Abstract. Key words: digital geophone, Jizhong depression, tight marl oil exploration, phase consistency, frequency. Introduction

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1 Bai Xuming, Yuan Shenghu, Wang Zedan, Chen Jingguo, Wang Xiaodong and Hu Qing, BGP, CNPC Abstract Geophone is one of the key equipment for seismic data acquisition and the quality of seismic data for prospecting depends directly on its performance. With the development of geophysical prospecting technology and high-resolution exploration requirement, especially in the tight marl oil exploration, increasing heavy demands of the accuracy and quality of seismic data acquired are presented, which include broad frequency band, high fidelity and high signal to noise ratio, so as to better identify lithology, pore fluids and fractured reservoir. In Jizhong depression, the tight marl oil is mainly located in Shulu sag, where the background interference is strong and the marl strata are deeply buried with variable occurrences. All these pose new challenges on detection technology of digital geophone. In this paper, the working mechanism and performance characteristics of digital geophone are described and then we focus on the first test conducted for tight marl oil exploration in Shulu depression. The test results show that compared with analog geophones, the seismic data acquired using digital geophone are characterized of less waveform distortion, broader frequency bandwidth and more abundant information, which lays a foundation for high-resolution processing and fractured reservoir prediction. Key words: digital geophone, Jizhong depression, tight marl oil exploration, phase consistency, frequency Introduction Marl, composed of clay minerals and carbonate particles, is a transitional sort sedimentary rock between clayrock and carbonate. By dropping dilute hydrochloric acid, marl can be distinguished from clayrock if gas bubbles occur, and from carbonate if dark argillaceous material appear where bubbles are produced (Hao et al, 2007). Tight oil is one of the unconventional oil and gas resources, widely distributed in China and around the world (Kang, 2012). In Huabei oilfield, the tight marl oil exploration targets are mainly located in the Shulu sag, Jizhong depression, where marl are widely distributed with considerable thickness, and favorable reservoir-forming conditions, and in addition, oil is full of its reservoir space laterally and oil-bearing segments distributes along the whole well longitudinally, so currently it is the most promising areas for tight oil exploration in Huabei oilfield. In Shulu sag, the marl reservoir space can be recognized as structural fracture, structural- dissolved fracture, pressure solution seam, inter-layer seam, dissolved pore, intergranular pore, induced fracture and etc., totally seven types. Among them, fracture is the majority and reservoir space is fracture-pore type, and cracks occur at very dip angle. In addition, the average reservoir porosity of marl ranges from 1.33 % to 4.88%, and the average permeability is between (0.08 ~ 14.5) 10-3 μm 2, so the reservoir exhibits ultra-low porosity with ultra-low permeability expressed and ultra-low porosity with low permeability (Wang et al, 2007). As marl reservoir is characterized of strong anisotropy and heterogeneity, the reservoir distribution is very complex. It poses high demands on seismic data fidelity and resolution to carry out fractured reservoir prediction, and to map fracture occurrence and distribution. Over the past years, analog geophones has been exclusive used in the region, and the data quality has not made an evident breakthrough. In addition, the great burial depth (3000 ~6000m) and variation in occurrence dip (30 ~50 ) of marl, and field serious interferences in this area present new challenges to digital geophones. In this paper, oriented to the geology demand for tight marl oil exploration in Shulu sag, the comparison test between digital and analog geophones is conducted for the first time. The results show that digital geophones have distinct advantages in ensuring the consistency of the raw data phase, frequency band, detection of weak signal from deep, and etc., although they are not powerful to resist random interference in field. The working mechanism and characteristics of digital geophones Based on the types of output signals, whether they are analog or digital, geophones can be divided into analog and digital types. Analog geophones are mainly speed detector and they also include acceleration and the piezoelectric detectors (Han et at, 2006). Digital geophones mainly belong to VectorSeis series or DSU series. The digital geophones are mainly DSU1 single- component digital detectors from SERCEL Company in this paper. Structure and working mechanism of digital detector Digital geophone is acceleration-type detector or so-called low-noise sensor based on semiconductor materials. Figure 1 demonstrates that its configuration consists of the following three parts (Ye, 2002): (1) Respectively, each component of one geophone made up of a microelectronic mechanical system, MEMS

2 Vibration axis (Micro Electro Machine System) forms a core body. (2) The microelectronic detection circuit and voltage feedback control IC, 24- bit ADC circuit and its data transmission and synchronous command control circuits form an assembly together with the core body. (3) High-strength plastic shell, metal cone tail, and the cables and plugs connecting geophones to acquisition station. sensor spring Mass body shell Electrode Cable manager framework Figure 1 Digital geophone configuration and working mechanism diagram MEMS is the most core technology for digital geophone. By detecting the change in capacitance on both sides, it works to adjust the amount of the power so that the balance of mass body is kep, and the amount of the power reflects the magnitude of vibration. Its working mechanism can be seen in Figure 2 in detail. The sensor is dormant when no voltage is present and gravity acceleration g pull the mass body down, then C1> C2. A cycle is started after electrifying by adjusting the size of V1 and V2 so that a force is produced to overcome the gravity until C1=C2, F1+F2=g, which means the sensor reaches equilibrium and it is ready to record signal. When receiving seismic signals vibrating along the axial direction, C1 and C2 values are continuously sampled and measured and their ratio keeps changing with the movement of the mass body. Simultaneously the negative feedback loop circuit alters the amount of V1 and V2 for compensation so that the mass body is kept at the center. The performance characteristics of digital geophone Digital geophone is much more powerful in performance than traditional analog geophone, and its remarkable features are as follows (Table 1): (1) A wide dynamic range. Digital detector s dynamic range is up to 105dB or more, which ensures the acquisition of high precision and is favorable for weak signal reception. (2) Slight distortion. Harmonic distortion index is less than 0.003%, at least an order of magnitude less than traditional analog receivers, which greatly improves the fidelity of raw field data. (3) Good linear response. The output of digital detector exhibits a very flat frequency amplitude and it always remains straight in the range of 1~500Hz. In addition, the output phase is zero phase, which is conducive to broaden data bandwidth. (4) High fidelity. Its perpendicular cross axis rejection ability is better than 46dB. The sensitivity error is small and the calibration accuracy can reach 0.3%. The quadrature signal isolation of the geophone is better than 40dB. (5) Direct digital signal output. Since the digital geophone has 24bit Σ -ADC circuit, it can directly output 24-bit digital signal with zero phase. (6) Free from electromagnetic signal interference. Because digital geophone is sensitive to gravity variations, it is not subject to the effects of external electromagnetic interferences, such as interferences of high- voltage power lines or underground cables. Table 1 Performance comparison between digital and analog geophones Item Digital Analog Output signal Digital Analog Linear response 0~800Hz 10~250Hz Dynamic range 105dB 60~70dB Harmonic distortion Less than 0.003% Greater than 0.03% Amplitude variation ±0.25% ±2.5% Sensitive vs. Stable Sensitive Temperature Power interference No Yes Instrument noise Low HF Low LF Testing data analysis Testing workflow design In order to carry out the feasibility study of digital geophone in tight marl oil exploration, detailed comparison tests are performed between digital and analog geophones in Shulu sag, Jizhong depression. The testing scheme is: a 2D wide line geometry with 4 wires of geophones, 2-shot and 600 traces is applied, and the spread length is 5990m

3 with the receiver interval of 20m and shot interval of 60m. Among 4 spreads, digital geophones are used in two and single-string 10 analog geophones are used in the other two. Moreover, the survey line runs across large-scale villages, provincial and country roads where serious interferences occur. nearly comparable. Therefore, field data acquisition is conducted using a small bin (eg. 10m 20m), and data processing can use super-bin by expanding bin size (eg. 20m 20m) to increase the fold number, which can serve as a solution to problems of weak digital-geophone shot record energy and low signal to noise ratio. Raw field data analysis According to the single shot records (Figure 2 ) using digital geophone and analog detector respectively, the energy of a shot record of digital geophone is weaker, usually as much as 1/6 of analog geophone and moreover the signal to noise ratio is lower with serious background interference; however, the bandwidth of digital-geophone shot record is 5~8Hz broader than analog-geophone record, and the digital-geophone shot record has more consistent phases. The raw data phase of digital detector is generally - 20~10, while that of analog geophones is -40~30., 200 times, 200 times, 400 times Figure 3 Stack sections when using digital and analog geophones during acquisition Migration section analysis In the pre-stack time migration sections (Figure 4) after noise reduction and migration, despite of same bin size and fold number, the section using digital geophones has more abundant information for weak interlayer reflection and higher resolution in the main target zones for tight marl oil exploration. And its frequency bandwidth is about 10Hz broader than that of analog geophone. Digital Figure 2 Shot records respectively using digital and analog geophones, and their energy, frequency spectrum and phase analysis Stack section analysis In the stack section before noise reduction (Figure 3), the SNR of digital- geophone data are lower than that of analog-geophone data when the fold is the same. But the fold number of digital- geophone data is as two times as that of analog-geophone data, the quality of both data is Anolog Figure 4 Migration sections and their frequency spectrum respectively using digital and analog geophones In addition, from impedance section (Figure 5), the resolution of digital- geophone impedance is higher than analog- geophone one, and the former can reflect the characteristics of the marl reservoir better.

4 sets of conglomerate interbedded with marls deposition can be recognized under Es3. From the bottom up, individual conglomerate layer thickness tends to be smaller, and the number of layers reduces as well. Moreover several sets of conglomerates, embedded in marl, are sweet spot in the marl reservoirs. Dongtan 1 H Figure 5 Impedance profiles respectively from data acquired using digital and analog receivers The above section comparison and analysis show that compared with analog receivers, seismic data acquired using digital-geophone have more abundant information for weak interlayer reflection, higher resolution, broader bandwidth and more consistency. In order to verify the reflections of digital geophone, the synthetic seismic record is inserted to migrated seismic section to calculate their correlation coefficient (Figure 6). The result is that the correlation coefficient of the digital-detector data is 0.306, while the coefficient of analog- geophone with synthetic record is Evidently, the correlation of digitalgeophone seismic data with the synthetic seismogram is 10% higher than that of analog-geophone data. Conclusions Figure 7 Newly acquired seismic data (1) Since digital detectors have wide dynamic ranges, good linear response, high sensitivity, slight distortion and etc, all-digital acquisition technology is the future trend for geophysical prospecting technology development. (2) The shot record acquired with digital geophone exhibits lower energy and signal to noise ratio than that with analog detector. But o when both fold numbers reach a comparable level, the two are roughly equal on the seismic section. (3) Compared with analog receivers, the data acquired using digital receivers contain more detailed information, and have distinct advantages in frequency bandwidth, fidelity and phase consistency, which provides an effective solution to technical problems facing tight marl oil exploration. Acknowledgements Figure 6 Seismic sections respectively using digital and analog receivers, and synthetic records Practical effectiveness In 2005, two 3D acquisitions were conducted with analog geophones, and the resulting seismic data exhibit narrow effective band and low resolution. Based on these data, it is difficult to identify small layers embedded in large sets of marl strata, and the demand for fracture prediction is not met for geophysical responses of fractures and caves cannot be identified. On the new data section (Figure 7), several This study was technically directed and supported by Mr. Deng Zhiwen, the general engineer of International Exploration Division, BGP, Mr. Qiu Yi, the general engineer of Oilfield Exploration Department, Huabei Oilfiel, and our colleges of Huabei Branch of Geophysical Research Institute, BGP, and we would like to express my heartfelt thanks.

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