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< Strip Logging

Red ball iconStrip Logging

During the earliest geologic mapping projects in Iowa, geologists had to rely completely upon exposures of earth materials at the land surface. From exposures in the northeastern corner and other scattered areas of the state, a geologic sequence of strata was established. The oldest rocks were found to occur at the surface in northeast Iowa, and progressively younger rocks were seen toward the southwest. The entire sequence of rock units was determined to be dipping downward to the southwest. However, with most of the landscape mantled by glacial deposits, other details of the underlying geology could not be well defined. With the advent of new drilling technology and the need for increased quantities of potable water, many of Iowa's communities, business, and home owners began drilling wells for their water supplies. Samples have been collected during the drilling of many of these wells, and in conjunction with drillers' logs, have provided the Iowa Geological Survey with a valuable source of data about Iowa's geology and water resources. Information derived from study of these samples and logs provides invaluable data, unavailable to early geologists, and have opened an era of subsurface investigations that continues today.


Microscope Production of Striplogs

Data acquired from drilling is studied and refined, and is copied onto geologic striplogs (diagram, right). The drill samples are processed and logged at a depth scale of 1 inch to 50 feet by Survey geologists using binocular microscopes (photo, left) and simple chemical tests. Normally, a sample interval of five feet is used on these logs. Each striplog contains a graphic column, with different colors and patterns used to describe various rock types, minerals, fossils, and rock porosity. To the right of this column is a detailed written lithologic description. To the left of the column, the names of rock formations and other stratigraphic units are noted. Information concerning water yields and well construction are also noted on the log. After a sample set is logged the information is entered into a computerized database GEOSAM system that provides rapid access to the broad range of recorded information on subsurface stratigraphy and hydrology in Iowa.


Uses of Striplogs

The log is then filed in the striplog library in the IGS's Main Office in Iowa City. The IGS striplog library presently consists of over 20,000 logs. Completed striplogs are used either individually or in groups for a variety of uses. The most common, and one of the most practical applications, is to assist well drillers and the general public in planning for water wells. Data on one or more existing wells near a proposed site are reviewed with the requirements of the new well in mind. A prediction of well depth, quantity and quality of water encountered, rock units penetrated at various depths, and estimates of casing necessary to line the well hole can be made. This is done either informally over the telephone or by an office visit, or by mailing a detailed well forecast. Such forecasts vary in accuracy with the quantity and quality of subsurface information available for a particular region and with the amount of structural and stratigraphic variation in the immediate area. However, when used with these limitations in mind, the forecasts can eliminate much guesswork and assist drillers and engineers in estimating costs and in determining well design. After a well is in use for some time, rock debris can leak into the hole because of a deteriorating casing. In these instances, a log of the well has proven valuable in determining where the debris is coming from and how to repair the well. The same is true for wells which have gone dry or where the water production has dropped off. Other important uses of well logs include determination of hydrologic conditions in local areas; for example, estimating whether large withdrawals of water can be made without serious impact on the production of adjacent wells. Variations in aquifers and rock units on a state-wide basis also can be examined. Regional cross-sections are constructed to gain better understanding of stratigraphic relationships and geological history in Iowa and adjacent states. These studies also help us gain knowledge of the distribution of economic deposits such as coal, limestone, dolomite, and gypsum.

Example of Striplog