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Geophysical Methods Applied For Cost Effective Site AssessmentsRaye Lahti, P.G. Associate - GeophysicistMay 2, 2013
Geophysical Methods Applied For Cost Effective Site Assessments
Non-intrusive geophysical methods
Uses in near surface oilfield investigations
Method Descriptions
Logistics, Survey, Advantages-Limitations
Geophysical Applications Legacy Site Characterization Brine Contamination
– Delineation
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EM31 Conductivity Survey USFWS – 2012 Sheridan Co. MT
Geophysical Methods Applied For Cost Effective Site Assessments
Benefits: detect subsurface features and unknowns Minimize equipment mobilization and
intrusive investigation cost Proactive measures supporting
environmental compliance is used as initial screening tool prior
to mobilizing or sending other equipment. Detect Underground Storage Tanks,
oilfield piping and pipelines, Utilities and delineation of brine spills/leaks
Can pinpoint piping and brine plumes is a fraction of the cost of other methods
(drilling, monitor wells, backhoe excavations)
Highly beneficial to site investigations
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Geophysical Methods Applied For Cost Effective Site Assessments
Focus: Electromagnetic Terrain Conductivity
Various Instruments, techniques and appplications
Electromagnetic (EM) Terrain Conductivity EM31, EM34, EM38 15, 25-200, 5 ft. ( 6, 8-60, 1.5 m)
Time Domain Electromagnetic Soundings 25 - >200 ft (8 - >60m) EM47
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Geophysical Methods Applied For Cost Effective Site Assessments
Electromagnetic Terrain Conductivity
Frequency Domain Systems
Transmitter (Tx)-Receiver (Rx)
Known Frequency
Direct Reading
Apparent Conductivity (mS/m)
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EM34 EM34 (Tx) EM31 EM38
Geophysical Methods Applied For Cost Effective Site Assessments
Time Domain Electromagnetic Soundings Transmitting time varying EM field. Data
received at multiple times after turn off Result is an apparent resistivity curve Computer modeling conducted Results in layered earth
Interpretation
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Transmitter
Receiver
Apparent Resistivity
Curve Model
Transmitter Wire
Geophysical Methods Applied For Cost Effective Site Assessments
Logistical Considerations Terrain, Vegetation, Geology, Power lines, Fences,
Piping, Access. (All may affect speed, accuracy and success)
These features must be addressed to assure the correct instrumentation is selected.
Survey Lay-Out EM Conductivity Surveys usually set out in grid with line
and station spacing defined by expected anomaly extent
TDEM Soundings laid out generally in a square with increasing size dependent upon desired exploration depth
Placement of survey - Previous Data– Well Data– EM Data– Drawings and property owner information
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Building Interference
Fencing Interference
0 FT 50 FT 100 FT
ONE INCH = 50 FEET
EM-31 Inphase Response Contour MapVertical Dipole
Mosaic Crop Nutrition - Area AMelbourne, Campbell County, KY
June 2007
Geophysical Methods Applied For Cost Effective Site Assessments
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NW North Dakota – EM Conductivity
Geophysical Methods Applied For Cost Effective Site Assessments
Glacial Overburden
Various Oilfield Infrastructure
1989 EM31Survey at reclaimed site
Discovered Pipeline Leak 2006
Additional and previous area surveyed in 2009
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1989 EM31 Survey
NW North Dakota (Goose Lake Study Area) EM Conductivity Example
Geophysical Methods Applied For Cost Effective Site Assessments
Follow on EM34 survey to obtain depth information
Exploration depths to 50 and 100 ft. (~15 and 30 m)
Brine contamination confined primarily to glacial overburden, but contamination is migrating and still present from 1989
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NW North Dakota (Goose Lake Study Area) EM Conductivity Example (USFWS and USGS Surveys)
Geophysical Methods Applied For Cost Effective Site Assessments
Legacy Site Decomissioning
5 ft. Exploration Depth – High Resolution
Objective: Accurate mapping of unknown subsurface piping and high salinity soils
Twenty Acre Site
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3-D Image of DataSite MapEM38 Conductivity
EM38
Geophysical Methods Applied For Cost Effective Site Assessments
Legacy Site Decommissioning
Use of near surface geophysics can minimize the unknowns
Identifying site infrastructure can save time and reduce damage to existing features
Twenty Acre Site
1-3 day survey
Two person crew
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3-D Image of DataSite Map
EM38
Geophysical Methods Applied For Cost Effective Site Assessments
Leaking Above Ground Brine Tanks Objective: Determine Vertical
and Lateral Extent of Brine Plume
Integrated non intrusive surveys Agricultural Area, Irrigation well EM34 – 50 ft. Exploration Depth Time Domain EM Soundings >100 ft. Depth of exploration Obtain data to provide both
vertical and lateral model
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IrrigationWell
Geophysical Methods Applied For Cost Effective Site Assessments
Leaking Above Ground Brine Tanks EM34 Terrain Conductivity Well Data Chloride Data Data from upper 50 ft.
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IrrigationWell
Shallow Well Data
EM34 Data
BorrowQuary
Geophysical Methods Applied For Cost Effective Site Assessments
Leaking Above Ground Brine Tanks Time Domain EM Soundings (~70) EM34 Terrain Conductivity –
Determined Lateral Extent Determined Vertical and Lateral
Extent (example 2-D Cross Section Shown)
Comparison with Well Data provided Ground Truth (shallow and deep)
100+ Acre Site
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Measured Chloride In Deep Wells
Geoelectric Cross Section (TDEM)3-D Image of Brine Zone
Geophysical Methods Applied For Cost Effective Site Assessments
Summary Electromagnetic Surveys provide a cost effective,
non –intrusive means of assessing sites in a time and cost effective means
Provide valuable information to limit the intrusive investigation by providing information for well placement and planning of future impacts and costs
Minimize potential damage to existing infrastructure during well placement, installation of reserve pits, piping, etc.
The user must be aware of advantages and limitations Simply deployed with 2 person crew Near real time results But, may be limited by site conditions
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Geophysical Methods Applied For Cost Effective Site Assessments
Questions and Answers
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Raye Lahti, P. G. Associate –Senior [email protected]