# Refraction interpretation by stripping

Series | Geophysical References Series |
---|---|

Title | Problems in Exploration Seismology and their Solutions |

Author | Lloyd P. Geldart and Robert E. Sheriff |

Chapter | 11 |

Pages | 415 - 468 |

DOI | http://dx.doi.org/10.1190/1.9781560801733 |

ISBN | ISBN 9781560801153 |

Store | SEG Online Store |

## Contents

## Problem 11.6a

Solve problem 11.5 by stripping off the shallow layer.

### Background

Stripping is a method of interpreting refraction data by removing the effect of upper layers, the removal being accomplished by reducing the traveltimes and distances so that in effect the source and geophones are located on the interface at the base of the “stripped” layer. Stripping can be accomplished by calculation or graphically, or by a combination.

### Solution

We wish to compare our results with those of problem 11.5, so we use the same measurements, namely km/s and

(To avoid triple subscripts, we denote intercept times at downdip and updip source locations by and .)

We start by using equations (4.24f) to get :

Equations (4.24b,d) can be written

hence , . These are the same as those in problem 11.5.

Next we calculate the distances perpendicular to the first refractor at and (Figure 11.6a). We use equation (4.24b) to get and :

These results are identical with those in problem 11.5. We verify the dip using these depths:

The first step in stripping is to plot the shallow refractor; we do this by swinging arcs with centers and and radii 1.07 and 0.53 km, the refractor being tangent to the two arcs. To get the “stripped” time values, we subtract the times down to and up from the first refractor, i.e., traveltimes along and for sources and . Although maximum accuracy would be achieved by stripping times for all geophones, the curves for the shallow refraction are so nearly linear that we calculate the stripped times only for each source and one intermediate point on each profile ( and ). We obtain the required distances by measuring the paths in Figure 11.6a. Calculation of the stripped times is given below. Path lengths: , , , km.

Stripping off the first refractor in effect moves sources and down to and and geophones at and down to and , so the stripped times are plotted above these shifted points, the new traveltimes curves being and . Measurements on these stripped curves give the following:

Item | Problem 11.5 | Problem 11.6 | Difference |
---|---|---|---|

4.92 | 4.88 | 0.8% | |

1% | |||

2% | |||

1.32 | 10% |

*Vertical depth measured at source A. |

We now get

This dip is relative to , so the total dip is

## Problem 11.6b

Compare the solutions by stripping with those using Adachi’s method (problem 11.5).

### Solution

To compare depths, we measured vertical depths below A. Results for the first layer are the same for both methods, those for the next layer are given in Table 11.6a.

## Problem 11.6c

What are some of the advantages and disadvantages of stripping?

### Solution

Advantages of stripping:

- Easy to understand
- Straightforward in application
- Can be used with beds of different dips if the strike is the same
- As rapid as other methods when done graphically
- Can be used to interpret irregular or curved surfaces

Disadvantages:

- Very sensitive to velocity errors
- Like most methods, assumes the same strike for all refractors
- Difficult to apply when dips are steep

## Continue reading

Previous section | Next section |
---|---|

Adachi’s method | Proof of a generalized reciprocal method relation |

Previous chapter | Next chapter |

Geologic interpretation of reflection data | 3D methods |

## Also in this chapter

- Salt lead time as a function of depth
- Effect of assumptions on refraction interpretation
- Effect of a hidden layer
- Proof of the ABC refraction equation
- Adachi’s method
- Refraction interpretation by stripping
- Proof of a generalized reciprocal method relation
- Delay time
- Barry’s delay-time refraction interpretation method
- Parallelism of half-intercept and delay-time curves
- Wyrobek’s refraction interpretation method
- Properties of a coincident-time curve
- Interpretation by the plus-minus method
- Comparison of refraction interpretation methods
- Feasibility of mapping a horizon using head waves
- Refraction blind spot
- Interpreting marine refraction data