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.

An E-W gravity profile across the La Bajada fault Zone in the Rio Grande Rift, North Central New Mexico

Rajesh GotetiUniversity of Rochester

SAGE 2007

OutlineOutline

IntroductionIntroduction

Gravity SurveyGravity Survey

Data Analysis and InterpretationData Analysis and Interpretation

ConclusionsConclusions

Introduction

La BajadaFault Zone

Profile Line

Lacoste Romberg analog gravity

meterScintrex digital gravity meter

Equipment

Data Reduction Tidal and Drift Correction

Latitude Correction

Free-air Correction

Bouguer correction

Terrain Correction

Drift & Tidal Corrections

Inner Zone Terrain Correction

Complete Bouguer Anomaly

Contouring the Anomaly

Gravity Method Flowchart

La Bajada Fault Zone

Complete Bouguer Anomaly Map on the geologic map

∆ρ = 0.47 g/cc

C. I. = 1 mGal

Steep gradient across the anomaly contours coincideswith the location of mappedfault zone trace

0 24 km

Complete Bouguer Anomaly Map

C. I. = 1 m. Gal

∆ρ = 0.47 gm/cc

High Low

Possible Location of the La Bajada Fault Zone

+ Gravity Stations

SFFLBF

TF

LBF

High Low

Differences Between CBA and RBA

SFF

Regional Aeromagnetic Survey Map

Gravity Profile Line

nT

High

Low

Steep gradient supports the possibilityfor a fault zone

La BajadaFault Zone

Profile Line

Geological Map showing the location of the profile

in the gravity survey

Residual Gravity Anomaly Profile

Easting (m)

RA(mGal)

So what do we have so far ?

Complete and Residual Bouguer Anomalies maps tohelp identify the La Bajada fault surface trace

Aeromagnetic map which predicts a fault trace that agreeswith the fault trace above

Residual Gravity anomaly profile for the line of interest Model Density contrast of 0.47 gm/cc

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)

TransoceanSeismic Line78-7 SP 90

TransoceanSeismic Line79-1 SP 70

N

12 km

2-D Forward Model using GM-SYS

TO lines

Model DerivedObserved

Misfit

78/7Sp 90

79/1Sp 70ρ = 2.2 gm/cc

ρ = 2.67 gm/cc“Basement”

3.6 Km

1.4 Km

0.5X

(km)

TO lines

Model DerivedObserved

Misfit

78/7Sp 90

79/1Sp 70ρ = 2.2 gm/cc

ρ = 2.67 gm/cc “Basement”

3.8 Km1.4 Km

0.5X

ρ = 2.4 gm/cc

ρ = 2.67 gm/cc “Basement”

Non- uniqueness

Conclusions

Model DerivedObserved

Misfit

78/7Sp 90

79/1Sp 70ρ = 2.2 gm/cc

ρ = 2.67 gm/cc“Basement”

3.6 Km

1.4 Km

0.5X

Conclusions (contd..)

Gravity surveys can be used to locate faults (e.g. La BajadaFault) and estimate approximate depths of layers based ondensity contrast.

Gravity surveying is a relatively inexpensive, fast techniquefor a first order insight into the subsurface geology.

Subsurface models based on gravity cannot yield uniquesolutions. Other geophysical techniques (e.g., seismics) cancomplement and constrain the gravity model better.

Acknowledgements

-Shawn, George, Darcy

-Scott for the wonderful field trips

-Team members

-SAGE faculty/staff for giving this enrichingexperience

Group Conclusions

1D Forward Model1D Forward Model

101

102

103

Depth(m)

101 102100

Rho(ohm m)

App Rho(ohm m)

Phase

Period(s)

45o

10-4 10-3 10-2

90o

0o

101

102

100

1D Occam Inversion, 8 layers forward modelStation 1 Station 2 Station 3

104

103

102

101

100

101 102

(m)

104

103

102

101

100

101 102

(m)

104

103

102

101

100

101 102

(m)

Gravity modeling can be used to locate faults and othergeophysical techniques can be used to constrain a gravitymodel and generate a reasonable subsurface geologicalinterpretation

Seismic reflection shows no evidence of Tanos fault. Creating a synthetic seismogram for Hawley’s geologicalcross-section may hint about the signatures of Tanos fault

The shallow depths along the survey line modeled withrefraction seismics does not reveal any faults

Thanks!!!!