Geophysical Assessment of Deep Karst in Dam in Florida Using the Multi-Electrode Resistivity Implant Technique (MERIT)

The use of geophysical assessment of dams in deep karst environments can yield valuable information for such complex geology. One of the most used methods in the assessment of dams and karst is electrical resistivity (ER). ER has progressed from 2D resistivity curves to inversion of complex array patterns that produce image. There are however limitations to the current data collection and inversion methodology that prevents higher quality images. These limitations are based on the geometry of array and data collection that results in trapezoidal inversion data set with decreasing data at depth.

This is due to as electrode spacing i.e. 1a, 2a, 3a, etc. increases, the number of measurements decreases for each progressive deeper data level. This is a fundamental limitation of the technique. This results with an inversion of progressively less data the deeper the survey. The bottom third of the surface ER represents only 10.5% of the data for the inversion. The reduced data with depth results in a trapezoidal image that has approximately 40% blank space of the total squared area of the image. In practice this results in the inability of the ER image to collect information on the abutments of dams and has low resolution at depth.

A unique technique the Multi-Electrode Resistivity Implant Technique (MERIT) overcomes some of the fundamental limitations of surface ER. MERIT utilizes a combined surface array and array of small permanent implants driven to depths of up to 50 feet. The tomographic configuration of the surface and deep buried array results in overlapping field density that significant increases data acquisition of up to five times and can increase the depth by third to twice that typical surface ER array. At depth the overlapping fields of MERIT optimized data collection significantly increases the data points in the lower third of the inversion that results high resolution than surface ER. The MERIT image is a full image and as such can be used to image abutments of a dam.

A case study of a dam in Florida where deep buried sinkhole features over 250 feet will be presented. Due to the higher resolution of the MERIT images the location and measurements of the sinkhole throats at depths of 100 feet. The permanent implants were utilized to perform 3D survey of the sinkhole features and were incorporated into the long-term monitoring plan for the site.

About the Speaker: 

Mr. Harro graduated from the University of South Florida in 1992. He is a Professional Geologist and Licensed Drilling Contractor in State of Florida and Tennessee for the G3 Group. He has performed numerous geophysical investigations for engineering, sinkholes and environmental projects over the last 20 years. Mr. Harro is the inventor and patent holder and is responsible for research and development for the Multi-Electrode Resistivity Implant Technique (MERIT). Mr Harro has published several papers on this technique with coauthors Dr M. H. Loke, Dr Sarah Kurse, Dr Henok Kilfu and Dr Paul Wilkinson of the British Geological Survey. Mr Harro over the past 3 years he has also been responsible for research and development of patent pending research project Sinkhole-Alert. Mr. Harro experience in geophysical investigations and drilling has provided him insight into an innovative solution for new geophysical imaging technology and sinkhole detection and is the on the ASTM sub-committee for karst.