Geophysical Exploration in Water Resources Assessment. John Mundell, P.E., L.P.G., P.G. Ryan Brumbaugh, L.P.G. Mundell & Associates, Inc.

Transcription

1 Geophysical Exploration in Water Resources Assessment John Mundell, P.E., L.P.G., P.G. Ryan Brumbaugh, L.P.G. Mundell & Associates, Inc.

2 Presentation Objective Introduce the use of geophysical survey methods to enhance standard site characterization practices and improve the ability to map and delineate water resources for complex and difficult geologic conditions.

3 Overview Brief Background Typical Water Resource Assessment Building a Conceptual Site Model (CSM) Geophysical Survey Methods Practical Application Examples Conclusion/Questions

4 Who Are We? Consulting professionals with geologic, geochemical, geophysical, and engineering backgrounds Focused on the use of geophysics integrated into environmental, engineering, water resources, and construction projects Personnel have performed geophysical surveys since Ryan Brumbaugh John Mundell

5 The Challenge - Geologic Complexity Can cause rapid material property changes in both the lateral and vertical direction in unexpected ways not able to be predicted by classic widely-spaced soil boring and drilling programs. Subsurface data density limits the development of an accurate Conceptual Site Model that can adequately describe groundwater availability.

6 Glacial Terrain Bedrock Terrain Karst Terrain

7 Midwestern Geologic Complexity

8 Geology and Hydrogeology of Marion County Flows, Soils, Urban Wells Glaciers formed landscape Melting glaciers filled areas with sand and gravel = aquifers Drinking water is stored in aquifers

9 THEORETICAL WELLFIELD AREA OPTIMIZATION OF WELLFIELD YIELD BASED ON PUBLISHED AQUIFER CONDITIONS TYPICAL WATER RESOURCE ASSESSMENT PRACTICE

10 WELLFIELD AREA

11 Water Supply Need Is there the potential for aquifers? WELLFIELD AREA

12 Desktop Regional Geologic Study In addition to our one boring, we have some nearby data WELLFIELD AREA

13 Aquifer Characteristics Hydraulic Conductivity, Thickness, 100 ft 100 Wellfield Drilling Program 1 Boring WELLFIELD AREA

14 Aquifer Characteristics Hydraulic Conductivity, Thickness, varies regionally 100 WELLFIELD AREA Wellfield Drilling Program 1 Boring REGIONAL AREA

15 Aquifer Characteristics Hydraulic Conductivity, Thickness, varies regionally WELLFIELD AREA Wellfield Drilling Program 1 Boring REGIONAL AREA

16 Aquifer Characteristics Hydraulic Conductivity, Thickness, varies regionally WELLFIELD AREA Wellfield Drilling Program 1 Boring REGIONAL AREA

17 Aquifer Characteristics 40 Hydraulic Conductivity, Thickness, varies regionally Interpolation of thicknesses WELLFIELD AREA Wellfield Drilling Program 1 Boring REGIONAL AREA

18 Aquifer Characteristics 40 Hydraulic Conductivity, Thickness, varies regionally Interpolation of thicknesses WELLFIELD AREA Wellfield Drilling Program 1 Boring REGIONAL AREA

19 Aquifer Characteristics 40 Hydraulic Conductivity, Thickness, varies regionally Interpolation of thicknesses WELLFIELD AREA Wellfield Drilling Program 1 Boring REGIONAL AREA

20 Aquifer Characteristics WELLFIELD AREA Hydraulic Conductivity, Thickness, varies regionally Interpolation of thicknesses REGIONAL AREA

21 WELL PLACEMENT VERSUS WELLFIELD CONCEPTUAL MODEL Wells are placed in areas of greatest thickness of aquifer material Wells are placed and screened in areas of greatest hydraulic conductivity Wells are spaced at distances sufficient that the effects of overlapping recovery zones are minimized (i.e., maximum separation for the wellfield size)

22 Aquifer Characteristics WELLFIELD AREA Hydraulic Conductivity, Thickness, varies regionally Interpolation of thicknesses REGIONAL AREA

23 Aquifer Characteristics WELLFIELD AREA Hydraulic Conductivity, Thickness, varies regionally Interpolation of thicknesses REGIONAL AREA

24 Aquifer Characteristics Hydraulic Conductivity, Thickness, 100 ft 100 Wellfield Drilling Program 1 Boring WELLFIELD AREA

25 Aquifer Characteristics homogeneous, isotropic Hydraulic Conductivity,,K2 Constant Thickness, 100 ft 100 K2 20 Wellfield Drilling Program 2 Borings WELLFIELD AREA

26 Aquifer Characteristics WELLFIELD AREA Hydraulic Conductivity, Thickness, varies regionally Interpolation of thicknesses 20 K REGIONAL AREA

27 EXISTING PRACTICE SUMMARY Dependence on widely-spaced regional geologic information relative to the aquifer thickness and expected scale of horizontal and vertical stratigraphic changes Its convenient, low-cost, and acceptable when water resources are plentiful and widespread The odds of achieving maximized production from the wellfield is remote

28 THEORETICAL WELLFIELD AREA OPTIMIZATION OF WELLFIELD YIELD BASED ON ACTUAL AQUIFER CONDITIONS BUILDING WATER RESOURCE CONCEPTUAL MODELS

29 Aquifer Characteristics Hydraulic Conductivity, Thickness, 100 ft 100 Wellfield Drilling Program 1 Boring WELLFIELD AREA

30 Aquifer Characteristics homogeneous, isotropic Hydraulic Conductivity, Constant Thickness, 100 ft Wellfield Drilling Program 1 Boring WELLFIELD AREA

31 Aquifer Characteristics homogeneous, isotropic Hydraulic Conductivity, Constant Thickness, 100 ft WELLFIELD AREA

32 Aquifer Characteristics homogeneous, isotropic Hydraulic Conductivity, Constant Thickness, 100 ft WELLFIELD AREA

33 Aquifer Characteristics homogeneous, isotropic Hydraulic Conductivity, Constant Thickness, 100 ft WELLFIELD AREA

34 Aquifer Characteristics homogeneous, isotropic Hydraulic Conductivity, Constant Thickness, 100 ft WELLFIELD AREA

35 Aquifer Characteristics homogeneous, isotropic Hydraulic Conductivity, Variable Thickness Wellfield Drilling Program 2 Borings WELLFIELD AREA

36 THEORETICAL WELLFIELD AREA OPTIMIZATION OF WELLFIELD YIELD BASED ON ACTUAL AQUIFER CONDITIONS WELL PLACEMENT DEPENDS ON CONCEPTUAL GEOLOGIC AND HYDROGEOLOGIC MODELS TO MAXIMIZE YIELD FROM WELLFIELD

37 Aquifer Characteristics >> K2 Hydraulic Conductivity, K2 K2 K2 K2 K2 WELLFIELD AREA

38 Aquifer Characteristics >> K2 Hydraulic Conductivity, K2 K2 K2 K2 K2 WELLFIELD AREA

39 Aquifer Characteristics Hydraulic Conductivity, Constant Thickness, 100 ft K2 K2 K2 >> K2 WELLFIELD AREA

40 Aquifer Characteristics Hydraulic Conductivity, Constant Thickness, 100 ft K2 K2 K2 >> K2 WELLFIELD AREA

41 Aquifer Characteristics 10 Hydraulic Conductivity, Variable Thickness K K2 0 K >> K2 WELLFIELD AREA

42 Aquifer Characteristics 10 Hydraulic Conductivity, Variable Thickness K K2 0 K >> K2 WELLFIELD AREA

43 Aquifer Characteristics Hydraulic Conductivity, Variable Thickness 50 K2 90 K2 130 K2 0 WELLFIELD AREA 0 >> K2

44 Aquifer Characteristics Hydraulic Conductivity, Variable Thickness 50 K2 90 K2 130 K2 0 WELLFIELD AREA 0 >> K2

45 Aquifer Characteristics K Hydraulic Conductivity, Variable Area 150 >> K2 WELLFIELD AREA K2

46 Aquifer Characteristics K2 50 Hydraulic Conductivity, Variable Area >> K2 WELLFIELD AREA K2

47 WELLFIELD OPTIMIZATION Depends on the accuracy of the conceptual model, that is, how it matches with reality The question is: Is drilling the only way to improve the conceptual model?

48 THE USE OF GEOPHYSICS TO IMPROVE WATER RESOURCE CONCEPTUAL MODELS If you had another method that would increase the ability to predict the horizontal and vertical distribution of aquifer materials, wouldn t you use it?

49 Technologies used to quantify properties of soils, bedrock and groundwater in an understandable manner. Technologies providing subsurface information without direct sampling. Technologies that can reduce time to collect information, drilling and cost.

50 A way of measuring the Earth s natural or induced properties for the purpose of characterizing its variability. Contrasts in surface or subsurface materials cause changes to these properties, and allow us to locate their position and depth.

51 The Commonly Applied Technologies Potential Field Methods - gravity & magnetics Transmitted Energy Source Methods - seismic & ground penetrating radar Electromagnetic Methods - conductivity, TEM, VLF Electrical Methods - resistivity & IP

52 Geophysical Methods Electromagnetic Metal Detection Electromagnetic Conductivity Electrical Resistivity Imaging Seismic (Refraction, Surface Wave) Microgravity Ground Penetrating Radar (GPR) Downhole Logging

53 Geophysics Water Issues Groundwater Investigations/Remediation Aquifer/Sand/Gravel Unit Mapping Fracture/fault/karst delineation Placement of Wells (Monitoring/Production) Enhanced Design of In-situ Water Treatment Monitoring of Remediation Progress

54 In the past, surgeons had to do exploratory surgery to locate objects in a human body similar to drilling and excavating to locate subsurface aquifer areas. Now days, applied geophysics is like scanning using MRI, except to image objects in the earth. MRI Machine

55 Wellfield Site 4000 ft by 3000 ft Area History indicates possible aquifer Objective - find one 250 ft x 450 ft aquifer area within this 4000 ft by 3000 ft grid

56 Approach: Soil Borings Here are 150 randomly chosen sampling locations (borings), surely we will find and characterize our single aquifer thickness and extent

57 Oops! We missed!

58 We also missed these 11!

59 And these 9!

60 And these 13 too!

61 Single row, Perimeter Medium Density, Double Low Density, Center Single Row Cross Medium density, Center Low density, Meandering Medium density, target High density, target

62 Wellfield Exploration Problems Areas where glacial drift overlies non-productive bedrock (poor yield or quality) Glacial drift is till-dominated but contains isolated pockets of sand and gravel aquifer materials Inadequate surface water locations are in upland areas Known water supply does/will not meet current or projected demand Hunt & Peck drilling has had poor or mixed results and is a costly method of exploration

63 Geophysical Mapping Criteria Effective at Detecting and Discriminating Aquifer Materials Able to Reliably Reach Desired Depth of Investigation Rapid Data Collection, Processing, Interpretation Easily Deployed Intuitive Cost Effective

64 Applicable Geophysical Methods Lateral Terrain Conductivity Mapping Vertical 2-D Resistivity Profiling Downhole Logging

65 EM31: Moderate Depth Conductivity Meter Lateral Conductivity Mapping EM34: Deepest Conductivity Meter GEM-2: Multi-Frequency Conductivity Meter

66 Conductivity Mapping Greatest Variety of Techniques Available Able to Scan Large Areas Quickly

67 Case History Conductivity Mapping Can Delineate Aquifers

68 Vertical 2D Resistivity Profiling Geophysical Services Division Mundell & Associates, Inc.

69 Resistivity Profiling High detail method for characterizing subtle variations in both shallow and deeper subsurface resistivity Often used in conjunction with more rapid mapping techniques, such as terrain conductivity

70

71 2D Electrical Resistivity Imaging System Here s an example of the subsurface coverage from a dipole-dipole array Current (I) Flows between B & A while Voltage (V) is Measured between M & N Apparent resistivity is calculated using ohm s law (R=V/I) and the electrode geometries

72 2D Electrical Resistivity Imaging System Once the apparent resistivity data is collected, it is modeled using an inversion technique which creates a true resistivity cross section

73 Resistivity for Sand and Gravel Aquifers Resistivity for Fractured Bedrock Aquifers

74 Downhole Logging

75 Downhole Logging Used for determining site geology from existing wells Can examine the structural integrity of existing wells

76 Downhole Logging

77 The Process 1. Preliminary Geologic Research is Done 2. An Aerial/Topographic Study of the Proposed Site is Done 3. Based on the Previous Two Studies, a Geophysical Technique/Techniques is Selected.

78 Geologic Setting Narrow Valley incised into Devonian Shale Valley filled with sand and gravel Till on either side Compare with setting to North where adequate sand & gravel and/or carbonate are available.

79 Aerial Study - Lineaments

80 Aerial Study - Lineaments

81 Conductivity Mapping East test hole produced >800 gpm, double any prior well yields >20 of aquifer than previously seen

82 Correlations: Logging

83 Once the preliminary study and lateral mapping are done, 2-D resistivity data can give vertical data

84 If multiple lines are collected, vertical resistivity data can also be sliced at different depths

85 Once an aquifer has been delineated laterally and vertically, water volumes can be approximated

86 Line 1 Waverly Wellfield Cross-Section Geophysical Pilot Test Area Line 3 Line 2 Geophysical Pilot Test Area

87 Line 1 Waverly Wellfield Cross-Section Geophysical Pilot Test Area Geophysical Pilot Test Area Line 3 Line 2

88 Waverly Wellfield - Geology

89 Waverly Wellfield Contour Maps Top of Bedrock Top of Aquifer

90 Waverly Wellfield Contour Maps Upper Aquifer Lower Aquifer

91 Waverly Wellfield Contour Maps Transmissivity

92 Waverly Wellfield Geophysical Pilot Test

93 Line 1 Resistivity Profiles Line 2 Line 3

94 Case History Ethanol Plant Central Indiana Site No surface or municipal supply 1000 gpm demand <50 gpm found on site* Geology: glacial till overlying Mississippian Siltstone & Shale (basically no aquifer) Linden Figure 4. Map showing location of Linden, Indiana. Linden is located between Lafayette to the north and Crawfordsville to the south. * Plant was already under construction before test wells were drilled because adequate water was taken for granted.

95 2D Resistivity Transects Planned Across Valley Research into available well log information indicated buried valley one mile in width (red). Few wells drilled within the valley - little known about the aquifer materials - Old Seismic Study (IDNR). A series of 1-mile long geophysical transects across the valley (blue). Method chosen was 2D resistivity imaging. N 1 mile Figure 6. Map of Study Area. Green area is about 1 mile in width and 3 miles along the inferred valley axis. Of the 10 resistivity lines acquired, 9 of them were located in the southernmost square mile.

96 Figure 5. Bedrock geology map showing location of possible bedrock valley to the northeast of Linden. Only apparent hope: Possible tributary to ancient Teays River several miles east of plant Buried Valley Ethanol Plant

97 First Lines crossed valley fill materials

98 Line 2 thickest accumulation of sand and gravel Geology Here similar to plant site Figure 7. Transect 2 (Lines 2A and 2B). Lines are oriented east-west and cross the buried valley on Line 2B. The shale bedrock is the low resistivity material on the lower portion of the section. The discontinous sand and gravel are the type of material encountered at the plant site which yielded less than 50 gallons per minute. The buried valley materials, which exist within a zone only about 500 meters wide, are complex in detail and appear to be a series of discrete, irregularly shaped channel bodies. Drilling has discovered that there are more than 50 meters of gravel in the center of the valley, and pumping test yields have exceeded 1,000 gallons per minute.

99 Cross lines appear to run along fill

100 Likely Best Location Location Chosen for Non-technical Reasons

101 Conclusions The use of geophysics can greatly aid the optimization of water production yields from a wellfield. The methods are relatively rapid and less expensive than extensive drilling programs. Drilling confirmation is always necessary to verify the validity of the wellfield conceptual model. The combined use of geophysics with standard drilling techniques have proven effective in areas where aquifers are limited.

102 The End Thanks for your Attention