Airgun Source Research Articles

Time-Lapse Monitoring of Daily Velocity Changes in Binchuan, Southwestern China, Using Large-Volume Air-Gun Source Array Data

Abstract Temporal changes of seismic velocities in the Earth’s crust can be induced by stress perturbations or material damage from reasons such as strong ground motion, volcanic activities, and atmospheric effects. However, monitoring the temporal changes remains challenging, because most of them generally exist in small travel-time differences of seismic data. Here, we present an excellent case of daily variations of the subsurface structure detected using a large-volume air-gun source array of one-month experiment in Binchuan, Yunnan, southwestern China. The seismic data were recorded by 12 stations within ∼10 km away from the source and used to detect velocity change in the crust using the deconvolution method and sliding window cross-correlation method, which can eliminate the “intercept” error when cutting the air-gun signals and get the real subsurface variations. Furthermore, the multichannel singular spectral analysis method is used to separate the daily change (∼1 cycle per day) from the “long-period” change (<1 cycle per day) or noise. The result suggests that the daily velocity changes at the two nearest stations, 53277 (offset ∼700 m) and 53278 (offset ∼2.3 km), are well correlated with air temperature variation with a time lag of 5.0 ± 1.5 hr, which reflects that the velocity variations at the subsurface are likely attributed to thermoelastic strain. In contrast, both daily and long-period velocity changes at distant stations correlate better with the varying air pressure than the temperature, indicating that the velocity variations at deeper depth are dominated by the elastic loading of air pressure. Our results demonstrate that the air-gun source is a powerful tool to detect the velocity variation of the shallow crust media.

The Wolfspar experience with low-frequency seismic source field data: Motivation, processing, and implications

The promise of fully automatic full-waveform inversion (FWI) — a (seismic) data-driven velocity model building process — has proven elusive in complex geologic settings, with impactful examples using field data unavailable until recently. In 2015, success with FWI at the Atlantis Field in the U.S. Gulf of Mexico demonstrated that semiautomatic velocity model building is possible, but it also raised the question of what more might be possible if seismic data tailor-made for FWI were available (e.g., with increased source-receiver offsets and bespoke low-frequency seismic sources). Motivated by the initial value case for FWI in settings such as the Gulf of Mexico, beginning in 2007 and continuing into 2021 BP designed, built, and field tested Wolfspar, an ultralow-frequency seismic source designed to produce seismic data tailor-made for FWI. A 3D field trial of Wolfspar was conducted over the Mad Dog Field in the Gulf of Mexico in 2017–2018. Low-frequency source (LFS) data were shot on a sparse grid (280 m inline, 2 to 4 km crossline) and recorded into ocean-bottom nodes simultaneously with air gun sources shooting on a conventional dense grid (50 m inline, 50 m crossline). Using the LFS data with FWI to improve the velocity model for imaging produced only incremental uplift in the subsalt image of the reservoir, albeit with image improvements at depths greater than 25,000 ft (approximately 7620 m). To better understand this, reprocessing and further analyses were conducted. We found that (1) the LFS achieved its design signal-to-noise ratio (S/N) goals over its frequency range; (2) the wave-extrapolation and imaging operators built into FWI and migration are very effective at suppressing low-frequency noise, so that densely sampled air gun data with a low S/N can still produce useable model updates with low frequencies; and (3) data density becomes less important at wider offsets. These results may have significant implications for future acquisition designs with low-frequency seismic sources going forward.

Reverse Time Migration of Vertical Cable Seismic Data to Image Hydrate-Bearing Sediments With High Resolution

Marine vertical cable seismic (VCS) is a promising survey technique for submarine complex structure imaging and reservoir monitoring, which uses vertical arrays of hydrophones deployed near the seafloor to record seismic wavefields in a quiet environment. Recently, we developed a new type of distributed VCS system for exploration and development of natural gas hydrates preserved in shallow sediments under the seafloor. Using this system and air-gun sources, we accomplished a 3D VCS yield data acquisition for gas hydrates exploration in the Shenhu area, South China Sea. In view of the characteristics of VCS geometry, we implement reverse time migration (RTM) on a common receiver gather to obtain high-resolution images of marine sediments. Due to the unique acquisition method, it is asymmetrical for the reflection path between the sources and the receivers in the VCS survey. Therefore, we apply accurate velocity analysis to common scatter point (CSP) gathers generated from common receiver gathers instead of the conventional velocity analysis based on common depth point gathers. RTM with this reliable velocity model results in high-resolution images of submarine hydrate-bearing sediments in deep water conditions. The RTM imaging section clearly shows the bottom simulating reflector (BSR) and also the reflection characteristics of the hydrate-bearing sediments filled with consolidated hydrates. Moreover, its resolution is relative to that of acoustic logging curves from the nearby borehole, and this imaging section is well consistent with the synthetic seismogram trace generated by the logging data. All these results reveal that VCS is a great potential technology for exploration and production of marine natural gas hydrates.

Temporal Variability of Thermohaline Fine-Structure Associated With the Subtropical Front Off the Southeast Coast of New Zealand in High-Frequency Short-Streamer Multi-Channel Seismic Data

Seismic oceanography generally makes use of multi-channel seismic reflection data sourced by air gun arrays and long hydrophone streamers to image oceanographic water masses and processes—often piggybacking on surveys that target deeper geological features below the seafloor. However, due to the acquisition methods employed, shallow (upper 200 m or so) regions of the ocean can be poorly imaged with this technique, and resolution is often lower than desirable for imaging fine-structure within the water column. In 2012, we collected a set of higher-resolution seismic lines off the southeast coast of New Zealand, with a generator-injector airgun source and hydrophone streamer configuration designed to improve images of shallower water masses and their boundaries. The seismic lines were acquired with coincident expendable bathythermograph deployments which provides direct ties between physical oceanographic data and seismic data, allowing for definitive identification of the Subtropical Front and associated water masses in the subsurface. Repeat acquisition along the same transect shows significant temporal variability on the scale of hours, illustrating the highly dynamic nature of this important ocean boundary. Comparisons to conventional low-frequency seismic data in the same location show the value of high-resolution acquisition methods in imaging the near-surface of the ocean and allowing subsurface features to be linked to their expressions at the surface.

Impacts of Reservoir Water Level Fluctuation on Measuring Seasonal Seismic Travel Time Changes in the Binchuan Basin, Yunnan, China

An airgun source in a water reservoir has been developed in the past decade as a green active source that had been proven effective to derive short-term subsurface structural changes. However, seasonal water level fluctuation in the reservoir affects the airgun signal, and thus whether the airgun signals can be used to derive robust seasonal variation in subsurface structure remains unclear. We use the airgun data observed in the Binchuan basin to estimate the seasonal variation of seismic travel time and compare the results with those derived from ambient noise data in the same frequency band. Our main observation is that seasonal change δt/t from airgun is negatively correlated to the variation of dominant frequency and water table fluctuation in the reservoir. One possible explanation is that water table fluctuation in the reservoir affects the dominant frequency of the airgun signal and causes significant phase shift. We also compute the travel time changes in P-wave from the empirical Green’s function after deconvolving the waveforms from a reference station that is 50 m from the airgun source. The dominant frequency after deconvolution still shows seasonal variation and correlates inversely to the travel time changes, suggesting that deconvolution cannot completely eliminate the source effect on travel time changes. We also use ambient noise cross-correlation to retrieve coda waves and then derive travel time changes in monthly stacked cross-correlations relative to a yearly average cross-correlation. We observe that seismic travel time increases to its local maximum in the end of August. The travel time changes lag behind the precipitation for about one month. We apply a poroelastic physical model to explain seismic travel time changes and find that a combined effect from precipitation and evaporation might induce the seasonal changes as shown in the ambient noise data. However, the pattern of travel time changes from the airgun differs from that from ambient noise, reflecting the strong effects of airgun source property changes. Therefore, we should be cautious to derive long-term subsurface structural variation from the airgun source and put more attention on stabilizing the dominant frequency of each excitation in the future experiments.

Observation of Higher-Mode Surface Waves from an Active Source in the Hutubi Basin, Xinjiang, China

ABSTRACTBasins with thick sediments can amplify and prolong the incoming seismic waves, which may cause serious damage to surface facilities. The amplification of seismic energy depends on the shear-wave velocity of the uppermost layers, which is generally estimated through surface wave analysis. Surface waves may propagate in different modes, and the mechanism of the mode development is not well understood. Exploiting a recently deployed permanent airgun source in the Hutubi basin, Xinjiang, northwest China, we conducted a field experiment to investigate the development of multimode surface waves. We observed surface waves at the frequency of 0.3–5.0 Hz with apparent group velocities of 200–900 m/s, and identified five modes of surface waves (three Rayleigh-wave modes and two Love-wave modes) through time–frequency and particle-motion analyses. We then measured 125 group velocity dispersion curves of the fundamental- and higher-mode surface waves, and further inverted the 1D S-wave velocity structure of the Hutubi basin. The S-wave velocity increases abruptly from 238 m/s at the surface to 643 m/s at 300 m depth. Synthetic seismograms with the inverted velocity structure capture the main features of the surface waves of the different modes. Synthetic tests suggest that the low velocity, high velocity gradient, and shallow source depth are likely the dominant contributing factors in the development of higher-mode surface waves.

3-D isotropic and anisotropic tomography of P-wave travel times from the Anhui airgun experiment in the Yangtze River

The Middle-Lower Yangtze River is a typical transition region between the nearly NW-oriented Tethys and NE-trending Pacific tectonic regimes. Structures of different periods and directions overlap strongly during these processes. The NE-trending Yangtze River compound structural belt and NW-trending Tongling-Hangzhou structural belt both control the magmatic activities and distributions of the metallogenic belts in the area. Here, we obtain 3-D high-resolution isotropic and azimuthally anisotropic velocity structures at depths of 1–10 km using the first arrivals from airgun sources. The velocity maps correspond well with the tectonic structures, with high-velocity anomalies distributed in ore-concentrated districts and low-velocity anomalies distributed along the Yangtze River. The fast directions are generally consistent with the fault strike, indicating that the azimuthal anisotropy is mainly dominated by the fault and fracture trends in the upper crust. The complicated fast directions near the Luzong and Tongling ore deposits reveal complex deformations in the upper crust, which are mainly caused by the intersection of the Yangtze River compound and Tongling-Hangzhou structural belts. The magma intrusion beneath the two ore deposits (Luzong and Tongling) are connected at depths of 5–10 km.

Distributed Acoustic Sensing Using a Large-Volume Airgun Source and Internet Fiber in an Urban Area

AbstractSeismological methods have been widely used to construct subsurface images in urban areas, for both seismological and engineering purposes. However, it remains a challenge to continuously operate a dense array in cities for high-resolution 4D imaging. In this study, we utilized distributed acoustic sensing (DAS) and a 5.2 km long, L-shaped, telecom, fiber-optic cable to record the wavefield from a highly repeatable airgun source located 7–10 km away. No P-wave signal was observed, but the S-wave signal emerged clearly on the shot-stacked traces, and the arrivals were consistent with collocated geophone traces. Because the signal quality is significantly affected by cable coupling and local noise, three methods can be employed to improve signal-to-noise ratio: (1) stacking contiguous, colinear channels to increase effective gauge length, (2) connecting multiple fibers within a single conduit and stacking collocated channels, and (3) using engineered fiber. In conclusion, the combination of DAS, using internet fiber and an airgun source with proven efficient signal enhancement methods, can provide frequent snapshots of the near surface across an urban area.

Illuminating high-resolution crustal fault zones using multi-scale dense arrays and airgun source

High-resolution imaging of fault zone structure and its temporal changes can not only advance our understanding of earthquake physics, but is also critical for better seismic hazard preparation and mitigation. In the past a few years, we deployed multi-scale dense arrays across the Chenghai fault system in Binchuan, Yunnan, China. The first array consisted of 381 intermediate-period three-component seismometers with an average station spacing of ∼2 ​km. The array has been deployed in the field for ∼3 months in 2017 and recorded numerous local and teleseismic earthquakes. Travel time analyses based on teleseismic earthquakes and an airgun source in the region indicated clear signature of low-velocity fault zones in the southern branch of the Chenghai fault system. In 2018 we deployed two other linear arrays using the same instruments with much smaller inter-station spacing, e.g. 30–50 ​m, across the southern branch the Chenghai fault. The profile lengths were 8 and 5 ​km, respectively. Record sections of the airgun source on the two linear arrays clearly marked a low-velocity zone (LVZ) within the southern array but no such signature in the northern array, suggesting along-strike variation of the LVZ. Although the instruments within our dense arrays had an intermediate frequency band, we demonstrated that they were capable of characterizing crustal structure with techniques commonly applied to broadband signals such as receiver functions. To our best knowledge, this was the first time to have multi-scale across-fault dense arrays with three-component seismometers in such apertures. These results lay out the pavement to comprehensively investigate fault zone structures as well as to derive subsurface structural changes using dense arrays and the airgun source.

A Modeling Comparison of the Potential Effects on Marine Mammals from Sounds Produced by Marine Vibroseis and Air Gun Seismic Sources

Concerns about the potential environmental impacts of geophysical surveys using air gun sources, coupled with advances in geophysical surveying technology and data processing, are driving research and development of commercially viable alternative technologies such as marine vibroseis (MV). MV systems produce controllable acoustic signals through volume displacement of water using a vibrating plate or shell. MV sources generally produce lower acoustic pressure and reduced bandwidth (spectral content) compared to air gun sources, but to be effective sources for geophysical surveys they typically produce longer duration signals with short inter-signal periods. Few studies have evaluated the potential effects of MV system use on marine fauna. In this desktop study, potential acoustic exposure of marine mammals was estimated for MV and air gun arrays by modeling the source signal, sound propagation, and animal movement in representative survey scenarios. In the scenarios, few marine mammals could be expected to be exposed to potentially injurious sound levels for either source type, but fewer were predicted for MV arrays than air gun arrays. The estimated number of marine mammals exposed to sound levels associated with behavioral disturbance depended on the selection of evaluation criteria. More behavioral disturbance was predicted for MV arrays compared to air gun arrays using a single threshold sound pressure level (SPL), while the opposite result was found when using frequency-weighted sound fields and a multiple-step, probabilistic, threshold function.

3-D isotropic and anisotropic shallow crustal structure on Pingtan Island, Fujian, southeastern coast of China

Pingtan Island is located on the southeastern coast of China between the Changle-Zhao'an and Binhai fault zones. We use P- and S-wave traveltime data from marine large-volume airgun sources for 3-D velocity structure inversion, and the spatial resolution is high at depths of 1–3 km. The isotropic inversion results show that the velocity in the island is higher in the midwestern area, which consists of sedimentary plains. The velocity is lower in the southern and eastern areas, which consist of eroded hills. Then we perform azimuthally anisotropic inversion using the obtained isotropic model as the initial model. The fast directions in the island are predominantly NE-SW and NNE-SSW, consistent with the main fault structures along the southeastern coast of China. The magnitude of anisotropy gradually decreases from south to north, consistent with the decreasing intensity of fault activity from south to north. On the southeastern coast of China, the fast direction in the upper crust is nearly orthogonal to that in the upper mantle from previous studies, which is nearly parallel to the absolute plate motion in the lower crust and upper mantle. The faults along the Fujian coastal area and Taiwan formed at different time period. But the fast directions in these two areas are similar. Therefore, the fast direction is mainly dominated by the trends of faults and fractures in the upper crust. And it is dominated by aligned olivine crystals induced by plate motion in the lower crust and upper mantle.

Utilization of airgun source signatures to improve seismic imaging of ocean water columns

Seismic oceanography is a new interdiscipline which uses “legacy” marine seismic reflection data collected by the oil industry to image ocean water columns like eddies and internal waves. Unlike seafloor reflections, ocean water-column reflections are typically weak due to the low acoustic impedance contrast, which challenges seismic oceanography. Utilization of the acoustic characteristics of low-frequency, broadband airgun source can enhance water-column signals for better ocean seismic imaging. Acoustics characteristics of received direct-path, water-column, and seafloor signals are analyzed and applied to design adaptive filtering process to enhance water-column signals. The processing increases the signal-to-noise ratio of water-column signal and attenuates the direct waves without changing the spectral content of the airgun source. The results can be useful to improve seismic oceanography by providing better imaging of ocean water columns.

Three-dimensional crustal Vp and Vs structures beneath the southern segment of the Tan-Lu fault revealed by active source and earthquake data

SUMMARY The 2400-km-long Tan-Lu fault, the largest deformation zone in eastern China, plays a decisive role in the seismicity, regional tectonics and mineral deposits distributions. However, the velocity structure beneath the Tan-Lu fault, particularly in the southern segment, is poorly imaged due to the lack of local earthquakes. To obtain a high-resolution crustal structure image, we carried out an active source experiment by firing mobile airgun sources along the Yangtze River in the Anhui Province in October 2015. We manually picked 4118 P wave and 1906 Swave first arrival times from the airgun signals. We also collected 28 957 P wave and 26 257 S wave first arrival times from local earthquakes in a larger area. 3-D crustal velocity images beneath the southern segment of the Tan-Lu fault and surrounding areas are studied using traveltime tomography. Compared with the local earthquake data, the active source data provide better constraints on the upper crustal structure, which further refines the resolution of the lower-crust structure. The Vp and Vs crustal structures are consistent with the local geological settings, and earthquakes are primarily clustered near faults and are spatially correlated with low-velocity zones. Strong velocity contrasts are observed across the Tan-Lu fault zone, which is the main factor controlling local anomalies. The high Vp, Vs and Vp/Vs beneath the Qinling-Dabie orogenic belt and the Middle-Lower Yangtze River Metallogenic Belt may relate to Mesozoic lithospheric delamination and asthenospheric upwelling. These results also demonstrate that the mobile large-volume airgun sources are promising tools for 3-D crustal structure surveys.

Delaying the source ghost to improve the ultralow-frequency response of a marine air-gun source

The competing effect between the fundamental bubble and its source-ghost response results in a strong attenuation of the lowest frequencies (below 7 Hz). This loss cannot be compensated easily by adjusting the source depth. Consequently, the low-frequency content in marine seismic data is not optimal, degrading the performance of low-frequency dependent processing approaches, such as full-waveform inversion. To overcome this, we have developed an additional source to counteract the ghost from the main source. In this situation, the fundamental bubble is characterized by the depth of the main source, whereas the ghost response is characterized by the summed depth of the main and additional sources. This source setup mitigates the competing effect and reduces the suppression of ultralow frequencies. Compared with a conventional horizontal source, our source design will reduce the mid- to high-frequency output, which may be beneficial in situations in which environmental constraints limit the maximum allowed output of a marine source.

Synchrosqueezing transform for geoacoustic inversion with air-gun source in the East China Sea

This paper presents a geoacoustic inversion method combining with warping transform and synchrosqueezing transform to improve the precision of extracting modal dispersion curves in shallow water waveguide. Dispersive characteristic is often used to invert geoacoustic parameters, while the dispersion curves are hard to be extracted in short range. Warping transform can compensate the nonlinearity of received signal but the frequency resolution is still inaccurate. Synchrosqueezing transform sharpens the frequency resolution of warped modes, which makes the extracted dispersion curves more accurate. The comparison of simulation shows that the result using combined method is closer to the preset value when implementing geoacoustic inversion of dispersion characteristics. Furthermore, this method is used to invert the sea bottom parameters of a shallow water experiment in the East China Sea. The Bayesian method and insitu measurement are used to verify the reliability of inversion results.

Dynamics of a toroidal bubble on a cylinder surface with an application to geophysical exploration

During the operation of a seismic airgun source, a certain amount of compressed high-pressure air is released from the airgun chamber into the surrounding water, generating an expanding toroidal bubble attached to the airgun-body. The subsequent oscillations of the bubble generate low-frequency pressure waves, which are used to map the ocean subbottom, e.g., to locate oil and gas reserves. The bubble dynamic behavior and the emitted pressure waves are inevitably influenced by the airgun-body. However, the bubble-airgun-body interaction is far from well understood. This paper investigates the strong interaction between a long cylinder and an attached toroidal bubble via hundreds of boundary integral simulations, aiming to provide new physical insights for airgun-bubble dynamics. Firstly, the overall physical phenomena are discussed and three types of bubble collapse patterns are identified, namely (i) upward jetting due to gravity, (ii) annular jet toward the cylinder body and (iii) weak/no jet. Thereafter, we investigate the effects of the cylinder radius, initial bubble pressure and Froude number on the bubble oscillation period and the pressure wave induced by the bubble. At last, the impact of a cylinder on a Sercel type airgun-bubble is discussed with a particular focus on the spectrum of the pressure waves.

Optimizing spatial distribution to minimize the inline directivity for a marine air-gun source

The spatial distribution of an air-gun array and the ghost reflection from the air-water interface cause a directional pattern of the source signature and introduce notches at certain frequencies. This significantly limits the bandwidth of the seismic data and the ability to obtain high-resolution subsurface imaging. With mathematical derivation and numerical simulation, we have determined that the inline directivity relies on the number of air guns and their spacing. Furthermore, the effective width of the take-off angles for an air-gun array is highly correlated with the reciprocal of the distance variance along the inline direction, which is subsequently used for optimizing the source design. For a normal horizontal source, one could adjust the distance between two consecutive air guns to improve the source signature. We optimize the spatial distribution for air guns along the inline and depth directions, that is, by using the synchronized multidepth stimulation mode, to minimize the inline directivity. Simulated results show that a six-gun composed multidepth source could be designed to achieve nearly identical energy distribution with up to ±30° take-off angle for the high-frequency end up to 250 Hz. This represents a significant improvement compared to a normal horizontal source and is more suitable for high-resolution seismic acquisition.

The impact of bubble curtains on seismic air-gun signatures and its high-frequency emission

In marine seismic acquisition, air guns are the most common source and, in recent years, research on their impact on the marine environment has increased. The main focus is on the reduction of emitted high frequencies, approximately greater than 200 Hz, which are normally not useful for seismic imaging. Therefore, potential ways to reduce the high frequencies from air guns are investigated and the development of alternative source types has increased. We have investigated the impact of bubble curtains on the source signature from seismic air guns because bubble curtains are known to mitigate high frequencies in other applications, e.g., pile driving for offshore wind farms. We have conducted tank experiments with two different configurations of bubble curtains around a single air gun and compared the results to the conventional source signature without a bubble curtain. The two different bubble curtains vary in size and in the way they are attached to the air gun. The amount of injected air into the bubble curtains is varied for both configurations. We compare the measured results to simulated data using a common model for air-gun source signatures. The results indicate a reduced peak amplitude with increasing air injection through the bubble curtain. This corresponds to a gradually decreasing frequency content for frequencies greater than 50 Hz. The frequencies of the source signal of less than 50 Hz are practically unaffected by the bubble curtain. In addition, the bubble time period of the source signal is slightly increased with an increasing amount of air injection through the bubble curtain. The main cause for the reduced peak amplitude is likely to be a buffer effect of the bubble curtain on the released air. Hence, a bubble curtain concentrated around the air-gun ports could be an efficient and practical solution to reduce the high-frequency acoustic emission from air guns.

Internal Structure of the Oceanic Lithosphere at a Melt‐Starved Ultraslow‐Spreading Mid‐Ocean Ridge: Insights From 2‐D Seismic Data

AbstractExtensive outcrops of serpentinized peridotite in melt‐starved spreading corridors of the ultraslow easternmost Southwest Indian Ridge are hypothesized to be due to slip on successive long‐offset normal faults that alternate polarity (flip‐flop detachment faults). We investigate the nature of the oceanic crust which forms under these conditions, using seismic reflection data acquired during the SISMOSMOOTH 2014 cruise. Using 3‐D binning, the seismic profiles were binned elastically, while three of the profiles shot closely were merged into one to take advantage of the larger air gun source volume. Using a poststack imaging sequence, we observe several types of reflectors at crustal and infracrustal depths, in the axial valley and off‐axis. Correlating our seismic observations with Residual Mantle Bouguer gravity anomalies and seafloor observations, we find that our results are explicable in the framework of the flip‐flop hypothesis of detachment faulting. Reflectors imaged down to 5 km into the basement and interpreted as due to damaged zones outlining the detachment faults dip 50° at the early stages, while at late stages after developing offsets >10 km, they dip 25°. Other reflectors observed in the crust are interpreted as moderate offset (<200 m) normal faults accommodating deformation and alteration in the hanging wall and channeling the sparse melt to the seafloor. We interpret these and other observed seismic reflectors in the frame of a two‐phase evolutionary sequence over the lifetime of two successive flip‐flop detachment faults: exhumation, footwall flexure, damage, serpentinization, and incipient magmatism in the footwall of one detachment fault; followed by further tectonic damage, alteration, and incipient magmatism in the hanging wall of the next detachment fault.

Diurnal and Semidiurnal P‐ and S‐Wave Velocity Changes Measured Using an Airgun Source

AbstractMonitoring the subtle seismic velocity changes provides a promising tool to study the Earth's dynamic processes. However, the precise monitoring of seismic velocity with high temporal resolution is still challenging especially at large distances. Here, we continuously monitor P‐wave and S‐wave velocities with a precision of up to 10−5 using an airgun source. During a 1‐week experiment conducted in the Yunnan Province, Southwestern China, unambiguous diurnal and semidiurnal velocity changes with amplitudes of ~10−4–10−3 were observed for both P‐ and S‐waves at distances up to 21 km. The amplitudes of P‐wave velocity (Vp) change are 1.2 to 3.8 times the amplitudes of S‐wave velocity (Vs) changes. The velocity variation decreases with the increase in distance and is larger in the basin region than that in the mountainous region. Seismic velocity change shows less correlation with volumetric tide strain but strong correlation with changes of air temperature and air pressure. We propose that the thermal strains from air temperature changes are the primary cause of the observed diurnal and semidiurnal seismic velocity changes. The stronger Vp than Vs change is explained by the higher Vp sensitivity to the saturation rate. The velocity changes in mountainous area are ascribed mainly to the crack density changes, while both crack density change and saturation change are required to account for the velocity changes in the basin.