Kingston Multi-hazard Assessment Gravity Survey
Trip Report

During the week of 8 June 1998, a team composed of researchers from the US National Ocean Survey (Miranda Chin, Doug Martin and Bob Moose), the Earthquake Unit of the University of the West Indies Mona Campus (Margaret Grandison), the Jamaica Mining and Geology Division (Norman Harris) and the CDMP (Stuart Nishenko and Bill McCann) undertook a Gravity/GPS survey of the Kingston Metropolitan Area. The following trip report describes the background, schedule and the initial results of this survey. See the Kingston Multi-hazard Assessment progress bulletin for more information on this activity. Figures 1-6 are available in a separate document to speed the downloading of this page. Each figure is approximately 50k for a total download of 300k to view the maps/picture page. The US Geological Survey has developed a description of gravity assessment, which is available from the USGS website (Adobe Acrobat format, 64k).

Trip Report

The need for a comprehensive map of the depth to bedrock in the Kingston metropolitan area was discussed at the February 6, 1998 Phase II planning meeting. Young alluvial deposits can selectively amplify strong ground shaking during earthquakes, with the period and degree of amplification being proportional to the thickness of the alluvium.

Phase I investigations of the Kingston Multi-hazard Assessment revealed how little thickness data was available for the Liguanea Plain. While engineering surveys are undertaken for new construction in the Kingston area, the majority of boreholes are only drilled to a depth of 20 meters. Those wells that have penetrated bedrock are few in number and are located in the eastern part of Kingston. Phase I of the Kingston Seismic Hazard Assessment had identified a deep alluvial basin (>120 m) in the Half Way Tree-New Kingston area and recommended additional study.

Subsequent research following the Phase II planning meeting has uncovered additional deep wells in the Kingston area. These wells were drilled during 1978-1979 as exploratory water wells. The majority of the wells were terminated at a depth of 200m (600 feet). Two wells, National Heroes Circle and Liguanea Club, penetrated 700 and 850 feet of Pleistocene sands and gravels. While none of these wells hit bedrock, they do demonstrate that the Liguanea Plain is at least 200 to 260m thick beneath downtown Kingston. A contour map of the minimum thickness of the Liguanea Plain, based on these well data, is shown in Figure 1.

A number of strategies to better map the depth to basement were discussed at the Phase II planning meeting, including well drilling and the use of geophysical exploration techniques (e.g., seismic or gravity profiling). Deep wells are expensive to drill and provide limited, site-specific information. Geotech Exploration Services provided a cost estimate for drilling and sampling a single 900 foot well at $J 1,998,865 (approximately $US 55,000). Paved roads, high noise levels, and urban development pose obstacles to many geophysical exploration techniques, such as seismic reflection/refraction or electrical resistivity methods. In contrast, gravity measurements are relatively unaffected by urban development, and only minimally impact the surrounding environment. In addition, high-precision gravity profiles provide a much greater spatial coverage at a significantly lower cost. Twenty kilometers of gravity coverage cost less than 1/10 of the price of drilling a single deep well.

The National Ocean Survey (NOS) was scheduled to install a sea level/climate monitoring network station in Port Royal during the first part of June 1998 as part of the OAS CPACC project. The availability of both GPS and gravity experts through the NOS project provided the ideal set of circumstances for undertaking a gravity survey in Kingston at that same time.

A survey-planning meeting was held at the Offices of the Jamaican Prime Minister on Monday, June 8, 1998. In addition to the principal investigators, representatives from the Land Surveyors Association of Jamaica, the Jamaica Survey Department, and the Office of Disaster Preparedness and Emergency Management were present. The objectives of the survey were discussed, actual survey routes were mapped out, and tasks assigned. Two profiles or survey routes were selected as part of an initial reconnaissance to demonstrate the feasibility of conducting these types of geophysical measurements in an urban environment.

The gravity / GPS survey consisted of four teams. The first two (lead by Doug Martin and Stu Nishenko) identified and marked the survey location points on June 8 and 9 (see Figure 2). Both routes were situated next to known deep well locations. Information from the well logs (lithology as a function of depth) will be used to calibrate the survey results. The 10-km east-west profile shown in Figure 1 consisted of 25 stations, spaced approximately 400 meters apart, and ran from Papine to Constant Spring Gully along Old Hope Road and Washington Boulevard. The 10-km north-south profile shown in Figure 1 had the same number of stations and spacing, and was run from Constant Spring to Ocean Boulevard at Kingston Harbor along Constant Spring Road, Halfway Tree Road, and Orange Street.

The third team (lead by Miranda Chin) surveyed these locations using the Global Positioning System (see Figure 3). Station locations were surveyed to an accuracy of approximately 5 cm. Finally, the fourth team (lead by Bob Moose) conducted the actual gravity measurements (see Figure 4 and Figure 6). Gravity measurements were made using two LaCoste-Romberg gravimeters (see Figure 5). The use of two instruments allowed an overall precision of less than 15 microgals per observation; one microgal represents less than one millionth of a percent change in the force of gravity.

Several radio stations aired public information broadcasts about the survey. These broadcasts, prepared by Margaret Wiggins-Grandison and Anestoria Shalkowski, explained what the field parties were doing on the streets and how this information was being used to help make Kingston a safer place to live. This message was especially well timed since June is Natural Disaster Awareness Month in Jamaica.

Data collected during the Survey have been reduced at the National Ocean Survey facilities in Silver Spring, MD. Gravity data reduction involved numerical corrections to the individual gravity observations to account for instrument drift, latitude, tidal effects, and the local terrain. GPS data reduction tied the individual measurements to the GPS reference station at Port Royal and the International Terrestrial Reference Frame 1996 coordinate system. The adjusted gravity data will be modeled, using a two-dimensional inversion program and the deep well data for local control, to estimate the thickness of the Liguanea Alluvial Plain. This thickness estimate will help improve the strong ground motion models that will be produced as part of the Kingston Seismic Hazard Assessment.

Acknowledgements

The success of this survey is due to the efforts of many individuals who worked long hours in the hot June sun. In addition to the primary project staff, thanks go to Rollin Alveranga and Jackie DaCosta (Prime Ministers Office) for assistance in setting up the planning meeting and expediting the clearance of the survey equipment through Customs; Leeroy Bulgin and Noel Francis (Jamaica Survey Department) for providing GPS equipment, transportation and personnel; Anestoria Shalkowski (Office of Disaster Preparedness and Emergency Management) for help with public relations; Glendon Newsome, Mr. McCook (Land Surveyors Association of Jamaica), Cynthia Edwards, John Marshall, Donna Scott (Jamaica Survey Department), Nagheib Miller, and Everald Scott for providing field support during the survey. Special thanks to the Kingston Constabulary for accompanying the survey teams through Kingston.