The main instrument used is called the sonde. A basic sonde consists of a source and two receivers one-foot apart. The sonde is lowered down the borehole and waves are generated and recorded continuously. The sonde is usually positioned in the borehole center using centralizing springs. Frequencies used are in the range of kHz. Typical investigation radius is 0.2 – 1.2 m. Acoustic Well Logging Operation

Acoustic Well Logging Sonde types

Acoustic Well Logging Transit time

Acoustic Well Logging Example

Seismic Activities Acquisition Interpretation Processing Inversion Re-Interpretation

Borehole Seismology

Borehole seismology involves the recording of seismic energy using one or more wells. Benefits of borehole seismology include higher vertical resolution and less near-surface effects. Drawbacks of this method: –Expensive because it requires drilling wells. –Only near-well area (0.1 – 100 m) is sampled. –Low spatial resolution because wells are usually drilled far from each other.

Uphole surveys Check shot surveys Vertical seismic profiling Acoustic logging Crosshole surveys Seismic while drilling Geophysical Consideration Types of Borehole Seismic Surveys

Weathering layer Subweathering layer R S1S1 S2S2 S3S3 Z1Z1 Z2Z2 Z3Z3 S R1R1 R2R2 R3R3 Z1Z1 Z2Z2 Z3Z3 Weathering layer Subweathering layer Uphole Survey Downhole Survey

S R1R1 R2R2 R3R3 Z1Z1 Z2Z2 Z3Z3 Z Interval Velocity Check Shot Surveys

It involves the recording and analysis of several arrivals along a well that penetrates target layers. Objective is estimating the velocity and thickness of subsurface layers. It is performed using receivers that are placed in the borehole at known depths and a source that is placed near the well head. It is similar to a check-shot survey but using a smaller receiver spacing. Vertical Seismic Profile (VSP)

S Layer 1 R Z H Z T Layer 2 D V Downgoing wave Upgoing wave Vertical Seismic Profile Geometry

The main advantages of VSP are: 1.High resolution (usable frequency up to 250 Hz) 2.Better control on multiples 3.Better control on attenuation effects 4.Ability to study converted waves 5.Ability to study areas closely below and above an interface The main disadvantages of VSP are: 1.Need a fairly deep borehole 2.Samples only rocks near the borehole ( m) Geophysical Consideration Vertical Seismic Profile Advantages and Disadvantages

Vertical Seismic Profile Zero-offset VSP

Vertical Seismic Profile VSP types

It involves two nearby wells (< 1 km), one is used for sources and the other for receivers. It gives a detailed model of the rocks between the wells, especially if tomography is used. Advantages include: –It bypasses the problems of the near surface because the source and receivers are below the weathering layer. –It delivers high –frequency data (~ kHz) leading to very high spatial resolution of the derived model. –Availability of shear and converted waves. Geophysical Consideration Crosshole Survey

Crosshole Survey Velocity computation

The noise from the drill bit is used as a source, while geophones on the surface are recording. Drill-bit waves are reflected off deeper interfaces and recorded at the surface. These reflections are used to predict the rock structure ahead of the drill bit. It is possibly the only seismic method that can be used for real-time decisions about the drilling operation. Seismic While Drilling

The noise from the drill bit is used as a source of a continuous random signal. A geophone at the top end of the drill string recording the drill-bit signal is used as a pilot. Several surface geophones around the well head are recording the drill-bit signal. The surface geophone signal is auto-correlated with the pilot signal in order to compress the drill-bit signal into a zero-phase wavelet. Seismic While Drilling Operation

Seismic While Drilling Geometry