Using Gravity Modeling to Understand the Subsurface Geology of the La Bajada Fault Zone

1
Using Gravity Modeling to Understand the
Subsurface Geology of the La Bajada Fault Zone

• Hussam Busfar
• The University of Texas at Austin
• SAGE 2004

2
Objectives

• Learn about gravity reduction and modeling
  process.
• Investigate the subsurface geology
• Locate Fault(s)
• Detect density contrasts
• Depth of units
• Structure
• Speculations and future improvement of data.

3
Data Collection

• Instruments used
• La Coste Romberg analog gravity meter
• Scintrix Autograv meter.
• Leica real-time differential GPS system
  (elevation-horizontal coordinates)
• 1 ft 0.3 m 0.06 mGal
• Data were collected by current/previous SAGE
  students/faculty, USGS, and oil companies.

4
Study Area

• Line of profile is 50 km.
• First the data were corrected then least square
  fit method was used along the profile.

5
Processing the data

• The gravity data colleted in the field is
  influenced by many factors.
• Raw data must be reduced
• The corrections are
• Instrumental (meter) drift
• Tidal effect
• Latitude (pole 9.83 m/s2, equator 9.78 m/s2)
• Free air
• Bouguer
• Terrain
• For Bouguer/Terrain correction we assumed a
  density of 2.2g/cc and 2.67g/cc for elevations
  from 0-2 km and gt2 km respectively.

6
Complete Bouguer Anomaly with Overlain Geology of
Study Area
7
W
E

• Inverse Model
• Residual Complete Bouguer Regional anomaly
• 3 density contrasts are plotted to fit the
  residual anomaly curve
• -0.35g/cc
• -0.45g/cc
• -0.55g/cc
• Each density contrast produces a different depth
  of sediments model
• Fault dip gt 60
• Fault 17 km from W
• Sediment thickness over fault?

regional
Bouguer
residual
8
W
E

• Forward Model
• Talwani Program
• Depth to sediment model from the inverse model is
  used to produce the forward model.
• Trial and error/horror!
• Densities assumed from geology.
• Fault
• dip gt 60
• 16 km from W
• depth 1km

9
Complete Bouguer Anomaly with Overlain Geology of
Study Area
10
Faults
W
E
0
2km
Inverse Model 350m to W
Forward Model 1.35km to W
11
Future Improvement of Study

• Fill in gravity data
• Constrain gravity models with well logs
• Use other geophysical techniques to aid in
  gravity modeling

12
Conclusion

• Gravity technique is relatively less expensive
  and fast.
• Gives us some idea of the subsurface geology.
• Gravity technique by itself gives us non-unique
  solutions
• Much more useful if coupled with other
  geophysical techniques