According to the United States Geological Survey (USGS), which also operates the global seismic network (GSN), 152 seismic stations serve as multi-use scientific facilities and societal resources for education, research, and monitoring.
That means seismic borehole data collected before, during, and after the drilling of deep boreholes provides valuable information for reservoir characterization.
Standard Geophysical Logs and their Use
Different types of ground movement have different seismic signal characteristics. Seismic sensors detect noise with sensitivity and are used for resource exploration and security surveillance.
Seismic activity, image-specific reservoir characteristics, and seismic noise data can help standardize results and improve your geophysical predictions. Moreover, geophysical logs complement preliminary seismological surveys, which could result from induced seismicity.
Geophysical borehole logging can be done at any time to gather the borehole’s lithological, groundwater, and numerical properties. As well, logs can be used during hydraulic fracturing to obtain energy technology data from seismic sources as well as microseismic monitoring.
Common physical logs include:
- Acoustic-televiewer logs capture a magnetically aligned, photographic image of the borehole wall’s acoustic reflectivity. Televiewer logs can only be collected from open holes covered in mud or water and work at a higher frequency than seismic waves. As a result, caution is advised when applying and directly comparing sonic log data to seismic data.
- Television logs capture a color optical image of the borehole and record it on video-cassette-recorder tape. Visual microseismic events can be viewed in real-time on a television monitor.
- Temperature logs keep track of how warm the borehole’s water is. Temperature logs help define water-bearing zones and determine vertical flow in the borehole between zones of various hydraulic heads penetrated by wells
- Electromagnetic induction records the resistivity or electrical conductivity of the water and rocks surrounding the borehole.
- Normal-resistivity logs – With the aid of variable-spaced potential electrodes on the logging probe, normal-resistivity logs capture the electrical resistivity of the borehole environment and nearby rocks and water.
- Electrical resistance logs record electrical resistance at a specific location inside the borehole and an electrical ground at the surface of the soil.
- Gamma logs record the quantity of gamma radiation that the rocks around the borehole naturally release or for nuclear explosion monitoring.
- Caliper logs are used to record borehole diameters. A caliper log is helpful in the interpretation of changes caused by caving or fracturing along the borehole walls.
The Best Baseline for Borehole Seismic Monitoring
These studies are part of NASA’s astrophysics data system seismic surveys.
1. 80-level, three-component, clamped borehole receiver array developed by P/GSI.
The most recent survey occurred in December 1999 in PanCanadian’s Weyburn Field in Saskatchewan.
It comprises 634 shots of a 3D VSP that generates 152,000 seismic data traces in 60 hours at a peak frequency of roughly 200 Hz.
The array’s 3C geophone pod spacing is 25 or 50 feet, both typical geophone spacings for VSP applications.
In contrast to what is feasible from a surface seismic survey, we have shown that data from the seismic array can be used to scan the whole drainage volume surrounding the well.
The array also enables high-resolution, real-time monitoring of production from a field.
2. 3D Reverse VSP (RVSP) survey recorded at the MIT test site in Michigan
During the MIT RVSP survey, the source depths range from 3,000 feet to 4,500 feet.
Direct source-receiver lines of more than 6,000 ft record high-quality data with a peak frequency of 360 Hz.
With travel paths of 15,000 feet, photographing geological formations 5,000 feet below the source, high-quality reflected data is captured.
The reverse VSP data create 3D images with a resolution around four times better than surface seismic techniques.
We have shown that we can record data with transmission lines as long as 15,000 feet and frequencies as high as 1,000 Hz from the source.
3. All fiber-optic hydrophone array
P/GSI worked with Litton to design and produce a 96-channel all-fiber-optic hydrophone array for boreholes.
A cross-well survey using the P/GSI downhole vibrator as the borehole source served as the initial survey using the all-fiberoptic hydrophone array.
The unconsolidated structure records 500 Hz data and the resulting images have a resolution of more than 5 feet.
