# Using refraction method to find depth to bedrock

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

Title | Problems in Exploration Seismology and their Solutions |

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

Chapter | 14 |

Pages | 497 - 503 |

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

ISBN | ISBN 9781560801153 |

Store | SEG Online Store |

## Contents

## Problem 14.1

To find the depth to bedrock in a damsite survey, 12 geophones were laid out at 15-m intervals from 15 to 180 m. Determine the overburden depth from the data in Table 14.1a assuming a single layer above the refractor. By how much does the depth differ if we assume two layers above the refractor?

### Background

The refraction method is discussed in problem 4.18.

15 | 19 |

30 | 29 |

45 | 39 |

60 | 50 |

75 | 59 |

90 | 62 |

105 | 65 |

120 | 68 |

135 | 72 |

150 | 76 |

165 | 78 |

180 | 83 |

### Solution

Figure 14.1a shows the plotted data. The single layer interpretation (fine lines) gives , , . The critical angle . Equation (4.18a) gives the depth to bedrock as

The three-layer solution (heavy lines) gives , , , . Note that is not reliable; it could be any smaller value. If it were smaller, the first-layer values would change slightly, but it would not significantly change the values for the other layers.

For the first interface, ,

For the second interface, , so

Thus we get from equation (4.18d)

so ,

The total depth is .

The difference between the two interpretations is 9 m or about 17%.

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Poission’s ratio from P- and S-wave traveltimes | Interpreting engineering refraction profiles |

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Specialized techniques | Introduction to Problems in Exploration Seismology and their Solutions |

## Also in this chapter

- Using refraction method to find depth to bedrock
- Interpreting engineering refraction profiles
- Interpretation of four-shot refraction data