GG 450 Spring 2008 Gravity Field Project

Kawainui Swamp

Purpose: Get acquainted with the gravity method of geophysical exploration by surveying, analysis, and interpretation of gravity data obtained around Kawainui Swamp, Oahu.

Background:

• Do a web and library search to obtain references to previous studies of the gravity field around Oahu and Hawaii.

• Obtain maps to permit accurate location and elevations of gravity stations.

• Be sure the gravity meters are “on heat” for at least 24 hours prior to the field survey, and the batteries charged.

• Determine possible path to be taken in the field, including a tie point.

Field Work:

• Obtain GPS units, GPS maps, cameras, field log sheets, cell phone contacts, etc.

• Take base station measurement.

•Proceed to field area and take another base measurement.

• Proceed from station to station taking gravity and GPS readings, and copious notes.

• For this survey, get at least one reading near sea level with both meters at the same time.

Data reduction: (COMPLETE BY LAB TIME, FEB 5)

• Collect all data from both field parties, including calibration constants for each meter.

• Determine whether a drift correction is needed, and correct the dial readings if necessary.

• Correct all data to mgals by applying the meter constant and correlation with the HIG base station value.

• As location data will be obtained in UTM coordinates, you will need to convert to Lat (and Long) to obtain the theoretical gravity.

• Compare values taken by both meters at the same location. What is the precision of the merasurements?

• Using the best possible estimate of elevation, calculate the free air anomaly for all points.

Data analysis: (COMPLETE BY LAB TIME, FEB 12)

• Compare the data collected with the historical data. Contour the FAA in the area of the field study.

• Based on your contour map, choose two or three profiles of interest for further study. Obtain the elevations and gravity values along that profile.

• Input these profiles into GM-Sys and model your data using geological insight and ‘parsimony” to obtain a geologically reasonable model. Use as few model parameters as possible - few layers, few model points. It is not necessary to exactly match every data point. Consider the uncertainty of your measurements.

•Print your results for comparison with others at lab time.

Data Interpretation: (Your final lab report MUST be completed by lab time, Feb 19)

• Using knowledge obtained from your geological background, references to previous work, and modeling results, infer possible structures that could explain the observed anomalies. Qualify your interpretations, and suggest possible next steps to more precisely define these structures.

• Have a written report to hand in (one for each person), prepared as if for a customer of a geophysical company, and prepare a 15-minute oral presentation on your findings. The oral presentation will be done in groups of two or three, but each person must give part of the presentation.