Chirp sub-bottom profilers are marine sonar systems which use a highly repeatable source signature to facilitate the acquisition of correlated data with decimetre vertical resolution in the top 20–30 m of sediments. Source signatures can be readily developed and implemented, but an applicable methodology for assessing resolution and attenuation characteristics of these wide-band systems did not exist. Methodologies are developed and applied to seven contrasting source signatures which occupy the same frequency band, but differ in their Envelope and Instantaneous Frequency functions. For the Chirp source signatures tested, a Sine-Squared envelope function is shown to produce seismic data with the optimum resolution and penetration characteristics.

Chirp sub-bottom profiler source signature design and field testing

A method for seismic exploration using nonlinear conversions (e.g., item 6, Fig. 10) between electromagnetic and seismic energy. Seismic returns from a source waveform (e.g., item 180, Fig. 7A) are correlated with a reference waveform (item 150, Fig. 5B), with both waveforms designed to minimize both correlation side lobes and interference from linear electroseismic effects. A waveform element (e.g., item 10, Fig 1A) is selected to be sequenced by a binary or similar digital code designed to generate an input sweep with the needed depth penetration and noise suppression. Correlation of the seismic response with the reference waveform in a data processing step mathematically aggregates the seismic response from the input sweep into a single wavelet (e.g., item 160, Fig. 6A). Preferred binary digital codes include prescribed Britains of maximal length shift-register sequences (e.g., item 100, Fig. 3A). Also, an apparatus (items 152-157, Fig. 15) for generating the desired waveforms.

Nonlinear electroseismic exploration

This invention provides for transmission of composite swept sine wave seismic signals wherein auto-correlation side lobes of such signals are reduced. The frequency spectrum of a desired swept sine wave seismic signal is divided into a number of consecutive segments, and the segments are arranged to form a sequence of segments by the order of frequency content. Component swept sine wave seismic signals each covering a segment in the sequence are generated simultaneously from the same location to produce the desired seismic signal. The component swept sine wave seismic signals are generated in a manner so that they alternate as upsweeps and downsweeps for consecutive segments in the sequence of segments. The seismic signal comprising the sum of the component swept sine wave seismic signals will have an auto-correlation function with reduced side lobes. Alternatively, the component swept sine wave seismic signals may be generated successively from the same location in a manner so that they alternate as upsweeps and downsweeps for consecutive segments in the sequence of segments. The reflections are successively recorded and stacked prior to correlation with the sum of the component swept sine wave seismic signals. Used in this manner, the sum of the component swept sine wave seismic signals will also have an auto-correlation function with reduced side lobes.

Composite seismic signal

A method of seismic exploration (140), using conversions between electromagnetic (144), and seismic energy (147), with particular attention to the electromagnetic source wave form used. According to the invention, source waveforms are correlated with reference waveforms selected to minimize correlation side lobes. Line power at 60 cycles may be used to provide a wave form element which is sequenced by a binary code to generate (141) an extended source wave form, segment with minimal correlation side lobes. Preferred binary codes include Golay Complementary Pairs, and maximal length shift register sequences.

Source waveforms for electroseismic exploration

Pseudorandom vibroseis sweeps have long been suggested as an alternative to standard linear sweeps due to their potential for having superior orthogonality, a lower likelihood for infrastructure damage, and increased low-frequency content. In the past, they were also attractive as they have a better autocorrelation shape, although that is less important today. Their use has been limited but the increasing popularity of simultaneous acquisition techniques has rekindled interest as they offer the ability to reduce interference noise. A wide variety of methods for generating pseudorandom sweeps have been developed over the last 45 years. This paper gives an overview of the motivations for their use before classifying and describing the different sweep types. Finally, the sweeps will be compared in terms of their major attributes including their suitability for simultaneous surveys.

The use of pseudorandom sweeps for vibroseis surveys

The linear swept-frequency sine wave has been the only seismic vibrator drive signal seriously used in almost two decades of operational vibrator activity. However, there are other drive signals and alterations of this commonly accepted signal which offer improved peak-to-side lobe ratios on signal autocorrelations, as well as significant savings in field data acquisition and subsequent data processing times. In fact, one class of input signals allows vibrator signal recordings to have the same time duration as equivalent dynamite signal recordings. Another class provides effective discrimination against periodic interferences such as 60-Hz power-line interference. Fortunately, these characteristics can be combined in the same input signal. The theory which suggests these improvements is outlined, and experimental seismic data which support the theory are shown. Data recorded at identical locations while using one conventional and three alternate input signals of equal bandwidths and energies are compared to show the advantages of the alternate signals.

Some alternate vibrator signals

One major effort in processing seismic data is that of correcting for the geometric effects or normal moveout introduced by multichannel data gathering procedures. Usually, this involves detecting coherent reflected signals, determining a time (t) versus moveout (Δt) relationship for these signals, time correcting the individual receiver traces, and combining selected traces into a stacked or composite section. Most of these operations are automated. Several velocity analysis techniques are available for detecting reflections automatically, and individual trace correcting and combining procedures are trivial. However, determination of a moveout function which will discriminate among valid reflections, multiple reflections, and noise is still largely a human operation. We describe a machine method devised to determine automatically these moveout, or t‐Δt, functions, as they are commonly called. The method makes extensive use of the observed phenomenon that the t‐Δt function is well approximated by a segmen...

Allen B. Cunningham

Papers

Some alternate vibrator signals

Interpretation of velocity spectra