1. What is borehole seismic testing (Seismic Test)?

Seismic down-hole and cross-hole services are geophysical methods used to characterize the subsurface properties of the Earth by measuring the propagation of seismic waves. These methods are often employed in geotechnical and environmental investigations, as well as in the oil and gas industry. Let’s explore the basics of seismic down-hole and cross-hole services:

1. Seismic Down-hole Testing:

   • Purpose:
     • Seismic down-hole testing involves the measurement of seismic wave velocities within a borehole drilled into the subsurface. It provides detailed information about the geological layers and their properties at depth.
   • Procedure:
     • A seismic sensor, or geophone, is lowered into the borehole at various depths, and seismic sources at the surface generate waves. The travel times of these waves are recorded, allowing the determination of subsurface velocities.

2. Seismic Cross-hole Testing:

   • Purpose:
     • Seismic cross-hole testing assesses the velocity and attenuation of seismic waves as they travel between two or more boreholes. This method provides information on subsurface continuity, layer thickness, and material properties.
   • Procedure:
     • Seismic sources and receivers are placed in different boreholes. Seismic waves are generated, and the travel times and amplitudes are recorded at various depths. The data are then used to create velocity profiles and identify subsurface features.

3. Key Applications:

   • Geotechnical Engineering:
     • Seismic down-hole and cross-hole testing are used to assess soil and rock properties, including shear wave and compressional wave velocities. This information is valuable for foundation design, slope stability analysis, and seismic site classification.
   • Environmental Studies:
     • In environmental investigations, these methods help characterize subsurface conditions, including the identification of contaminants and the assessment of aquifer properties.
   • Oil and Gas Exploration:
     • In the oil and gas industry, seismic down-hole testing provides data on subsurface formations, assisting in reservoir characterization and the optimization of drilling programs.

4. Advantages:

   • High Resolution:
     • Seismic down-hole and cross-hole testing offer high-resolution data compared to surface seismic methods, especially in urban environments.
   • Direct Measurement:
     • The measurements are taken directly in the borehole, providing accurate information about the subsurface conditions at depth.

5. Challenges:

   • Cost and Complexity:
     • These methods can be more expensive and technically complex compared to surface seismic surveys.
   • Borehole Logistics:
     • The success of these tests depends on the successful drilling and completion of boreholes, which may face logistical challenges.

6. Data Interpretation:

   • The collected data are processed and interpreted to create subsurface models, velocity profiles, and other geotechnical or geological information.

Seismic down-hole and cross-hole services are valuable tools for gaining insights into subsurface conditions, and they play a crucial role in engineering, environmental studies, and resource exploration. Consulting with geophysical experts is recommended to ensure the proper design and execution of these tests based on project-specific requirements.

2. Basic borehole seismic  testing  methods

• The seismic Down-Hole Test is widely used in Vietnam.

Apply ASTM D7400-08 standard.  

The source of transmitted waves is created from wooden and iron hammers and anvils, causing vibrations.

P and S waves created when hammering on a wooden anvil are transmitted in the soil medium to the geophones.
The receiver geophones used in the measurements are the same 3-dimensional geophones.

Each 3-dimensional geophone is composed of 2 horizontal geophones and 1 vertical geophone. The two horizontal geophones are arranged perpendicular to each other and lie in a horizontal plane, while the vertical geophone is arranged perpendicular to the horizontal plane. The frequency response of geophones ranges from 10 Hz to hundreds of Hz, enough to satisfy the recording of elastic vibration waves within the frequency range of interest.

The geophones are attached to a baseboard, behind the baseboard is an airbag connected to the surface by an air tube that is used to press the geophones tightly against the borehole wall during the measurement process.

• Seismic Cross-Hole Test

Apply ASTM Standard D4428/ D4428M-91.

The cross-hole method is performed to record elastic waves in the medium between two boreholes, at selected depths and distances.

The vibration source and receiver (geophone) were placed at the same elevation in the drill holes during each measurement.

Using a conventional transceiver system in series creates maximum efficiency for measuring wave velocity in the field in the radiating direction.