A PROPOSAL FOR GEOPHYSICAL MAPPING OF FRACTURE ZONES FOR WATER SUPPLY

Charles T. Young, Associate Professor of Geophysical Engineering, MTU

We propose MS thesis research involving geophysical measurements of fracture zones near proposed and completed water wells. The object is to improve water well siting and production with geophysics.

The locations for wells drilled in the UP are usually chosen by the property owner and the well driller based on the driller's practical knowledge. Many of these wells are drilled into fractures, either in volcanic/igneous or sedimentary rock. A conceptual model is shown in Figure 1. Often a well as deep as several hundreds of feet is required for a domestic waters supply. There is abundant rainfall and recharge, but the advance knowledge of the depth to a suitable fracture or fracture zone is usually unavailable. Choices about well location are typically made as follows: The well location is chosen by the property owner to be near the proposed building, and the drilling is started. If adequate water is not found at first, the driller asks the property owner for authorization to drill deeper. This procedure usually results in a usable water supply; eventually the drill will penetrate a fracture zone below the "water table". Clearly, the depth of the hole and thus the cost of drilling could be reduced if the location and orientation of the fracture zone were estimated by geophysics before drilling.

THE GEOPHYSICAL TECHNIQUES

VLF Wave Tilt and VLF Resistivity

This technique uses government radio signals in the frequency range 15 to 25 KHz; which are broadcast for navigation. A conceptual diagram is shown in Figure 2. The primary signal from the distant transmitter travels through the earth-ionosphere wave guide. It is refracted into the Earth and penetrates into the earth typically 100 to 500 meters in the rocks in the UP. The waves interact with electrically conductive water-bearing fractures and create a secondary signal. The primary signal alone creates a horizontal magnetic field. When a secondary signals near narrow conductors have a vertical component. Thus the combined signal is tilted from horizontal, thus the method is termed "wave tilt". The primary and secondary signals are detected by a specialized VLF receiver, which allows the anomalous zones in the subsurface to be mapped. The data may be processed to create a two-dimensional cross section of the subsurface, as shown in Figure 3. MTU owns a 1967 version of a geophysical VLF receiver (Geonics EM-16 with a resistivity attachment). It receives signals from a maximum of two VLF stations (switch selectable). Each data point is obtained by tilting the instrument and adjusting knobs to obtain an audio null, then the operator records the tilt and knob reading in a notebook. The more modern equipment (e.g. ABEM WADI) operates by automatically tuning all receivable stations and recording data on an internal electronic notebook. The older apparatus will work for this research "in a pinch", but we want to rent a ABEM WADI, which will allow more data of higher quality to be obtained.

VLF resistivity uses the same signals as VLF wave tilt. The Geonics EM-16R is an attachment to the EM-16, which receives the electric field component of the incident wave using a pair of grounded metal electrodes. The magnetic and electric fields are combined in the instrument to read out in earth resistivity. The resulting value is a weighted average of earth resistivity within the penetration depth of the signal. Thus, it provides a slightly different view or interpretation of the Earth along the measurement line. It is capable of detecting a narrower conductive zone than the wave tilt method.

Multielectrode Resistivity

Electrical Resistivity has been used for groundwater exploration for many decades. Measurements are made by circulating electric current through the soil through a pair of electrodes and measuring the resulting potential between another pair of electrodes. Typically, manual deployment of these four terminal arrays provides only a one-dimensional view of earth resistivity (along either a horizontal or vertical line). MTU has equipment purchased in 1982. The equipment for measuring earth resistivity has evolved rapidly. State of the art equipment is costly; the equipment we want to rent sells for $30,000. Modern equipment allows multiple electrodes to be used (e.g. up to 256), and the measurement is automatically sequenced between the electrodes. Modern computer programs allow two and three-dimensional data presentation and interpretation, as shown in Figure 4. This improves the data quantity or field efficiency by about a factor of ten over the manual methods. The new generation of equipment literally adds two new dimensions to data acquisition. At recent meetings, the only papers presented using the older four electrode equipment were from scientists in third world countries. Impressive examples of data from complex geological and environmental engineering sites may be seen in the 2D Brochures and Customer Cases section of the URL for Advanced Geosciences Inc. and www.agiusa.com and for the Syscal line of equipment at http://www.terraplus.com/resis.htm.

In situations where a borehole is present that intersects a water bearing fracture, it is possible to place a current electrode in the borehole and send current into the fracture zone. This method is shown in textbooks as a method is used in situations where drilling encounters conductive minerals and information is desired about the subsurface extension (direction of strike) of the mineral (Reynold, 1997 pp 467-470). The method is known as "mise a la masse" (French for "place it on the mass"). Textbooks show simple empirical interpretation of data. We expect to be able to use the same technique, except that we would place an electrode in a well which intercepts an electrically conductive fracture zone, make measurements at the surface and infer the direction of the fracture. The three dimensional modelling programs provided with the rented multielectrode equipment would provide a significant advantage over the older empirical interpretation techniques.

It is quite clear from examples in the technical literature and in advertising brochures that multielectrode resistivity can visualize fracture zones that are likely to contain water. There are three options for the use of resistivity in this project:

1. Rent state-of-the-art multielectrode equipment.

2. Request the loan of state-of-the-art multielectrode equipment from the Terraplus scholarship equipment loan program (Jerry McJunkin, Terraplus, private communication).

3. Use MTU's existing four-terminal equipment do limited work.

The MTU - UP Small Business Connection

The department has friends in the environmental and consulting business in the UP. We propose coordinating this research activity with these small businesses. In particular, we need to locate sites to carry out the research and coordinate with water well drilling operations.

Mr. Gregg Johnson

Professional Consulting, Inc.

1009 West Ridge St.

Marquette, MI 49855

gaj@up.net

Mr. Harry Klieman

Kleiman Drilling

P.O. Box 704

Iron Mountain, MI 49801

906 774-1955

This geophysical approach should, of course, be preceded by reviewing existing information:

a knowledge of the geologic and hydrologic setting and drillers logs from adjacent wells (if any) should reveal the bedrock type and the nature of the occurrence of water.

FIGURES

Figure 1. Conceptual model of the occurrence of water in fractures.

Schematic section of Precambrian geology and hydrogeological conditions in Burkina Faso. Perched water table sinks and rises depending on season. Economic aquifers are associated with fracture zones in granites and more easily weathered lithologies, such as schists, in the volcano-sedimentary sequences. Low-yielding shallow wells (w) are dug; productive wells in hard rock (D) must be drilled. (after Palacky, Ritsema, De Jong: Electromagnetic prospecting for groundwater in precambrian terrains in the republic of Upper Volta, Geoph. Prospecting VOL.29, P.932-955, 1981) From: http://www.geophysicsgpr.com/abem/wadi/wadi.htm

Figure 2. Conceptual diagram showing the interaction of VLF signals with a waterbearing fracture zone.

From: http://www.geophysicsgpr.com/abem/wadi/wadi.htm

Figure 3. Current density cross section computed from VLF data. The areas of high current density (high positive numbers or "hot" colors) are interpreted as water bearing fractures (although, depending on the setting, they could be conductive minerals or zones of contaminated groundwater). A free demo version of this software is available from http://www.giscogeo.com/pages/empwrmag.html , and CTY has duplicated the computation using Matlab.

Figure 4. Resistivity cross section obtained with multi-electrode instrumentation indicating water bearing fracture zones in bedrock. A fracture zone in bedrock was located with multi-electrode resistivity imaging. A well was drilled and it produced more than 100 gallons/minute from the fracture zone at 145-150 feet. The program for computing this cross section from data is provide with the rental of multielectrode resistivity equipment and a limited version is available as a free download. Image is from http://www.agiusa.com/2Dwater.shtml

REFERENCES:

http://www.waterlocating.com/ A business in Southern California using VLF to locate fracture zones for water supply.

http://www.abem.se/vlf/wadi.htm Brochure for ABEM WADI VLF instrument

www.expins.com Exploration Instruments (geophysical equipment rental) The WADI rents for $40/day+$85 preparation

http://www.geol.vt.edu/gradstu/wseaton/research.html

http://www.spectrum-geophysics.com/cases/duarte.htm Example in southern California using

seismic, multi-electrode resistivity

Reynolds, J.M, 1997, An Introduction to Applied and Environmental Geophysics, John Wiley& Sons, New York, 796 p.

These costs are for approximately two weeks work away from Houghton.

Personnel costs/profit needs to be added.