Solutions for a complex world: Gravity Surveys
How low can your basin-fill go? Determining depth-to-bedrock.
Article by: Moira Poje | hydroGEOPHYSICS
Gravity surveys are one of the most widely used techniques to determine the depth to bedrock and structural framework for broad hydrological characterization.
As engineering and consulting professionals, we grapple with the challenges of project design associated with visualizing trends and structures beneath the earth’s surface. Desk-based studies are typically limited, elicited from sparse historical databases and the extent of instrumentation. Invasive drilling programs can be expensive and limited in scope, with frequently no way to optimize placement or quantity of borings required. Geophysical tools, such as the gravity method, can offer an inexpensive way to understand large subsurface areas quantitatively.
Gravity is well suited to hydrologic and hydrogeologic problem-solving as well as the exploration of ore bodies.
The gravity method is one of the most widely used techniques in the industry to determine the depth to bedrock and structural framework for broad hydrological characterization. It is complementary to traditional invasive methods of investigating groundwater and hydrologic resources and can help support the development of a groundwater pumping program.
Gravity surveys are a non-invasive geophysical method that uses a gravimeter to record changes in acceleration due to the earth’s gravitational pull. Gravity measurements across a survey area are compiled to create contoured maps of the earth’s gravity in a given area. Combined with rock density measurements, we can model the depth to bedrock and thickness of overlying sediments. Rock samples are collected at outcrops across the survey area, and density values are calculated from these rocks to confirm model assumptions. Density changes within a basin can correspond to fault zones, and thus gravity measurements are useful in defining the structural framework of a basin. Additionally, changes in density can be related to the edges of buried ore deposits or volcanic flows. For example, note the two examples below.
Residual Bouguer Anomaly Contoured Map
Residual Bouguer Anomaly contoured map of processed gravity data (above) overlain on a raised-relief map of the survey area. Blue shades, for example in the top right, are interpreted as deeper bedrock zones where alluvium thickness is greatest, and red and brown shades indicate shallower bedrock zones. The green shades are areas of more moderate alluvial depths.
Profile of Residual Gravity Data
A profile of residual gravity data showing the slice-derived contour intersections (above) and modeled values in milligals. The lower plot is the modeled cross-section, with blue dotted polygon representing the bedrock and basin-fill represented by the green shaded polygon. The gravity method was used to determine the depth to bedrock in a basin where the gravel fill exceeded expected depths during the drilling campaign.
Gravity is well suited to hydrologic and hydrogeologic problem-solving as well as the exploration of ore bodies. It is a very non-invasive technique with low environmental impact, and measurements can be collected rapidly and efficiently across the survey area. The gravimeter is portable, and data can be acquired by a single field operator, which provides excellent flexibility for updating survey design when anomalies are discovered. HGI has experience using gravity for hydrogeological characterization and mineral exploration. With the implementation of gravity surveying, another tool can be added to your site-characterization toolbox.
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About the Author: Moira Poje | hydroGEOPHYSICS
Moira Poje is a Staff Geoscientist with hydroGEOPHYSICS, Inc. She works on contaminant delineation, void detection projects for mine reclamation, subsurface fluid flow mapping, and depth-to-bedrock delineation via seismic, gravity, and electrical methods.
Moira has presented at the AGU Annual Meeting, Arizona Hydrological Society Annual Meeting, CUR REU symposium, and published in EEGS’s magazine FastTIMES. Moira serves on the Board of Directors for the Environmental and Engineering Geophysical Society (EEGS). Moira holds a B.A. in Geophysics and Planetary Science from Boston University.
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