As Silixa explains, several fundamental reasons show why using borehole seismic techniques is better than conventional surface seismic methods. Borehole seismic techniques are more precise and offer various benefits. These include synthetic seismogram, check shot, vertical seismic profiles (VSP) – offset VSP and three-component VSP. Borehole seismic techniques also provide a detailed description and extrapolation regarding the lithologic parameters of geological formations around the borehole.
VSPs are used in wells to help in correlating surface seismic information. This article will shed light on the science behind borehole seismic techniques and outline multiple techniques for conducting borehole surveys.
Synthetic Seismograms
For ages, synthetic seismograms have been used in correlating surface seismic segments. Like other theoretical models or techniques, synthetic seismograms experience some of the challenges associated with the simplifying assumptions applied in the model. Luckily, it’s possible to obtain a precise fit of surface seismic lines. Synthetic seismograms usually provide a critical link to understanding the seismic relationship with the well log.
Borehole Geophysics
Borehole geophysics refers to the science of collecting and analysing measurements of physical properties obtained from test holes or wells. The probes that measure these physical properties are lowered into the well or borehole to collect point or continuous information about the site. This information is displayed graphically on a geophysical log. Multiple geophysical logs are recorded to maximize their synergistic nature.
Borehole geophysics is applied in environmental and ground-water investigations to collect data on well construction, rock porosity and permeability, lithology and fractures, and water quality. The logging system comprises cables, probes, data recording units, processing modules, and draw-works. A computer manages these logging systems and collects various logs with just a single pass of the probe.
Borehole Seismic Techniques
Borehole seismic surveys are conducted in various ways. Some major borehole seismic techniques comprise:
- Vertical seismic profiling
- Cross-hole seismic profiling
- Parallel seismic technique
1. Vertical seismic profiling (VSP)
Vertical seismic profiles are presently the easiest and most cost-efficient borehole seismic technique. This process uses a seismic energy source – like a hammer hitting a plate – applied at predetermined distances from the well or borehole. Multiple geophones are then inserted into that borehole. Consequently, a one-way travel time of the arrival of the produced seismic energy is measured.
Shear-wave investigations can be done with a tri-axial geophone, coupling it to the borehole walls at different depths. Shear-wave energy is produced by hitting the ends of a massive wooden block held in position by a heavy weight or vehicle. When the two ends of the massive wooden block are hit, it generates both negative and positive shear waves. The negative and positive shear waves help seismic interpreters to identify various shear wave arrivals from extraneous signal noise and compressional waves.
2. Cross-hole seismic profiling
Cross-hole seismic techniques are used to measure the travel time of seismic energy generated by the boreholes to identify the properties of the intervening materials. One borehole uses an external seismic energy source, and receiving geophones are lowered into the other boreholes.
The travel times of the seismic waves are influenced by the energy arrival at each receiving geophone. The separations of every borehole and the depths of the seismic source and the receiver are then measured. When these pieces of information are collected, the compressional seismic velocities for every depth interval can be measured.
This information is used to obtain a profile of intervening geological boundaries and produce detailed reports on various rock properties, like rippability.
3. Parallel seismic (PS) technique
The parallel seismic technique is a borehole test technique mainly used for determining the depths of different foundations. Moreover, this technique can determine the thickness of a scour zone after being filled with soft sand or mud. Parallel seismic tests are conducted on concrete, steel, masonry, and wood foundations. Therefore, a section of the structure linked to the foundation has to be left exposed for hammer impacts.
This technique involves hitting the exposed part of the structure and then using a three-component geophone or hydrophone to record the shear or compressional waves produced from the foundation. The PS data is analysed in the time domain.
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