Jesse Crews, P.G. 1 Ahmad A. Behroozmand 2 Rosemary Knight 3. Senior Geophysicist, GEM Center*, Stanford University

Transcription

1 Jesse Crews, P.G. 1 Ahmad A. Behroozmand 2 Rosemary Knight 3 1 Senior Geophysicist, GEM Center*, Stanford University 2 Posdoctoral Researcher, Stanford University 3 Professor of Geophysics & Director of GEM Center*, Stanford University *Center for Groundwater Evaluation and Management August, 2016 Outline Project background Study Area Hydrogeology Field Study Geophysical methods Time-domain Electromagnetics (TEM) Surface NMR (SNMR) and downhole NMR (DNMR) Results Conclusions and recommendations 2 1

2 Project Background 3 Project Background Stanford was introduced to this project area in early 2015 by a hydrogeology consultant working with the Indian Wells Valley Cooperative Groundwater Management Group. The Indian Wells Valley Groundwater Basin is currently in a state of overdraft, and in order to comply with the new Sustainable Groundwater Management Act (SGMA), basin stakeholders are exploring various groundwater management alternatives. Basin stakeholders are interested in assessing both the fresh and brackish portions of their groundwater system, and are currently evaluating what tools and methods should be used in this assessment Brackish water may be treated, blended, or used as-is for certain industrial or mining applications 4 2

3 Declining Water Levels Source: Todd Engineers (2014) 5 Project Stakeholders The Indian Wells Valley Cooperative Groundwater Management Group Includes: US Bureau of Land Management (BLM) City of Ridgecrest Kern County Board of Supervisors Eastern Kern County Resources Conservation District Indian Wells Valley Airport District Indian Wells Valley Water District Inyokern Community Services District Kern County Water Agency Naval Air Weapons Station China Lake, Environmental Project Office Searles Valley Minerals 6 3

4 Pilot Study Timeline Initial discussions between Stanford and the Indian Wells Valley Cooperative Groundwater Management Group (IWVCGMG) occurred in summer of 2015, and a Pilot Study was designed. Initial field data acquisition, including surface Time-Domain Electromagnetics (TEM) and surface Nuclear Magnetic Resonance (SNMR) occurred in October This effort was funded by the Center for Groundwater Evaluation and Management (GEM) at Stanford. Initial results indicated that better resolution of NMR data was desirable, so a second field acquisition, using downhole NMR logging (DNMR) was undertaken in April/May This effort was largely funded by the Indian Wells Valley Water District. 7 Study Area 8 4

5 Study Area The Indian Wells Valley is a closed basin situated in the Mojave Desert. It is bounded on the west by the Sierra Nevada, with other smaller uplifts of igneous and metamorphic rock bounding its other sides. This basin is very large (over 600 sq. miles), but has a low population density, and much of the area is free from infrastructure-related electromagnetic noise sources. A large portion of the basin area falls in Federally-owned lands, including the Naval Air Weapons Station China Lake, and land administered by the Bureau of Land Management. 9 Indian Wells Valley Groundwater Basin 10 5

6 Urban Areas Ridgecrest 11 Urban Areas Ridgecrest 12 6

7 Federal Lands 13 Geology Image source: California Geological Survey 14 7

8 Hydrogeology This actively forming basin is filled with alluvial sediments derived from the surrounding mountains, with coarser deposits closer to the mountains and interbedded fine-grained deposits further toward the center and eastern portion of the basin. Fine-grained units are often discontinuous. Unconfined conditions near basin margins grade into semi-confined toward basin center. Image Source: Tetra Tech EM (2003) Dutcher and Moyle (1979) 15 Estimated Groundwater Elevations (2012) Source: Kern County Water Agency (2013) 16 8

9 Estimated Depth to Groundwater (2012) Source: Kern County Water Agency (2013) 17 Field Study 18 9

10 Problem Basin-scale characterization of a complex alluvial aquifer system with both fresh and brackish water Water table depth varies from very near surface in the northeast to > 150m Transition to brackish water is poorly characterized, occurring near the surface in some areas (typically to the east) and beyond typical well depth in other areas Wells typically have large screened intervals, and may be blending fresh and brackish water in some cases 19 Proposed Geophysical Approach Time-Domain Electromagnetics (TEM) combined with Surface and Downhole Nuclear Magnetic Resonance (SNMR, DNMR) TEM is sensitive to the electrical conductivity of earth materials, and can provide indications of changing lithology (clays are more conductive than coarse sediments) or changes in water quality (brackish water is more conductive than fresh water). NMR methods are sensitive to the presence and amount of water in earth materials, without regard to electrical conductivity. NMR data can be used to resolve ambiguities in the TEM data, helping to distinguish changes in water content from changes in lithology. Because of large basin size, this initial Pilot Study was designed to test these methods in several discrete areas of the basin to determine applicability, assess noise conditions, and observe responses in different geologic/hydrologic conditions

11 Field Site Selection Criteria Physically accessible by road On Navy or BLM land Outside protected Wilderness areas As far as possible from power lines and other infrastructure Each site should represent different geologic and/or hydrologic conditions Where possible, sites should be near existing wells Sites within the NAWS should be in areas relatively free from unexploded ordnance 21 Field Sites 22 11

12 Field Sites 23 Field Sites NAV A NAV B BLM B NAV C BLM A 24 12

13 Geophysical Methods 25 TEM background TEM involves applying a magnetic field to the subsurface by running a DC electric current through a loop (most often a square) of wire at the surface, referred to as the transmitter loop (Tx). This induces electric currents in the subsurface (so-called eddy currents), which subsequently generate a secondary magnetic field that can be measured with another loop, referred to as the receiver loop (Rx) These measurements are then inverted to provide an estimate of the electrical resistivity of materials in the subsurface Rx loops 26 13

14 TEM background EM induction measurements respond to the bulk subsurface electrical resistivity/conductivity and, in particular, the spatial distribution of conductive/resistive anomalies ( (r) / (r)) The formation resistivity depends on factors that influence the bulk resistivity of the formation, including sediment type (sand or clay or a mixture), porosity, clay type (in clay rich sediments), and salinity of the saturating fluid. 27 Geology, hydrology and formation resistivity Low resistivities Clay dominated sediements Impermeable Resistivity for different sediments High resistivities Sandy sediments permeable Resistivity Hydrogeology Hydraulic conductivity Low High 28 14

15 Geology, hydrology and formation resistivity Application areas and targets of EM induction geophysics Resistive targets Permafrost zones Crystalline rock Intermediate targets Faults, fracture zones Archeological structures Conductive targets Seawater intrusion Saline and inorganic plumes Caves, karst Freshwater aquifers Clay (lenses, soils)... Table modified from Everett (2013) 29 TEM background Typical configuration: central loop Rx loops WalkTEM system Tx loop 30 15

16 TEM recorded signal in the receiver loop Observed data Inversion results Stacked Rhoa data Smooth model Layered model Low moment Rhoa [ohmm] High moment Depth [m] Depth of investigation Time [s] Resistivity [ohmm] Resistivity [ohmm] 31 TEM recorded signal in the receiver loop Observed data Inversion results Stacked Rhoa data Smooth model Layered model Low moment Rhoa [ohmm] High moment Depth [m] Time [s] Resistivity [ohmm] Resistivity [ohmm] The position of these layer boundaries can be fixed or constrained in the inversion, when well data is available for control 32 16

17 TEM recorded signal in the receiver loop Observed data Inversion results Stacked Rhoa data Smooth model Layered model Rhoa [ohmm] Low moment High moment Depth [m] Error bars show uncertainty Time [s] Resistivity [ohmm] Resistivity [ohmm] 33 Ground-based TEM in a few words Easy to setup (our best time was 40 minutes, start to finish) Point measurement with a small layout Resolution A few meters below the surface Up to ~ m (depending on the Tx loop size, overall formation resistivity and the Rx loop type) Very sensitive to conductive clay layers & salt water interfaces High production rate - Surveying large areas at a reasonable cost For larger-scale hydrogeological investigations, this method can be performed from a helicopter-mounted system that quickly covers large areas and provides a 3D resistivity model of the area

18 NMR background Hydrogen atoms within water molecules can act as magnetic dipoles, and generally align themselves to any static magnetic field they are within. Nuclear Magnetic Resonance (NMR) involves subjecting earth materials to specific radio frequency pulses, which cause the magnetic dipoles of water molecules to align in a different direction than their normal orientation. When the signal is turned off, the hydrogen dipoles will re-orient themselves to the ambient magnetic field. The process of this re-orientation generates an exponentially decaying signal that can be measured, which provides information about the total amount of water present, and indications of permeability. 35 NMR background NMR has been used in the medical field for decades. For geophysical applications, NMR can be used at the surface or with specialized logging instruments lowered into wells and boreholes. Surface NMR (SNMR) involves laying out loops or squares of wire, similar in appearance to TEM. The loop size is generally larger (we used 100 x 100 meter squares), and the measurements take longer. Depth of investigation is typically between 50 and 100m. Vertical resolution varies depending on the depth and geology, but generally ranges from +/ m near the surface to a few meters at the depth limit (60-100m). Downhole NMR (DNMR) measures a shell of material around a well or borehole, with vertical resolution down to 0.5 meters

19 Surface NMR data and model Observed data Inversion results Figure from Behroozmand et al. (2016) 37 NMR in a few words Sensitive to the presence of water, without influence from electrical properties of the formation. Surface NMR can be deployed in areas without wells, and can estimate water content to depths of meters. Downhole NMR is deployed in PVC-cased wells or encased boreholes, and can provide high-quality measurements of total water content (porosity, if fully saturated), and indications of permeability with resolution down to 0.5 meters

20 Results 39 Field Sites NAV A NAV B BLM B NAV C BLM A 40 20

21 Field Sites NAV A NAV B BLM B NAV C BLM A 41 Site NAV A TEM SNMR 42 21

22 Site NAV A SNMR NAV A 01 Primary Loop Noise Loop 43 Site NAV A SNMR results Water Table 44 22

23 Site NAV A Downhole NMR Near-surface Clay Unit easily seen on log, even in unsaturated zone Water Table Water Table Depth (m) 45 Site NAV A Surface NMR Comparison Downhole NMR Surface NMR Depth (m) Water Table Water Table 46 23

24 Site NAV A TEM Results NAV A NAV A 02 TEM results 0 Smooth Model Lithology Estimated Water Quality 0 Layered Model Water Table Depth (m) Gradual transition toward brackish water? Depth (m) Depth of Investigation Resistivity (Ohmm) Lithology (interpretted from well data and TEM) 400 Estimated Water Quality (from TEM data) ,000 Resistivity (Ohmm) Coarse Fine Fresh Brackish 48 24

25 Site NAV A TEM Results NAV A NAV A 03 TEM results 0 Smooth Model Lithology Estimated Water Quality 0 Layered Model Water Table Depth (m) Gradual transition to brackish water? Depth (m) Depth of Investigation Resistivity (Ohmm) Lithology (interpretted from well data and TEM) ,000 Resistivity (Ohmm) Estimated Water Quality (from TEM data) 0 Coarse Fine Fresh Brackish 50 25

26 Site NAV A TEM Results NAV A NAV A 06 TEM results 0 Smooth Model Lithology Estimated Water Quality 0 Layered Model Water Table Depth (m) Better water quality at depth? Depth (m) Depth of Investigation Resistivity (Ohmm) Lithology (interpretted from well data and TEM) ,000 Resistivity (Ohmm) Estimated Water Quality (from TEM data) 0 Coarse Fine Fresh Brackish 52 26

27 Site NAV A TEM Results 53 Site NAV A TEM Results Near-surface Clay Unit increasing to the west Deep units are less resistive to the west, possible increase in salinity 54 27

28 Site NAV A Preliminary Interpretation Good agreement between TEM and SNMR data Both techniques indicate a clear transition at 55-60m, right at the depth of the water table, as measured in wells, and indicate a coarsegrained fresh-water aquifer. The two western sites show a clay-rich layer near the surface, which is absent to the east. Where this clay layer is present, the TEM also indicates an increase in resistivity at the water table rather than a decrease, as would be more commonly expected. Water retained by capillary forces in the near-surface clay layer allowed it to also show up clearly on the DNMR logs, and it likely could be identified on SNMR soundings. The three western TEM soundings show an interval of less-resistive material at approx m depth, generally corresponding with finer-grained intervals on nearby well logs. The two western TEM soundings show a gradual decrease in resistivity with depth that is not reflected in the other two soundings to the east, possibly indicating a transition to more saline groundwater. 55 Field Sites NAV A NAV B BLM B NAV C BLM A 56 28

29 Site NAV B TEM SNMR 57 Site NAV B SNMR Results No viable NMR signal Considerable noise issues at this site (broad-band noise) 58 29

30 Site NAV B NAV B 03 TEM SNMR 59 Site NAV B 03 Downhole NMR Water Table Depth (m) Bedrock 60 30

31 NAV B 03 TEM results 0 Smooth Model Lithology Estimated Water Quality 0 Layered Model Water Table Depth (m) Transition to bedrock Depth (m) Depth of Investigation Resistivity (Ohmm) Lithology (interpretted from well data and TEM) ,000 Resistivity (Ohmm) Estimated Water Quality (from TEM data) 0 Coarse Fine Bedrock Fresh Brackish 61 Site NAV B TEM Results Thin clay layer near surface 62 31

32 Site NAV B Preliminary Interpretation TEM soundings and downhole NMR log were excellent. No viable Surface NMR signal Considerable noise at this site Site NAV B 03 (the northern one) was near a well, and the transition to less-resistive material at ~ 30m corresponds with measured water level. Very low resistivity appears to correspond with finer-grained materials rather than brackish water. Transition to bedrock unit at ~ 90m, shows as resistive on the TEM sounding and very low porosity (2-3%) on the Downhole NMR log The two western TEM soundings appear quite similar, while site eastern site appears to have a conductive layer near the surface, likely a thin clay layer related to the playa lake. 63 Field Sites NAV A NAV B BLM B NAV C BLM A 64 32

33 Site NAV C Surface NMR did not receive any signal, likely due to lack of water in the sensitive zone. Six TEM soundings were taken, but data recording issues made three of them unusable. Downhole NMR logs were taken in shallow wells to the north and to the south of the TEM soundings. 65 Site NAV C TEM SNMR 66 33

34 Site NAV C Downhole NMR Depth (m) Water Table Entire Log Interval Unsaturated 67 Site NAV C TEM Results NAV C

35 NAV C 02 TEM results 0 Smooth Model Lithology Estimated Water Quality 0 Layered Model 100 Transition to bedrock 100 Depth (m) Depth (m) Depth of Investigation Resistivity (Ohmm) Lithology (interpretted from well data and TEM) ,000 Resistivity (Ohmm) Estimated Water Quality (from TEM data) 0 Coarse Fine Bedrock Fresh Brackish 69 Site NAV C TEM Results NAV C

36 NAV C 04 TEM results 0 Smooth Model Lithology Estimated Water Quality 0 Layered Model Depth (m) Fracture zone? Transition to bedrock Depth (m) Depth of Investigation Resistivity (Ohmm) Lithology (interpretted from well data and TEM) ,000 Resistivity (Ohmm) Estimated Water Quality (from TEM data) 0 Coarse Fine Bedrock Fresh Brackish 71 Site NAV C TEM Results More conductive at depth; possible fracture zone Highly resistive bedrock 72 36

37 Site NAV C Preliminary Interpretation TEM Good quality soundings Indicated a transition to highly-resistive material at depth, interpreted as bedrock. Zone of lower resistivity at site NAV C 04 interpreted as possible fracture zone No clear indication of brackish-water transition in this area Surface NMR No viable NMR signal Noise seems to have been cancelled successfully. Very likely no water signal to be measured within the SNMR-sensitive zone, based on resistive TEM soundings and water levels in nearby wells Downhole NMR Good quality logs, but limited by the depth of available wells Encountered an obstruction (likely garbage) in one well 73 Field Sites NAV A NAV B BLM B NAV C BLM A 74 37

38 Site BLM A TEM SNMR 75 BLM A photos 76 38

39 BLM A photos 77 Site BLM A SNMR Primary Loop Noise Loop 78 39

40 BLM A SNMR results Water Table 79 Site BLM A 04 TEM Results BLM A

41 BLM A 04 TEM results 0 Smooth Model Lithology Water Table Estimated Water Quality 0 Layered Model Depth (m) Increasing Salinity? Depth (m) Depth of Investigation Resistivity (Ohmm) Lithology (interpretted from well data and TEM) ,000 Resistivity (Ohmm) Estimated Water Quality (from TEM data) 0 Coarse Fine Bedrock Fresh Brackish 81 Site BLM A TEM Results 82 41

42 Site BLM A Preliminary Interpretation Unsaturated, resistive material from the surface to the water table, at approximately 60m Saturated, fresh-water aquifer Transition at m to less resistive interval Likely a change to finer-grained material Gradual transition at m to higher conductivity Possibly an increase in salinity Good agreement between TEM and SNMR data Water levels match general estimates from contoured well data. 83 Field Sites NAV A NAV B BLM B NAV C BLM A 84 42

43 Site BLM B TEM SNMR 85 BLM B photos 86 43

44 BLM B photos 87 Site BLM B SNMR Results No viable NMR signal Considerable noise issues at this site (main reason) Deep water table (~ 80m) may have exceeded the resolution depth of the SNMR system 88 44

45 BLM B 06 TEM results Possible finegrained Estimated Lithology unit near Smooth Model Water Quality 0 surface 0 Layered Model 100 Water Table 100 Depth (m) Depth (m) Resistivity (Ohmm) Lithology (interpretted from well data and TEM) ,000 Resistivity (Ohmm) Estimated Water Quality (from TEM data) 0 Coarse Fine Fresh Brackish 89 Site BLM B TEM Results Similar resistive material throughout Thin clay layer near surface 90 45

46 Site BLM B Preliminary Interpretation Unsaturated, resistive material from the surface to the water table, at approximately 100m Saturated, fresh-water aquifer Thin, low-resistivity layer seen near surface, most prominent at BLM B 03 Likely a clay layer that is not laterally extensive No additional significant changes down to resolution depth Likely means that there is no transition to brackish water or to finergrained materials at this site (except possibly BLM B 04, to the southeast) No SNMR Data due to high noise, but water table may also have been too deep to resolve. Water levels within general estimate range from well data 91 Conclusions and Recommendations 92 46

47 Conclusions Five test sites surveyed Measured: 23 TEM soundings 6 Surface NMR soundings 5 Downhole NMR logs 93 Conclusions The TEM method was extremely successful in this pilot study. Soundings generally correlated well with both geologic and water level data in nearby wells. Even at sites with higher noise, reasonable interpretations were able to be made at every site. Depth of investigation with TEM was typically beyond 300m. Downhole NMR proved to be a valuable compliment to the TEM data, providing finer vertical resolution and important constraints on the inversion and interpretation of the TEM data. The SNMR method was successful where there was sufficient water signal shallower than 70m, but had some issues at sites BLM B and NAV B with noise. Use of the shielded receiver cable appears to improve S/N. This technique would be useful in assessing areas with shallow groundwater where there are no wells that could be logged using downhole NMR

48 Conclusions Gradual transition to lower resistivity at several sites suggests that the transition to higher-salinity groundwater can be measured with TEM. Tying TEM soundings to well control, especially downhole NMR logging, will be crucial to generate valid interpretations. Significant lateral variations in geologic units were observed over distances of 1 km or less, so any survey design should be sufficiently dense to characterize changes on this scale. Overall, this basin is geologically complex, with considerable lateral variation in basement topography, distribution of coarse vs fine sediments, and changes in water quality. 95 Recommendations This pilot study indicated that the TEM method, constrained by well log information (particularly downhole NMR logs) is a viable method for assessing both stratigraphy and water quality across Indian Wells Valley, to a depth of at least meters. Given the large aerial extent of Indian Wells Valley and the significant lateral variations within it, the ideal geophysical method for investigating hydrostratigraphy in this region would be an airborne TEM survey. Similar surveys have been performed across Denmark, and recently a successful survey was flown by our team over the Tulare Irrigation District in the Central Valley. Airborne surveys have the advantage of greater depth of investigation (a stronger current is used), and the data from continuous flight lines can be processed together, greatly reducing uncertainty in the inversion. For large areas, airborne surveys are less expensive than surface-based surveys for the amount of data obtained

49 Recommendations For more targeted studies over sub-regions of Indian Wells Valley, surface TEM surveys may be utilized for lower cost, however the overall value and quality of airborne surveys are generally greater. Members of our team have successfully implemented surface TEM surveys for groundwater characterization in Egypt. Lines or grids of individual soundings can be processed and inverted together, greatly reducing uncertainty in the inversion. TEM surveys should be tied to well data, ideally including lithologic, electrical, gamma, and NMR logs. TEM surveys will allow the information from individual wells to be correlated/migrated over larger areas with much greater confidence. Given the regular drilling schedule of the Navy Sea Bees, new drilling locations should be coordinated with any planned studies. 97 Aerial TEM in the Central Valley Stanford researchers have recently completed an aerial TEM survey over the Tulare Irrigation District in the Central Valley with great success. The primary target of investigation, the Corcoran Clay, was clearly resolved, and deep stratigraphic patterns were revealed that may have significant impact on groundwater management. Photo: Lisa M. Krieger Electrical Resistivity Geologic Interpretation Figures: Knight, et al. (2016) 98 49

50 Surface TEM and NMR in Egypt Surface TEM could be deployed in a grid, generating a pseudo-3d survey In Indian Wells Valley, surface stations should be spaced 0.5 to 1 km apart. Soundings with larger loops (100 x 100 m) could be strategically added to the survey to increase depth of investigation. Figures: Behroozmand et al. (2016) 99 Surface TEM and NMR in Egypt Example data outputs from the Egypt TEM survey: Figures: Behroozmand et al. (2016)

51 Acknowledgments This Pilot Study would not have been possible without the support of: Indian Wells Valley Water District Naval Air Weapons Station China Lake Indian Wells Valley Cooperative Groundwater Management Group Vista Clara, Inc. Tim Parker 101 Field Crew Special thanks to all our field crew: Emily Fay Ian Gottschalk Alex Kendrick Elliot Grunewald Dave Walsh