Using Gravity Modeling to Using Gravity Modeling to Understand the Subsurface Geology of Understand the Subsurface Geology of

the La Bajada Fault Zonethe La Bajada Fault Zone

Hussam BusfarHussam BusfarThe University of Texas at AustinThe University of Texas at Austin

SAGE 2004SAGE 2004

ObjectivesObjectives Learn about gravity reduction and modeling process.Learn about gravity reduction and modeling process. Investigate the subsurface geology:Investigate the subsurface geology:

Locate Fault(s)Locate Fault(s) Detect density contrastsDetect density contrasts Depth of unitsDepth of units StructureStructure

Speculations and future improvement of data.Speculations and future improvement of data.

Data CollectionData Collection Instruments used:Instruments used:

La Coste – Romberg analog gravity meterLa Coste – Romberg analog gravity meter Scintrix Autograv meter.Scintrix Autograv meter. Leica real-time differential GPS system: Leica real-time differential GPS system:

(elevation-horizontal coordinates)(elevation-horizontal coordinates) 1 ft 1 ft ≈ 0.3 m ≈ 0.06 mGal≈ 0.3 m ≈ 0.06 mGal

Data were collected by current/previous Data were collected by current/previous SAGE students/faculty, USGS, and oil SAGE students/faculty, USGS, and oil companies.companies.

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

profile.profile.

ProcessingProcessing the datathe data o The gravity data colleted in the field is influenced by The gravity data colleted in the field is influenced by

many factors.many factors.o Raw data must be reducedRaw data must be reducedo The corrections are:The corrections are:

• Instrumental (meter) driftInstrumental (meter) drift• Tidal effectTidal effect• Latitude (pole ≈ 9.83 m/sLatitude (pole ≈ 9.83 m/s22, equator ≈ 9.78 m/s, equator ≈ 9.78 m/s22) ) • Free airFree air• BouguerBouguer• TerrainTerrain For Bouguer/Terrain correction we assumed a density of 2.2g/cc and For Bouguer/Terrain correction we assumed a density of 2.2g/cc and

2.67g/cc for elevations from 0-2 km and >2 km respectively. 2.67g/cc for elevations from 0-2 km and >2 km respectively.

Complete Bouguer Anomaly with Complete Bouguer Anomaly with Overlain Geology of Study AreaOverlain Geology of Study Area

Inverse ModelInverse ModelResidual = Complete Bouguer Residual = Complete Bouguer – Regional anomaly– Regional anomaly

3 density contrasts are plotted 3 density contrasts are plotted to fit the residual anomaly curveto fit the residual anomaly curve

-0.35g/cc-0.35g/cc

-0.45g/cc-0.45g/cc

-0.55g/cc-0.55g/ccEach density contrast produces Each density contrast produces a different depth of sediments a different depth of sediments modelmodel

Fault dip >= 60°Fault dip >= 60°

Fault ≈ 17 km from WFault ≈ 17 km from W

Sediment thickness over fault? Sediment thickness over fault?

W E

regional

Bouguer

residual

La Bajada fault

Forward Model Talwani ProgramTalwani Program

Depth to sediment Depth to sediment model from the inverse model from the inverse model is used to produce model is used to produce the forward model.the forward model. Trial and error/horror!Trial and error/horror!

Densities assumed Densities assumed from geology.from geology.Fault:Fault:• dip >= 60dip >= 60• ≈ ≈ 16 km from W16 km from W• depth ≈ 1kmdepth ≈ 1km

W E

Another Fault?

Complete Bouguer Anomaly with Complete Bouguer Anomaly with Overlain Geology of Study AreaOverlain Geology of Study Area

FaultsW E

0 2km

Inverse Model ≈ 350m to W≈ 350m to WForward Model ≈ 1.35km to W≈ 1.35km to W

Future Improvement of StudyFuture Improvement of Study

Fill in gravity dataFill in gravity data

Constrain gravity models with well logsConstrain gravity models with well logs

Use other geophysical techniques to aid Use other geophysical techniques to aid in gravity modelingin gravity modeling

ConclusionConclusion Gravity technique is relatively less Gravity technique is relatively less

expensive and fast.expensive and fast. Gives us some idea of the subsurface Gives us some idea of the subsurface

geology.geology. Gravity technique by itself gives us non-Gravity technique by itself gives us non-

unique solutions unique solutions Much more useful if coupled with other Much more useful if coupled with other

geophysical techniquesgeophysical techniques