Engineering geophysics

Most Geophysicists are trained in exploration problems which are different from engineering problem.

Exploration geophysicists design and conduct cost effective geophysical surveys to acquire desired resource in economic quantities.

The engineering geophysicists are concerned with resolution, detail and quantification of specific subsurface condition. It target areas are usually difficult and success may not be possible.

The major application areas for engineering geophysics :

Site Assessment - determine bedrock depths

Correlation - correlation of geological units between boreholes.

Hazards - location of faults, geological contacts, shear zones, cavities and man-made objects.

Engineering - Determination of dynamic elasticity, rippability

Properties

Resources - Location of burrow materials such as sand, gravel, clay and hard rock

Groundwater - Determination of ground water conditions, location of leakage zones.

Monitoring - Blasting efectiveness assessment.

Role and function of engineering geophysicist.:

FUNCTION AND OPERATIONS FLOWCHART FOR ENGINEERING GEOPHYSICAL SURVEY

- Budget - interpreted drilling log - Initial physical model - Field operations

- Problem - geological classifications - Selection of methods

- Site information - Initial geological model - Selection of field procedures

- Deadline &timing - Selection of contractor

- Monitoring - Supervision of contractor

Civil Engineer Geologist Geophysicist Contractor

Completion/evaluation

- Critical evaluation - Final geological model - Data assessment - Supply of field data

- Integration - Report - Physical interpretation - Operation report

- Final Opinion - Correlation

- Geolog. & engineering

Interpretation

- Ambiguities and limitation

- Report

Common problems in the operation of engineering geophysical surveys

Civil engineer

· Budget for geophysics inadequate or inappropriate

· Geophysical survey poorly integrated with other investigstion

· Scale and implication of geophysical interpretation unclear

· Geophysical interpret. Disagree with other nformation

Engineering geologist

· Poor core recovery in drilled holes

· Geological classification difficult or inappropriate

· Unresolved ambiguities in geological model

Engineering geophysicist

· Subsurface condition very different from those expected

· Inadequate and poor quality data

· Interpretative tools inadequate for highly variable subsurface condition

· Unresolved ambiguity

Geohysical contractor

· Site conditions not as expected

· Decreased coverage owing to equipment failure, weather etc.

· Other site work interfering with operation

Engineering geophysics has an important role to play in geotechnical investigations for civil engineering and mine development projects, however geophysicists need to realize that greater care is required in interpretation and reporting geophysical results/

In particular geophysicists need to develop an increased sensitivity to alternative interpretations, consistent with a measured data set an their engineering implications.

Comparison of different interpreted models of seismic refraction interpretation

Original Intercept time Intercept time Raytrace Velocity function

Interpretation Re-interpretation re-interpretation interpretation interpretation

With blind zone

340 350

- 0m ======= ------------------ ____350________ __300__________ ___340______

550 800 800 730 800

________

1600 __________ _____________ _____________ ____________

-50 _________ 2200 ___1600______ V(s)= 1650(1+0.01z)

_____________ 2200 2450

----------------------

-100 3300 3000 3000 _____________

3250

_____________

-150 4800 _______________

4800

-200m

Engineering Geophysics : A Geologist’s view

Three remarks given by geologists:

1.Great! Provides quantitative data which our geologists can’t do. We are saving a lot of drilling

2.Hopeless! Misled us badly on several jobs. We don’t use it any more.

3.Useful, as an extra tool in site investigation, but requires very careful interpretation, and calibration.

Case 2 is during time when Seismic refraction survey was oversold by geophysical companies and engineers (Late fifties)

Today seismic survey mostly yield Case 3

Geophysicists usually involved in all the investigation stages.

Common applications:

· Location of features with anomalous physical properties

· Delineation of boundaries between features with contrasting physical properties

· Assessment of physical properties of rock masses in situ

o Rippability

o Elastic properties

o Effects of grouting

Location of features with anomalous physical properties:

· Basic dyke/ plugs

· Deeply weathered zone

· Major fault zones

· Buried channels cavities

· Land slipped masses

Delineation of boundaries between features with contrasting physical properties

· Transported overburden

· Residual overburden

· Transported overburden over residual overburden

Assessment of physical properties of rock masses in situ

· Rippability

· Strength and compressibility

· Effectiveness of grouting

Challenges in future research:

· Location of hazardous features in offshore situations by any geophysical method, in particular, high reflection seismic

· Location of thin continuous, weak seams in rock masses. Single seams of this kind have often been the cause of major rockslides.

· Location and delineation of residual fresh boulders within masses of extremely weathered rock (residual soil)

· Location and delineation of hidden cavities in rock or soil masses

· Development of a method for the remote-reading of instruments (eg. Piezometers, extensometers) installed at great depth in rock masses. This is required for effective monitoring of rock mass behaviour during subsidence, during and after mining activities.

· Assessment of the depth of mechanically loosened rock over compact rock, both in surface exposures, and around tunnels and other underground excavations

· Looking of points or areas of leakage from the floor of reservoirs.