Geophysical Choices for Karst Investigations

Abstract

It is not unusual for a geologist or engineer to be called upon to investigate and address problems related to sinkholes in karst environment. Sinkholes in a karst environment can be a dream, or nightmare. Most professionals know that geophysics can help, but the geophysical method and parameters that may be best suited for the problem at hand are a less certain choice. Geophysical technology and standards have evolved tremendously over the last 10-years. This paper briefly examines geophysical standards that have been developed. A discussion of the methods that are commonly applied to karst investigations, the advantages and disadvantages of the methods, acquisition parameters, and the kinds of results to be expected will be presented. Common geophysical methods to be examined in detail include electromagnetic terrain conductivity, gravity, electrical imaging and seismic. INTRODUCTION It is not unusual for a geologist of engineer to be called upon to evaluate, investigate or address existing or perceived problems related to a sinkhole in a karst environment. Most professionals recognize that geophysics can help quantify the problem so appropriate actions may be developed. However, the geophysical method and parameters represent a less certain choice for many professionals due to the lack of familiarity with many geophysical methods. Methods for selecting surface geophysical methods are discussed in the context of existing consensus standards. The method, advantages and limitations, interferences, common acquisition parameters means of geophysical information presentation are discussed for a number of geophysical methods. When these are recognized, the right geophysical method can be applied to the right kind of problem. This can lead to a more successful application of geophysical methods, and a more successful karst investigation project. 1 Senior Geophysicist, Science Applications International Corporation, 6310 Allentown Boulevard, Harrisburg, Pennsylvania 17112 Richard.A.Hoover@saic.com Geophysical Choices for Karst Investigations Page 1 of 10 CHOOSING A GEOPHYSICAL METHOD For those who are not geophysicists, ten years ago, the choice of a geophysical method for application to a particular engineering or geologic problem frequently depended on experience or guesswork. Today, standards have evolved in the engineering community that assist non-geophysical professionals in understanding the method theories and how to select and apply each one. The American Society of Testing and Materials (ASTM) have published a guide to selecting surface geophysical methods (ASTM-6429). The guide represents a consensus standard for the selection and use of geophysical methods for a variety of subsurface problems. The guide also provides a standard for selecting surface geophysical methods in addition to an overview of the methods. The guide established a simple index table for the selection of geophysical methods with either an A or B ranking. The meaning of Aor Bindicates a primary (A) or secondary (B) consensus geophysical method for a particular application. For the investigation, of sinkholes and voids, the consensus standard includes five methods with either an A or B ranking. These recommendations are shown on Table 1. Table 1. ASTM Consensus Standard Methods for assessing sinkholes and voids Method Consensus Standard Seismic Refraction B Electrical (DC) B (A) Frequency Domain Electromagnetic A Ground Penetrating Radar A Gravity A Other geophysical methods may work at a given site however these methods identify the consensus methods that are most likely to succeed in most geologic settings. Similarly, these methods may fail given the proper setting, however they form the basic methods that geophysicists will consider when assessing sinkholes and voids. All geophysical methods apply physical measurements to answering geologic questions. With geophysical applications, a volume of the subsurface is measured. It is necessary to recognize the physical properties of the feature being measured as well as the effective volume of measurement in order to define survey objectives. The investigation of a small sink feature causing soil piping adversely affecting a residential building footer will require a different scale of geophysical information than the design needs for a proposed shopping mall in a karst setting. The need to scale the geophysical method to the size of the problem becomes an important consideration (Benson, 1992). Preparing for a geophysical survey, it is necessary to identify the volume of the subsurface of interest and be able to contrast the size of the features of interest with the resolution of the geophysical methods considered. Geophysical limitations related to resolution relative to the target, and site constraints from interferences can Geophysical Choices for Karst Investigations Page 2 of 10 dictate the method of investigation. The scale of the project will determine the cost effectiveness of the geophysical methods employed SEISMIC Seismic refraction methods measure the velocity of energy transmitted through the soil and rock. Typical seismic velocities range from 500 to 6,000 feet per second for soils, and 6,500 to 18,000 feet per second for bedrock (Haeni, 1984). Lower soil velocities can be expected when unconsolidated soils are present near sinkhole features. Seismic surveys provide a mechanical measurement of energy movement. Therefore the method has an advantage of less interference common with other methods. The seismic method can be applied to investigations near power lines, areas with saturated clay soils, or significant topographic irregularities. Nearby heavy equipment, causing vibrations can cause a noise source in developed areas. When frozen ground, is present, creating a faster velocity layer the seismic refraction method may not be viable. A typical karst investigation involves placing 12 to 24 sensors called geophones along a line on the ground surface (Figure 1). Geophone separation commonly Figure 1. Seismic refraction spread and resulting cross section developed from ten spreads, each with 24 geophones spaced 20-feet apart. Geophysical Choices for Karst Investigations Page 3 of 10 nges from five to twenty feet. The geophone spacing is a critical factor in etermining the resolution to be attained by the seismic methods. Very small actures or narrow bedrock pinnacles cannot be resolved with widely spaced g w along a lin for more ophisticated data interpretation methods. Three or five source locations are common presented as a profile along the line being urveyed. If several seismic lines are acquired in the same area, the information can lectrical resistivity surveys, sometimes called electrical imaging, measure the ability pparent bedrock resistivity; while mud filled voids will lower the apparent bedrock resistivity. Therefore, changes or anomalous y values are usually the feature of interest. wenty eight to fifty six electrodes are commonly placed into the ground at 1, 2, 3 or eld day. ra d fr eophones. Common energy sources include sledge hammer, mechanically assisted eight dropping devices, or shot-gun shells. The number of energy source locations e of geophones determines data redundancy necessary s in a karst survey, while two and seven source locations are used at some sites. The number of source locations has a significant bearing on cost, due to the time required to both acquire and interpret the data. The depth of penetration of a seismic refraction survey is typically 1/3 the length of the geophone spread. Other seismic methods can provide a deeper investigation with a more limited surface area. With reasonable site access, a small refraction survey team of two persons can collect 4 to 6 lines of data during an 8 or 10 hour field day. Seismic survey results are commonly s be provided in map form, however normal software limitations preclude this as a normal presentation of the data. ELECTRICAL E to pass an electric current through the earth. Soil resistivities range from 5 to 8,000 ohm meters while bedrock resistivities range from 200 to 4,000 ohm meters (Ward, 1990). The presence of a void in the subsurface represents an infinite resistivity, and will increase the a