Time to depth

Question 1

Group velocity

Answer 1

speed at which the peak of the signal envelope travels

Question 2

Instantaneous Velocity

Answer 2

Over very small interval, i.e. a sonic log reading

Question 3

Interval Velocity

Answer 3

Over a larger interval i.e. the Tertiary

Question 4

Effective Velocity



Answer 4

Root Mean Square Velocity

Question 5

Principle of sonic logs

Refraction at the borehole wall

Answer 5

R and T dans le borehole

Refraction at the borehole wall

Question 6

Characteristics of sonic logs for TZ

Answer 6

• High frequent (12-24 kHz) source
• Usually not over full well - shallow part often missing
• Standard tool unreliable in unconsolidated sediments
• Readings in altered formation yielding incorrect values
• —> Calibration needed, i.e. with wellshoot

Question 7

Well shoot (Checkshot) survey

Answer 7

• Typically 10- 20 levels, usually taken at TD
• Cheap
• Calibration of sonic only purpose
• Breaktime determination only
• Corrections often only geometrical
• Horizontal wells complicated

Question 8

Drift curve

Answer 8

Drift = Wellshoot time
–
integrated sonic times

Negative drift -> Sonic velocities lower than wellshoot velocities
Zero drift -> Sonic velocities equal to wellshoot velocities
Positive Drift -> Sonic velocities faster than wellshoot velocities

Question 9

Reasons for drift
Negative drift (Sonic slower than WST)

Answer 9

• Sonic taken in altered formation (drilling)
• Low velocities difficult to measure
• Wellpath does not constitute a “minimum time path”
• Minor amounts of gas in borehole
• Very common at shallow levels

Question 10

Reasons for drift 
Positive drift (Sonic faster than WST)

Answer 10

• High frequency velocities higher than low-frequent velocities
• Wellshoot breaks more difficult to pick at larger depths – tendency to pick too late
• Relatively rare, mostly at deeper levels

Question 11

Depth conversion methods/techniques

Answer 11

• Use of velocities increasing with depth - Faust and “Vnot and k”
• “Layer Cake” depth conversion
• Individual layers may have increasing
velocities with depth
• Based on seismic velocities
• Based on well velocities
• Based on Depth Migrated Data
• Use of geostatistics

Question 12

Layer-cake TZ conversion

Answer 12

• Widely used in multi-lithology settings
• A ”geologically plausible” model
• Identification and mapping of
boundaries is vital
• Search for “large” contrasts in Vint
• Individual layers > 100 - 200 ms
• Use of geostatistics possible

Question 13

given a multi-lithology environment. What boundary conditions should be met for the successful application of a layer cake depth conversion.

Answer 13

 Boundaries should be clearly mapable and identifiable

 Must be able to map the boundries and correlate them over a big region without any doubt

Question 14

what are the advantages of using seismic times and depths from wells for making a time-to- depth conversion model.

Answer 14

Layer Cake Model
Seismic times and depths from wells
 Not “real” velocities
 Accurate at well locations
 Subject to incorrect imaging
 Anisotropy compensated
 Dependent on adequate coverage
Sonic and checkshot velocities
 Accurate, real
 Corrections at well locations probable
 Independent from migration
 Subject to anisotropy
 Independent of seismic coverage