropped by step-faults to the west of Fruta del Norte and so the potential gold target located at Rio Zarza is at depths of 700-800 m (www.tempusresources.com.au).
During December, 2019, Geotech carried out a helicopter borne ZTEM (z-axis tipper electromagnetic) EM, magnetic and radiometric survey over Rio Zarza on behalf of Tempus Resources, with the objectives to define Fruta del Norte style signatures at depth. A total of 640 line-km of data were acquired on 200m spaced, east-west lines and 2km spaced north-south tie-lines. Both 2D and 3D inversions were performed on the ZTEM data. In Summer 2020, Insight Geophysics provided additional analyses on the ZTEM-magnetic-radiometric results
The Insight analysis showed that the similar EM and magnetic signatures observed over Fruta del Norte were also defined at Rio Zarza ZTEM. Indeed, in spite of low bulk resistivities and weak contrasts it appears that both the 2D and 3D smooth model plausibly define de FDN economic package and similar resistivity patterns at great depth (>500m) at Rio Zarza. The conclusion is that the resistivity contrast was sufficient for successful electromagnetic exploration.
Applications of Alaska Division of Geological & Geophysical Surveys AEM data, Mapping, Mining, Permafrost
Speaker: Abraham Emond, Alaska Division of Geological & Geophysical Surveys
Summary: DGGS is celebrating 30 years of public domain AEM data. Our AEM data is a favorite with DGGS Mineral’s group geologists and mineral explorers. DGGS has leveraged the AEM data to improve geologic maps and identify sub regional features. Mineral explorers use our data to target areas and plan follow up surveys. Recently DGGS has collaborated with other agencies on permafrost and infrastructure support studies with AEM data. Scientists and citizens all took great interest in the Goldstream AEM data set’s ability to map permafrost. Rapid inversion models allowed scientist to target potential talics for follow up study. Drilling results, isotope analysis, vegetation, and surficial morphology were used to expand interpretations and confirm resistivity models from the AEM data.
3D Geophysical Inversion and Integration combined with a structural geological interpretation: An Innovative workflow for the Keats Discovery Zone, Queensway Project Newfoundland
Speakers: Brenda Sharp, Goldspot Discoveries
Summary: Geophysical surveys are widely used to obtain information about both the surface and subsurface of the Earth. Geophysical inversions provide 3D petrophysical models which are consistent with survey data, and are intended to model the geology of the subsurface. These models are a powerful tool for the geoscientist to improve their explicit or implicit geological models.
In 2020 the GoldSpot team developed a geophysical inversion package (minusOne), and inverted and integrated the high-sensitivity aeromagnetic and HeliFALCON Airborne Gravity Gradiometer (AGG) survey data in Newfoundland and Labrador, Canada. First, we assessed the uncertainty of the gravity gradient tensor components and their combinations by leveraging stochastic methods. Second, five independent gravity gradient components, Tij, were inverted. Third, to find groupings with similar traits, GoldSpot combined the results using the self-organizing map (SOM) and clustering. Conductivity-depth images, CDI, and a 3D susceptibly recovered model (from a previous HELITEM multi-pulse survey) were additionally integrated before performing the SOM and clustering. As expected, the inversion of the Tzz component emphasizes the feature’s location and the inverted Txx and Tyy show lineation, while the remainder of the components define edges. The stochastic inversion can help in the uncertainty analysis of these components. Deterministic techniques, on the other hand, are computationally faster but do not provide additional benefits like the stochastic method.
A structural geology interpretation based on the inversion was created by first drawing form-lines along continuous geophysical-cluster trend margins, which thus illustrate the geophysical fabric of the model. The next stage was to identify the curved, low-angle truncated and high angle form-lines which may represent the geological equivalents of folds, ductile faults which shear stratigraphy, and truncated folds limbs and brittle faults (respectively). These structural features host the main mineral deposits, and so identification of these features provides a practical guideline for exploration work.
The structural geology study from the inversion compared favorably to a 2D review conducted on the original datasets. The 2D interpretation identified intersections between the Appleton Linear and previously unidentified NE-SW / ENE-WSW structures. The identification of these structures encouraged New Found Gold to conduct field follow up and drilling which in turn lead to the discovery of the Keats “Discovery” Zone deposit.
Connecting the dots with airborne geophysics: Mapping aquifer structure at new scales in the Mississippi Alluvial Plain
Speakers: Burke Minsley, United States Geological Survey
Summary: The Mississippi Alluvial Plain hosts one of the most prolific aquifer systems in support of agricultural resources in the United States. The surficial aquifer, spanning more than 92,000 square kilometers in parts of seven states, provides over 9.2 billion gallons per day primarily for irrigation that supports a $12 billion agricultural economy. Despite its economic importance, there is insufficient knowledge of the belowground architecture of the aquifer system over this vast area with the spatial resolution needed for effective management of water resources. Here, we introduce the results of a large airborne geophysical survey that uses a combination of electromagnetic (AEM), magnetic, and radiometric sensors to map the aquifer system and deeper sediments at an unprecedented scale for a regional aquifer system in the United States. More than 40,000 flight-line-kilometers of airborne geophysical data supplement more than 9,000 existing borehole logs in the region to provide new three-dimensional insight into the three-dimensional arrangement of sediments that comprise the surficial aquifer and surrounding units. Interpretations of aquifer structure derived from the AEM data will be used to inform the development of regional groundwater models, and to guide decisions on where additional data are needed in the future.
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