assessing recharge characteristics of recharge ponds using time
TRANSCRIPT
ABSTRACT & POWERPOINT PRESENTATION
Assessing Recharge Characteristics of Recharge Ponds Using Time Domain Electromagnetics (TDEM)
Martin Miele, PGp, PG Lead Geophysicist
United Water Conservation District Santa Paula, California
Managed Aquifer Recharge Symposium January 25-26, 2011 Irvine, California Symposium Organizers:
• National Water Research Institute • Orange County Water District • Water Research Foundation
www.nwri-usa.org/rechargesymposium2011.htm
Assessing Percolation and Recharge Characteristics of Recharge Ponds Using Time Domain Electromagnetics (TDEM)
Martin Miele PGP, PG
United Water Conservation District Santa Paula, California 93060
The United Water Conservation District (UWCD) is a public agency within Ventura County, California that is charged with conserving the water of the Santa Clara Rivers and tributaries. UWCD works to manage the surface water and groundwater resources within seven groundwater basins. The groundwater basins are within the more regional Ventura Basin, which is within the Transverse Range physiographic province. The geology of the Transverse Range is primarily elongate east to west trending folds and complex faulting which creates the elongate mountains and valleys that dominate Ventura County. UWCD stores surface water in a surface reservoir, diverts natural and reservoir released water, replenishes groundwater through percolation ponds, and delivers both diverted surface water and pumped groundwater to those areas vulnerable to overdraft and saline intrusion. UWCD has been percolating diverted river water in the vicinity of Saticoy and El Rio spreading grounds since the 1920’s. The percolation of water in the ponds is for the purpose of replenishing (recharge) groundwater supplies to the Upper Aquifer System (UAS) and Lower Aquifer System (LAS) in the Oxnard Forebay (recharge area) and surrounding Oxnard Plain in Ventura County. The Saticoy and El Rio spreading grounds consist of more than 20 ponds that are located in a geologically complex area (several faults/folds in the area). Time‐domain electromagnetics (TDEM) is a geophysical exploration technique used to map subsurface geology and hydrogeologic conditions. TDEM methods are able to determine subsurface electrical properties of earth materials. It is a very common surface EM technique for groundwater exploration/definition, and for subsurface geologic mapping, used throughout the world. Two fundamental electromagnetic principles are required to derive the physics behind TDEM surveys: Faraday's law of induction and Lenz's Law. A loop of wire is energized by a direct current. At some time (t0) the current is cut off as quickly as possible. Faraday's law dictates that a nearly identical current is induced in the subsurface (eddy currents). The net result is a downward and outward diffusion of currents in the subsurface. These currents produce a secondary magnetic field. At the surface, the change in secondary magnetic field with time is measured. The way the currents diffuse in the subsurface is related to the resistivity distribution in the ground which is used to determine the subsurface geology. Resistivity changes are related to grain size, composition, water content, consolidation, aquifer characteristics, fracturing, etc. TDEM measurements were taken within the 20 recharge ponds to assess the general subsurface geology and subsurface conditions of each pond. These data were then compared to the recharge and percolation characteristics of each pond, and well logs in the area. The results are then correlated to the subsurface geology as assessed with TDEM. Since the ponds are in an area of complex geology the results show various characteristics. This data may be used to assess the percolation and recharge characteristics for the development of future recharge ponds. Results suggest the development of additional recharge facilities in the Oxnard Forebay will likely benefit the UAS more than the LAS.
Assessing Recharge Characteristics Assessing Recharge Characteristics of Recharge Ponds Using Time of Recharge Ponds Using Time
Domain Electromagnetics (TDEM)Domain Electromagnetics (TDEM)
Martin Miele PGP, PG
United Water Conservation District
Santa Paula, California
PresentationPresentation
• Overview of United Water Conservation District (UWCD) recharge basin facilities and
functions.
• Discuss Time Domain Electromagnetic Induction theory and methodology (TDEM or TEM).
• Discuss and present results of TDEM surveys regarding subsurface
conditions, characteristics
of recharge ponds and percolation.
This project was a test reconnaissance project. It was conducted during a pause/break from a
much larger TDEM project for saline water intrusion along the coast. A high resolution
geophysical survey is planned for this same overall recharge pond area this summer.
UWCD Boundaries, Basins, and UWCD Boundaries, Basins, and FacilitiesFacilities
RechargeBasins
Forebay to Oxnard Plain Forebay to Oxnard Plain Basin CrossBasin Cross‐‐SectionSection
Saticoy and Noble recharge basins
SW
NE
NW SE
The Facility The Facility ––
Freeman Diversion to Freeman Diversion to Recharge PondsRecharge Ponds
Saticoy and Noble Recharge PondsSaticoy and Noble Recharge Ponds
Recharge CycleRecharge Cycle
• Basins filled and recharge rate decays exponentially (clogging
and mounding
factors).
• Drying period can significantly rejuvenate infiltration rates (desiccation of clays and
dispersion of groundwater mounds).
• Additional rejuvenation through discing, ripping, or scraping sediment on pond
bottom restores higher recharge rates.
TDEM Measures ResistivityTDEM Measures Resistivity
Resistivity ‐
The property of a material which resists the flow of electrical current (reciprocal of conductivity)
For typical aquifer applications resistivitymeasurements for a given formation aredependent upon:
• grain size• composition• degree of compaction or lithification• porosity• water content• water quality
Resistivity Used to Assess Recharge Resistivity Used to Assess Recharge Basin CharacteristicsBasin Characteristics
Assess Overall Geology/Hydrogeology of the Area • Define subsurface conditions related to groundwater (define geology for
determining the efficiency of basin to recharge aquifer)
• Aquifer delineation and groundwater storage characteristics (Managed Aquifer
Recharge)
Assess Surface/Shallow Subsurface Materials (typically sediments)• Sandy/gravelly materials – high resistivity – high permeability and good
percolation characteristics
• Clay/silt – low resistivity – low permeability and poor percolation characteristics
Time Domain Electromagnetic Time Domain Electromagnetic Induction (TDEM)Induction (TDEM)
• Square transmitter loop laid out on land surface (typically 20 m to 200 m per side).
• Square wave current signal of several amps (3 to 18 amps) applied.
• Current is cut off nearly instantaneously for each square wave (ramp time). During this time the
primary magnetic field is time variant.
• In accordance to Faraday’s Law the time variant nature of the primary magnetic field induces a
secondary current in the ground beneath the loop. It is an image of the transmitter loop.
• The secondary current begins to decay. The decaying current similarly induces more current to flow
(eddy currents) that travel downward and outward into the subsurface like a series of smoke rings.
The eddy currents have a associated secondary magnetic field.
Time Domain Electromagnetic Time Domain Electromagnetic Induction (TDEM)Induction (TDEM)
• During time off periods, the receiver measures the decay of the secondary magnetic field from
eddy currents over time.
• To reduce background noise and increase depth larger currents and larger transmitter loops are
used (multiple wire turns can be used).
• Data is converted into apparent resistivity and is computer modeled into layered system.
TDEM Transmission and TDEM Transmission and MeasurementsMeasurements
Transmitted Signal
Eddy Currents
TDEM Field SetupTDEM Field Setup
TDEM Data CharacteristicsTDEM Data Characteristics
• Each “sounding”
(measuring location) consists of various frequency sweeps
• High frequencies represent the shallower materials and low frequencies represent deeper materials
• Forward and Inversion model each sounding (match observed and calculated data)
• Each model consists of several layers, each with a different true resistivity.
TDEM ModelingTDEM Modeling
Saticoy and Noble Recharge Basin Saticoy and Noble Recharge Basin SurveysSurveys
• Conducted one sounding for each accessible pond (Ponds were mostly
dry).
• Survey was conducted during a pause from a much larger TDEM survey for
saline water intrusion. More of a reconnaissance survey of ponds.
• Protem
47 (Geonics
LTD) – Higher Frequency System– Repetition Rates (frequency sweeps) 285 Hz, 75 Hz, 30 Hz
– Investigation depths from 2 to 200 meters (650 feet)
– Used for Upper Aquifer System
– Used 60 meter (200 feet) transmitter loop (battery)
TDEM Sounding Locations with TDEM Sounding Locations with Saticoy and Noble Recharge BasinsSaticoy and Noble Recharge Basins
Geophysical Geophysical CrossCross‐‐Section ASection A‐‐AA’’
Noble Basins
Geophysical Geophysical CrossCross‐‐Section BSection B‐‐BB’’
Noble Basins
Geophysical Geophysical CrossCross‐‐Section CSection C‐‐CC’’
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Saticoy Basins
Geophysical Geophysical CrossCross‐‐Section DSection D‐‐DD’’
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Saticoy Basins
ForebayForebay
FaultsFaults
Percolation DataPercolation Data
4.85 3.033.103.55
Percolation Values (feet/day) Per Basin(Cross‐Section C‐C’) Saticoy
S N
Percolation DataPercolation Data
S N
1.530.681.21
Percolation Values (feet/day) Per Basin(Cross‐Section A‐A’) Noble
TDEM Used to Assess Recharge Basin TDEM Used to Assess Recharge Basin CharacteristicsCharacteristics
Assess Overall Geology/Hydrogeology of the Area • Define subsurface conditions related to groundwater (define geology for
determining the efficiency of basin to recharge aquifer)
• Aquifer delineation and groundwater storage characteristics (Managed Aquifer
Recharge)
Assess Surface/Shallow Subsurface Materials (typically sediments)• Sandy/gravelly materials – high resistivity – high permeability and good
percolation characteristics
• Clay/silt – low resistivity – low permeability and poor percolation characteristics
The EndThe End
Questions?Questions?