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used was a 25-channel Texas Instruments Model Photographic Recording Oscillograph . The energy source used consisted of a 33-pound sledge hammer equipped with seismograph /oscillograph drive switch and piezoelectric trigger mechanism ( time break ) . Limitations The subsurface velocity profiles presented in this report represent the most reasonable interpretation of seismic refraction survey data based on our knowledge of the existing geologic conditions at the site . The results are presented for design feasibility information only and are not intended to serve as information for determining construction procedures . Interpretations were made in accordance with generally accepted geophysical methods and practices . This warranty - is in lieu of all other warranties , express. or implied . The quality of seismic refraction data for this survey was good ; but . in cases was affected by electrical disturbance from a nearby power transmission lines Cheavy 60 Hz noise ) , and In some cases was affected by background noise from passing vehicles and aircraft , slightly irregular terrain , wind , and lateral inhomogeneity ( scattered resistant blocks ) . These factors produced noise signals and /or scatter in the recorded data , I .imiting the accuracy of first break compression wave picks and interpretation . The seismic refraction method used has some inherent limitations such as the possibility for undetectable hidden layers , blind zones , and velocity inversions . The absence of exploratory borings and other existing subsurface geologic information in the area surveyed may also limit the reliability of interpretation , since more than one interpretation is often possible for a given set of data , and correlation with borings or other subsurface information is important in 'establishing accuracy . The maximum depth of reliable seismic information obtained during this survey can be assumed to be approximately one- third of the length of the individual lines , with information at a maximum depth underlying the middle one- third of the lines . For example , a seismic refraction line 300 feet in length will typically yield reliable data on subsurface materials to a depth of about 100 feet beneath the middle 100 feet of the line .