An EW gravity profile across the La Bajada fault

1
An E-W gravity profile across the La Bajada fault
Zone in the Rio Grande Rift, North Central New
Mexico
Rajesh Goteti University of Rochester SAGE 2007
2
Outline

• Introduction
• Gravity Survey
• Data Analysis and Interpretation
• Conclusions

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Introduction
Profile Line
La Bajada Fault Zone
4
Equipment
Lacoste Romberg analog gravity meter
Scintrex digital gravity meter
5
Data Reduction

• Tidal and Drift Correction
• Latitude Correction
• Free-air Correction
• Bouguer correction
• Terrain Correction

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Gravity Method Flowchart
Drift Tidal Corrections Inner Zone Terrain
Correction Complete Bouguer Anomaly Contouri
ng the Anomaly
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Complete Bouguer Anomaly Map on the geologic map
?? 0.47 g/cc C. I. 1 mGal
La Bajada Fault Zone

• Steep gradient across the
• anomaly contours coincides
• with the location of mapped
• fault zone trace

24 km
0
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Complete Bouguer Anomaly Map C. I. 1 m. Gal
?? 0.47 gm/cc
Possible Location of the La Bajada Fault Zone
Gravity Stations
High
Low
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Differences Between CBA and RBA
SFF
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Regional Aeromagnetic Survey Map
Gravity Profile Line

• Steep gradient
• supports the possibility
• for a fault zone

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Geological Map showing the location of the profile
in the gravity survey
Profile Line
La Bajada Fault Zone
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Residual Gravity Anomaly Profile
RA (mGal)
Easting (m)
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• So what do we have so far ?
• Complete and Residual Bouguer Anomalies maps to
• help identify the La Bajada fault surface trace
• Aeromagnetic map which predicts a fault trace
  that agrees
• with the fault trace above
• Residual Gravity anomaly profile for the line of
  interest
• Model Density contrast of 0.47 gm/cc

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• In addition
• Transocean Oil Company seismic lines to (1)
  constrain the depth to
• basement in both the footwall and hanging wall
  of the La Bajada fault
• and (2) throw on both the fault segments
• Velocity estimates from Baldridge et al (1994)

N
Transocean Seismic Line 79-1 SP 70
Transocean Seismic Line 78-7 SP 90
12 km
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2-D Forward Model using GM-SYS
(km)
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Non- uniqueness
? 2.4 gm/cc
Basement
? 2.67 gm/cc
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Conclusions
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• Conclusions (contd..)
• Gravity surveys can be used to locate faults
  (e.g. La Bajada
• Fault) and estimate approximate depths of layers
  based on
• density contrast.
• Gravity surveying is a relatively inexpensive,
  fast technique
• for a first order insight into the subsurface
  geology.
• Subsurface models based on gravity cannot yield
  unique
• solutions. Other geophysical techniques (e.g.,
  seismics) can
• complement and constrain the gravity model
  better.

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• Acknowledgements
• Shawn, George, Darcy
• Scott for the wonderful field trips
• Team members
• SAGE faculty/staff for giving this enriching
• experience

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Group Conclusions
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1D Forward Model
Depth(m)
App Rho(ohm m)
102
101
101
102
100
Phase
103
90o
45o
101
102
100
10-4
10-3
10-2
0o
Rho(ohm m)
Period(s)
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1D Occam Inversion, 8 layers forward model
Station 1
Station 3
Station 2
100
100
100
101
101
101
(m)
(m)
(m)
102
102
102
103
103
103
104
104
104
101
102
101
102
101
102
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• Gravity modeling can be used to locate faults
  and other
• geophysical techniques can be used to constrain a
  gravity
• model and generate a reasonable subsurface
  geological
• interpretation
• Seismic reflection shows no evidence of Tanos
  fault.
• Creating a synthetic seismogram for Hawleys
  geological
• cross-section may hint about the signatures of
  Tanos fault
• The shallow depths along the survey line modeled
  with
• refraction seismics does not reveal any faults
• 
  Thanks!!!!