Geophysical Case studies From Texas

Transcription

1 Geophysical Case studies From Texas 1

2 PRESENTATION OUTLINE Active Growth Faults in Metropolitan Houston 1. Hockley 2. Tomball 3. Long Point 4. Pearland Foundation Case Studies Karst Geophysics in Austin: Cave, Conduit and Fault Mapping Conclusions 2

3 Definitions of Geophysics Geophysics is the study of earth materials and processes using the methods of physics. It is, in general, a non-invasive and relatively inexpensive technology that provides unique constrains which are NOT obtainable using traditional engineering geology for site characterization. REMINDER: Geophysics is just another 3

4 Definitions of Geophyicist Geophysicist: One who studies and applies geophysics to the solution of geological, environmental and geotechnical problems. Alternative Definition of a Geophysicist from Google: A slightly below average classical physicist, a slightly below average geologist, a slightly below average engineer.!! 4

5 Resistivity Method -High-resolution image of shallow subsurface -Targets: bedrock vs. faults, voids, internal stratigraphy ) Depth Exploration: ¼ of profile length 5

6 N EXAMPLEs OF RESISTIVITY IMAGING PROFILE WITHOUT AND WITH ANOMALY S Limestone Marl 6

7 Natural-Potential Method (NP) Utilizes the earth s naturalelectric field at the ground surface to detect and map groundwater pathways and geologic features, such as faults, fractures, conduits, caves Movement of water Target: Seepage, Voids 7

8 mv 8

9 Natural-Potential Method (NP), cont. Long-Line Method Gradient method 9

10 How GPR Does Work? 10

11 Tomball LPF-1 LPF-2 Pearland Lidar elevation map showing major Houston Faults 11

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13 A schematic cross-section of a growth fault Figure 1a 13

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15 MAIN HOCKLEY FAULT AT HWY. 290 N Up Down 15

16 Shopping Mall being built UP Down L1 Highway 290-East Bound 16

17 2005 field mapping with resistivity survey locations L6 L7 U FAULT SCARP D N L3 L5 FAULT Westbound 290 L2 L1 FAULT SCARP L4 Patched Asphalt Eastbound (A GPR profile is taken along Line 1) Union Pacific Railroad 17

18 W Fault Scarp E L1 Profile Major Crack Location Resistivity profiles at Hockley Fault L2 Profile 18

19 Resistivity Profiles Across the Main Hockley Fault NW SE Sand Silt/Silty Sand Clay/Silty Sand Sand L5 Profile Sand Silt/Silty Sand L6 Profile 19

20 NW Fault Scarp SE NW Silt/Silty Sand Sand SE Resistivity profile across the Main Hockley Fault 20

21 NW Ft Fault Scarp Ft SE High L1 Amplitude GPR Profile across the fault scarp L1-This scarp did not have any resistivity anomaly. So it is NOT considered the main Hockley fault. Low 21

22 NW Ft SE Ft GPR Data Along Line 1 Low Amplitude High 22

23 FS L7 U D L6 U D N Westbound 290 L3 L5 U D L2 L4 U D L1 FS U D Patched Asphalt Eastbound Feet Location of Hockley fault determined by resistivity surveys 23 Union Pacific Railroad

24 Location of Fault Zone based on geophysics results 24

25 A April 2010 August 2010 B Hockley Fault Deformation POSTSCRIPT TO GEOPHYSICAL SURVEYS 25

26 Main Hockley Fault and its minor fault along east feeder MALL Minor Fault Main Fault 26

27 D C Minor faults immediately to the west of the main fault along the wet feeder 27

28 If you follow the main fault on the road towards the newly built building you will see the crack at the corner of the building! 28

29 LONG POINT FAULT AT MOOREHEAD STREET 29

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31 NE Observed Fault Location Driveway SW Clay Sand Clay Resistivity imaging section across the LPF. 31

32 Down UP 32

33 A mansion is being built few houses west of Long Point Fault! 33

34 HWY. 249 Resistivity Resistivity Line Line 34

35 A View of Tomball Fault at Demolished Beckondorf Middle School-View to East RL UP Down 35

36 Major cracks and subsidence School West Entrance N 125 Ft Fault Zone S Sand Caliche Resistivity imaging data across the Tomball Fault 36

37 View looking SW A BLIND TEST OF RESISTIVITY SURVEY OVER PEARLAND FAULT IN SE OF HOUSTON 37

38 SW Borehole locations FAULT ZONE NE Sand Clay Silty Clay Clay Resistivity Line 1 SW 10 feet spacing NE Sand Clay Silty Clay Clay Resistivity Line 2 Resistivity imaging data across Pearland Fault 38

39 Gamma Ray data overlaid on the resistivity profiles 39

40 Foundation problems explored via GPR in one of these houses in Spring, Texas 40

41 N EX-1 EX-2 EX Decayed wood floor in the living room! 41

42 EX-1 N EXCAVATIONS EX-2 EX-3 Living room with wood flooring 3D GPR survey line Concrete Patio GPR SURVEY DESIGN F/P Post-tension cables embedded in the concrete foundation 42

43 N DAISY GPR survey with 1500 MHz antenna 43

44 N High Amplitude Scale Low 3D GPR DATA FROM 1500 MHz ANTENNA 44

45 GPR SURVEY CART WITH 400 MHz Antenna 45 N

46 N Amplitude Scale EX-1 High EX-2 EX3 Excavated Void Figure 6 Low 3D GPR DATA for 400 MHz Antenna: 3 ft depth slice 46

47 N EX-1 High EX-2 EX-3 Low Amplitude Scale 3D GPR Data for antenna 400 MHz: 5 Ft depth slice 47

48 EX-3 Void 48

49 October 8, 2008 October 28, 2008 Water depth is one inch Water depth is 4 inch Excavated void location based on the GPR data 49

50 2008 Annual Precipitation at Northwest Houston. Houston: Bush Intercontinental Airport Average Temp Departure Precipitation Departure January " +0.94" February " +1.02" March " -0.95" April " -2.14" May " -0.58" June " -3.29" July " -2.09" August " +3.62" September " +7.74" October " +4.17" November " -1.27" December " -2.01" ANNUAL " +5.16" 7.74 INCH 4.17 INCH 50

51 FRENCH DRAINAGE INSTALLATION 51

52 Foundation problems few miles away from The Alamo, San Antonio 52

53 Foundation Case Study GPR Unit View to the NE 53

54 R1 Geophysical Profiles Around the Building N R2 Air-conditioning unit BUILDING X R3 R4 STAIRS R5 54

55 ACU GPR surveys around the building: Note the significant cracks 55

56 Note the significant cracks Stairs 56

57 N Ft Ft Poor quality GPR data due to the high conductivity of the soil S Two GPR Profiles Ft Poor quality GPR data due to the high conductivity of the soil 57

58 Micro-resistivity survey along the building 58

59 NW Corner Air-Conditioner SW Corner Void Line R1 Conductivity data-converted from the resistivity data 59

60 Resistivity survey along Line R3 60

61 W SW Corner Next to stairs E Clay Subsidence Clay Sand Conductivity profile along R3 61

62 Line 5 A view to the NW 62

63 SE Corner B-2 NE Corner Sand Clay Sand Sharp Contact- Anomaly? Conductivity profile along R5 63

64 RESULTS OF THE FOUNDATION STUDY Using micro-resistivity surveys, we not only located a number of areas of granular soil where substantial amounts of moisture have accumulated. The resistivity data also indicated the presence of an abandoned sewer main under the building that was not known to be present at the time, but has been verified on old base maps. GPR results did not yield any information. 64

65 D U USGS Well Where is all the water coming from into the Barton Springs Pool?! 4th largest spring system in Texas Water temperature: 68 F (22 C) Mean discharge: 53 cfs (1,500 lps) (105 acre-feet/day) 65

66 Geological Map of Barton Springs Pool U D Eliza A Parthenia Zenobia B 66

67 Dye tracing as far as 13 miles away 67

68 In the 1830s, William Barton owned the land, which included the three springs to which he gave his three daughters names: Parthenia, Eliza and Zenobia. 68

69 Natural-Potential Method (NP) Utilizes the earth s naturalelectric field at the ground surface to detect and map groundwater pathways and geologic features, such as faults, fractures, conduits, caves Movement of water Target: Seepage, Voids 69

70 mv 70

71 Locations of NP and Resistivity Profiles at Barton Springs Eliza N1 Old Mill NP NP and Resistivity Springs Fault U D L2 L3 L4 L5 L6 L7 L8 L9 L10 L11 71

72 Endangered Salamanders of Barton Springs 72

73 NP Data at the Barton Springs Pool mv W South Bank Positive NP Anomaly E mv North Bank Water main hole Feet Fault 73

74 NP Profiles Over Aerial View of Pool N N1 Eliza Main Barton Old Mill NP profile data Springs Fault Flow-path Gate to the Pool 74

75 Pecan Trees L4 L3 L2 L6 Star Kitchen Tent L5 L4 50-Ft 75

76 W South Gate Line 2-Resistivity Data E Clay-filled Cave? Clay layers Resistivity Data mv Line 2-NP Data NP anomaly Feet 76

77 W E LINE 2 LINE 3 Significant fracture or fault LINE 4 LINE 5 77

78 mv NP Data along Line 8 on a dry day NP Data Line 8 after a big storm Correlation of an NP anomaly taken along Line 8 on a dry day and on a day after a stormy rain 78

79 Extra Resistivity Line 79

80 Resistivity Imaging Data West South Gate P E C A N T R E E S East 80

81 3D Resistivity Data in the E-W Direction (view from north to south) N South Gate 81

82 Concluding Remarks We think we located number of conduits and caves in the Barton Springs area; We also think that we located a significant fracture or fault that appears to control the flow of the water into the pool. Now, it is pool time! 82

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84 FRONT YARD GEOPHYSICS IN AUSTIN 84

85 Fixed Sinkholes R2 NP line Resistivity line R1 Road patches 1 Location of Geophysical Lines 85

86 Natural Potential Survey Design NP Data Point

87 Location of Resistivity Line R1 87

88 W RES ISTIVITY PROFILE R1 E mv NP DATA Both data sets indicate no anomaly! FEET 88

89 N RESISTIVITY PROFILE R2 S mv NP DATA Both data sets indicate significant anomalies! Feet 89

90 Q: Do karstic features represent a hazard to the Austin Community? Q: What do you think? 90

91 Q: Or do we represent a hazard to the Karstic Community? Taken from 2010 NSS Journal with permission from Dr. George Veni 91

92 Final Remarks-1 EGA A key factor conducting a successful survey is to select the proper geophysical methods for the site conditions. There is no one geophysical tool that can be considered the proverbial silver bullet which can uniquely locate, delineate, and characterize all hazards.. 92

93 Final Remarks-2 EGA But clearly geophysics can play a role in advance in reducing the risk many hazards present to public safety and health. Properly planned and executed, a geophysical survey can provide a wealth of information cost effectively that could NOT be obtained otherwise, such as drilling and/or surface sampling. 93

94 EGA Let s remember: Geophysics is just another 94