Glenn S. Sasagawa

A portable iodine stabilized He-Ne laser and its use in an absolute gravimeter

A portable, robust, and easy to use iodine-stabilized laser operating at a wavelength of 633 nm was realized at the BIPM. The size of the laser and its electronic servo are such that they can be installed in a suitcase having dimensions acceptable in an aircraft cabin as hand baggage. The performance of this laser is comparable to that of stationary lasers. A frequency repeatability of a few parts in 10/sup 11/ and a frequency stability that can reach 2 parts in 10/sup 13/ on a 300-s sample time are achieved. The radiation emitted by the laser can be used as a reference for optical interferometry for several years without the need for periodic calibration and its life is limited by that of the laser tube. A prototype was installed in an absolute gravimeter and continuous sets of g measurements extending over more than 30-h periods were achieved with standard deviations on the average value of g of about 2.5 parts in 10/sup 8/. >

Absolute gravity measurements in California, 1984–1989

Repeated absolute gravity measurements have been made at 12 sites in California between 1984.3 and 1989.7. As determined in laboratory tests, the instrument used has an estimated accuracy of 10 μGal (approximately 10−8 g). The repeatability of the measurements is consistent with this accuracy assessment. No gravity changes above the limits set by instrumental uncertainty and environmental noise are observed in California during this period; the field observations provide upper limits on the rates of secular gravity changes which could be attributed to crustal deformation with a resolution corresponding to vertical displacement rates of 1–2 cm/yr.

Short- and Long-Term Stability of the JILAG-4 Absolute Gravimeter

Variations of absolute gravity measured with the JILAG-4 absolute gravimeter at intervals ranging from 2 hours to 5 years are reviewed to ascertain short- and long-term instrument stability. We find that the standard deviation of the twenty-four 2-hourly drop set means taken during a given station occupation is 1-2/xGal when natural or man-induced microseismic conditions are low and 3-5/xGal when the microseismic activity is high. The standard deviations of the station gravity values obtained by repeated occupations weeks or years apart are within these same ranges, with lower standard deviations found again at bedrock sites where the microseismic noise is low. Based on the repeatability of observations since the beginning of the measurement program in 1987, there is no indicate!on for drift, gradual deterioration, or aging of the instrument. However, because of the degraded performance of the lasers used since 1990, the standard deviation of repeated station occupations increased from 2.27 to 2.87/xGal, and data had to be rejected at several sites. Individual station gravity Values in excess of +_3/xGal from the station mean are found mostly at those sites where density Variations between reoccupations are expected on the basis of geological conditions, usually due to groundwater table fluctuations and/or soil moisture changes.

Data from: Drift-Corrected Seafloor Vertical Deformation at Axial Seamount 2013-2014

This collection contains data from a Self-Calibrating Pressure Recorder (SCPR) deployed at Axial Seamount from 2013 September to 2015 August. The data include seafloor height change (cm) that have been converted from seafloor pressure (kPa). Corrections of ocean tides and pressure gauge drift have been applied to the data.

A SEA FLOOR GRAVITY SURVEY OF THE SLEIPNER FIELD TO MONITOR CO2 MIGRATION

Since 1996, excess CO{sub 2} from the Sleipner natural gas field has been sequestered and injected underground into a porous saline aquifer 1000 m below the seafloor. In 2002, we carried out a high precision micro-gravity survey on the seafloor in order to monitor the injected CO{sub 2}. A repeatability of 5 {micro}Gal in the station averages was observed. This is considerably better than pre-survey expectations. These data will serve as the baseline for time-lapse gravity monitoring of the Sleipner CO{sub 2} injection site. A repeat survey has been scheduled for the summer of 2005. This report covers 3/18/04 to 9/19/04. During this time, we participated in several CO{sub 2} sequestration-related meetings and conferences. On March 29, 2004, we participated in the 2004 Carbon Sequestration Project Review Meeting for the Department of Energy in Pittsburgh, PA. During the week of May 2, 2004, we attended and presented at the Third Annual Conference on Carbon Capture and Sequestration in Alexandria, VA. Finally, during the week of August 8, 2004, we took part in the U.S.-Norway, CO{sub 2} Summer School in Santa Fe, NM. Additional modeling was also completed, examining the seismic velocity pushdown estimates from the gravity models and the expected deformation of the seafloor due to the injected CO{sub 2}.